Insect nicotinic receptor interactions in vivo with neonicotinoid,organophosphorus, and methylcarbamate insecticides and a synergist

The nicotinic acetylcholine (ACh) receptor (nAChR) is the principal insecticide target. Nearly half of the insecticides by number and world market value are neonicotinoids acting as nAChR agonists or organophosphorus (OP) and methylcarbamate (MC) acetylcholinesterase (AChE) inhibitors. There was no previous evidence for in vivo interactions of the nAChR agonists and AChE inhibitors. The nitromethyleneimidazole (NMI) analog of imidacloprid, a highly potent neonicotinoid, was used here as a radioligand, uniquely allowing for direct measurements of house fly (Musca domestica) head nAChR in vivo interactions with various nicotinic agents. Nine neonicotinoids inhibited house fly brain nAChR [³H]NMI binding in vivo, corresponding to their in vitro potency and the poisoning signs or toxicity they produced in intrathoracically treated house flies. Interestingly, nine topically applied OP or MC insecticides or analogs also gave similar results relative to in vivo nAChR binding inhibition and toxicity, but now also correlating with in vivo brain AChE inhibition, indicating that ACh is the ultimate OP- or MC-induced nAChR active agent. These findings on [³H]NMI binding in house fly brain membranes validate the nAChR in vivo target for the neonicotinoids, OPs and MCs. As an exception, the remarkably potent OP neonicotinoid synergist, O-propyl O-(2-propynyl) phenylphosphonate, inhibited nAChR in vivo without the corresponding AChE inhibition, possibly via a reactive ketene metabolite reacting with a critical nucleophile in the cytochrome P450 active site and the nAChR NMI binding site.The nicotinic nervous system has two principal sites of insecticide action, the nicotinic receptor (nAChR) activated by acetylcholine (ACh) and neonicotinoid agonists (1–6), and acetylcholinesterase (AChE) inhibited by organophosphorus (OP) and methylcarbamate (MC) compounds to generate and maintain localized toxic ACh levels (Fig. 1) (7). The nAChR and AChE targets have been identified in insects by multiple techniques but not by direct assays of the ACh binding site in the brain of poisoned insects. Here we use the outstanding insecticidal potency of the nitromethyleneimidazole (NMI) analog of imidacloprid (IMI) (8) as a radioligand (9), designated [³H]NMI, to directly measure the house fly (Musca domestica) nAChR not only in vitro but also in vivo, allowing us to validate by a previously undescribed method the neonicotinoid direct and OP/MC indirect nAChR targets (Fig. 2). This approach also helped solve the intriguing mechanism by which an O-(2-propynyl) phosphorus compound strongly synergizes neonicotinoid insecticidal activity (10) by dual inhibition of cytochrome P450 (CYP) (11–13) and the nAChR agonist site (described herein). Insecticide disruption at the insect nAChR can now be readily studied in vitro and in vivo with a single radioligand allowing better understanding of the action of several principal insecticide chemotypes (Fig. 3).Open in a separate windowFig. 1.The insect nicotinic receptor is the direct or indirect target for neonicotinoids, organophosphorus compounds and methylcarbamates, which make up about 45% of the insecticides by number and world market value (2, 7).Open in a separate windowFig. 2.In this study, Musca nicotinic receptor in vivo interactions with major insecticide chemotypes are revealed by a [³H]NMI radioligand reporter assay. *Position of tritium label.Open in a separate windowFig. 3.Two neonicotinoid nicotinic agonists and two anticholinesterase insecticides.     

Groundwater sapping as the cause of irreversible desertification of Hunshandake Sandy Lands,Inner Mongolia,northern China

In the middle-to-late Holocene, Earth’s monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation–atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China’s Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.Earth’s climate is subject to abrupt, severe, and widespread change, with nonlinear vegetation–atmosphere feedbacks that produced extensive and catastrophic ecosystem shifts and subsequent cultural disruption and dispersion during the Holocene (1–7). In the early and middle Holocene, northern China’s eastern deserts, including much of the currently sparsely vegetated and semistabilized Hunshandake (Figs. 1 and ​and2),2), were covered by forests (8), reflecting significantly wetter climate associated with intensification of monsoon precipitation by up to 50% (6).Open in a separate windowFig. 1.Geographical location of the Hunshandake Sandy Lands (A) and its area (encircled by red line in B). The black rectangle in B marks the location of the enlarged maps C and D on the Right, and the green rectangle shows the location of Fig. 2. Map C shows the localities of water samples, and map D shows the localities of sections with stratigraphy presented in Fig. 3. The sand–paleosol section P (Fig. 3) is on the southern margin, and the site Bayanchagan marks the coring site of ref. 8. Rivers with headwaters in the Hunshandake likely formed by groundwater sapping are marked in blue. Drainages to the southwest and west are currently undergoing groundwater sapping, with substantial spring-driven flow found at the current river base level.Open in a separate windowFig. 2.(Left) Holocene lakes and channels in the Hunshandake and lake extent at selected epochs. Upper, middle, and lower lakes are indicated by points A, B, and C, respectively. Xilamulun River (point D) drains to the east. Groundwater-sapping headcuts at the upper reaches of incised canyons (point E) suggest a mid-Holocene interval of easterly surface flow, followed by groundwater drainage beginning at the ca. 4.2 ka event. Northern and central channels at point E are currently abandoned, and groundwater sapping has migrated to the southerly of the three channels shown. (Right) Cross-sections of the predrainage shift, northerly drainage into Dali Lake (Localities shown on the Left), showing the increase in widths of channels downstream (Vertical exaggeration ∼30:1).Monsoonal weakening, in response to middle-to-late Holocene insolation decrease, reduced precipitation, leading to a green/sandy shift and desertification across Inner Mongolia between ca. 5,000 and 3,000 y (years) ago (6). However, variations in the timing of this transition (9, 10) suggest local/regional thresholds or possibly environmental tipping by stochastic fluctuations. The impacts of this wet-to-dry shift in the Hunshandake, expressed as variations in surface and subsurface hydrology coincident with the termination of the formation of thick and spatially extensive paleosols, and the impacts of a ca. 4.2 ka (1 ka = 1,000 years) mid-Holocene desiccation of the Hunshandake on the development of early Chinese culture remain poorly understood and controversial (6, 11). Here we report for the first time to our knowledge on variability in a large early-to-middle Holocene freshwater lake system in China’s Hunshandake Sandy Lands and associated vegetation change, which demonstrates a model of abrupt green/desert switching. We document a possible hydrologic trigger event for this switching and discuss associated vegetation and hydrologic disruptions that significantly impacted human activities in the region.     

Little evidence for enhanced phenotypic evolution in early teleosts relative to their living fossil sister group

Since Darwin, biologists have been struck by the extraordinary diversity of teleost fishes, particularly in contrast to their closest “living fossil” holostean relatives. Hypothesized drivers of teleost success include innovations in jaw mechanics, reproductive biology and, particularly at present, genomic architecture, yet all scenarios presuppose enhanced phenotypic diversification in teleosts. We test this key assumption by quantifying evolutionary rate and capacity for innovation in size and shape for the first 160 million y (Permian–Early Cretaceous) of evolution in neopterygian fishes (the more extensive clade containing teleosts and holosteans). We find that early teleosts do not show enhanced phenotypic evolution relative to holosteans. Instead, holostean rates and innovation often match or can even exceed those of stem-, crown-, and total-group teleosts, belying the living fossil reputation of their extant representatives. In addition, we find some evidence for heterogeneity within the teleost lineage. Although stem teleosts excel at discovering new body shapes, early crown-group taxa commonly display higher rates of shape evolution. However, the latter reflects low rates of shape evolution in stem teleosts relative to all other neopterygian taxa, rather than an exceptional feature of early crown teleosts. These results complement those emerging from studies of both extant teleosts as a whole and their sublineages, which generally fail to detect an association between genome duplication and significant shifts in rates of lineage diversification.Numbering ∼29,000 species, teleost fishes account for half of modern vertebrate richness. In contrast, their holostean sister group, consisting of gars and the bowfin, represents a mere eight species restricted to the freshwaters of eastern North America (1). This stark contrast between teleosts and Darwin''s original “living fossils” (2) provides the basis for assertions of teleost evolutionary superiority that are central to textbook scenarios (3, 4). Classic explanations for teleost success include key innovations in feeding (3, 5) (e.g., protrusible jaws and pharyngeal jaws) and reproduction (6, 7). More recent work implicates the duplicate genomes of teleosts (8–10) as the driver of their prolific phenotypic diversification (8, 11–13), concordant with the more general hypothesis that increased morphological complexity and innovation is an expected consequence of genome duplication (14, 15).Most arguments for enhanced phenotypic evolution in teleosts have been asserted rather than demonstrated (8, 11, 12, 15, 16; but see ref. 17), and draw heavily on the snapshot of taxonomic and phenotypic imbalance apparent between living holosteans and teleosts. The fossil record challenges this neontological narrative by revealing the remarkable taxonomic richness and morphological diversity of extinct holosteans (Fig. 1) (18, 19) and highlights geological intervals when holostean taxonomic richness exceeded that of teleosts (20). This paleontological view has an extensive pedigree. Darwin (2) invoked a long interval of cryptic teleost evolution preceding the late Mesozoic diversification of the modern radiation, a view subsequently supported by the implicit (18) or explicit (19) association of Triassic–Jurassic species previously recognized as “holostean ganoids” with the base of teleost phylogeny. This perspective became enshrined in mid-20th century treatments of actinopterygian evolution, which recognized an early-mid Mesozoic phase dominated by holosteans sensu lato and a later interval, extending to the modern day, dominated by teleosts (4, 20, 21). Contemporary paleontological accounts echo the classic interpretation of modest teleost origins (22–24), despite a systematic framework that substantially revises the classifications upon which older scenarios were based (22–25). Identification of explosive lineage diversification in nested teleost subclades like otophysans and percomorphs, rather than across the group as a whole, provides some circumstantial neontological support for this narrative (26).Open in a separate windowFig. 1.Phenotypic variation in early crown neopterygians. (A) Total-group holosteans. (B) Stem-group teleosts. (C) Crown-group teleosts. Taxa illustrated to scale.In contrast to quantified taxonomic patterns (20, 23, 24, 27), phenotypic evolution in early neopterygians has only been discussed in qualitative terms. The implicit paleontological model of morphological conservatism among early teleosts contrasts with the observation that clades aligned with the teleost stem lineage include some of the most divergent early neopterygians in terms of both size and shape (Fig. 1) (see, for example, refs. 28 and 29). These discrepancies point to considerable ambiguity in initial patterns of phenotypic diversification that lead to a striking contrast in the vertebrate tree of life, and underpins one of the most successful radiations of backboned animals.Here we tackle this uncertainty by quantifying rates of phenotypic evolution and capacity for evolutionary innovation for the first 160 million y of the crown neopterygian radiation. This late Permian (Wuchiapingian, ca. 260 Ma) to Cretaceous (Albian, ca. 100 Ma) sampling interval permits incorporation of diverse fossil holosteans and stem teleosts alongside early diverging crown teleost taxa (Figs. 1 and ​and2A2A and Figs. S1 and ​andS2),S2), resulting in a dataset of 483 nominal species-level lineages roughly divided between the holostean and teleost total groups (Fig. 2B and Fig. S2). Although genera are widely used as the currency in paleobiological studies of fossil fishes (30; but see ref. 31), we sampled at the species level to circumvent problems associated with representing geological age and morphology for multiple congeneric lineages. We gathered size [both log-transformed standard length (SL) and centroid size (CS); results from both are highly comparable (Figs. S3 and ​andS4);S4); SL results are reported in the main text] and shape data (the first three morphospace axes arising from a geometric morphometric analysis) (Fig. 2A and Figs. S1) from species where possible. To place these data within a phylogenetic context, we assembled a supertree based on published hypotheses of relationships. We assigned branch durations to a collection of trees under two scenarios for the timescale of neopterygian diversification based on molecular clock and paleontological estimates. Together, these scenarios bracket a range of plausible evolutionary timelines for this radiation (Fig. 2B). We used the samples of trees in conjunction with our morphological datasets to test for contrasts in rates of, and capacity for, phenotypic change between different partitions of the neopterygian Tree of Life (crown-, total-, and stem-group teleosts, total-group holosteans, and neopterygians minus crown-group teleosts), and the sensitivity of these conclusions to uncertainty in both relationships and evolutionary timescale. Critically, these include comparisons of phenotypic evolution in early crown-group teleosts—those species that are known with certainty to possess duplicate genomes—with rates in taxa characterized largely (neopterygians minus crown teleosts) or exclusively (holosteans) by unduplicated genomes. By restricting our scope to early diverging crown teleost lineages, we avoid potentially confounding signals from highly nested radiations that substantially postdate both genome duplication and the origin of crown teleosts (26, 32). This approach provides a test of widely held assumptions about the nature of morphological evolution in teleosts and their holostean sister lineage.Open in a separate windowFig. 2.(A) Morphospace of Permian–Early Cretaceous crown Neopterygii. (B) One supertree subjected to our paleontological (Upper) and molecular (Lower) timescaling procedures to illustrate contrasts in the range of evolutionary timescales considered. Colors of points (A) and branches (B) indicate membership in major partitions of neopterygian phylogeny. Topologies are given in Datasets S4 and S5. See Dataset S6 for source trees.Open in a separate windowFig. S1.Morphospace of 398 Permian–Early Cretaceous Neopterygii. Three major axes of shape variation are presented. Silhouettes and accompanying arrows illustrate the main anatomical correlates of these principal axes, as described in Open in a separate windowFig. S2.Morphospace of 398 Permian–Early Cretaceous Neopterygii, illustrating the major clades of (A) teleosts and (B) holosteans.Open in a separate windowFig. S3.Comparisons of size rates between (A) holosteans and teleosts, (B) crown teleosts and all other neopterygians, (C) crown teleosts and stem teleosts, (D) crown teleosts and holosteans, and (E) stem teleosts and holosteans. Comparisons were made using the full-size SL dataset, a CS dataset, and a smaller SL dataset pruned to exactly match the taxon sampling of the CS dataset. Identical taxon sampling leads the CS and pruned SL datasets to yield near identical results. Although the larger SL dataset results often differ slightly, the overall conclusion from each pairwise comparison (i.e., which outcome is the most likely in an overall majority of trees) is identical in all but one comparison (E, under molecular timescales).Open in a separate windowFig. S4.Comparisons of size innovation between (A) holosteans and teleosts, (B) crown teleosts and all other neopterygians, (C) crown teleosts and stem teleosts, (D) crown teleosts and holosteans, and (E) stem teleosts and holosteans. Comparisons were made using the full-size SL dataset, a CS dataset, and a smaller SL dataset pruned to exactly match the taxon sampling of the CS dataset. Comparisons of size innovation are presented for K value distributions of the three datasets resemble each other closely.     

Amphitheater-headed canyons formed by megaflooding at Malad Gorge,Idaho

Many bedrock canyons on Earth and Mars were eroded by upstream propagating headwalls, and a prominent goal in geomorphology and planetary science is to determine formation processes from canyon morphology. A diagnostic link between process and form remains highly controversial, however, and field investigations that isolate controls on canyon morphology are needed. Here we investigate the origin of Malad Gorge, Idaho, a canyon system cut into basalt with three remarkably distinct heads: two with amphitheater headwalls and the third housing the active Wood River and ending in a 7% grade knickzone. Scoured rims of the headwalls, relict plunge pools, sediment-transport constraints, and cosmogenic (³He) exposure ages indicate formation of the amphitheater-headed canyons by large-scale flooding ∼46 ka, coeval with formation of Box Canyon 18 km to the south as well as the eruption of McKinney Butte Basalt, suggesting widespread canyon formation following lava-flow diversion of the paleo-Wood River. Exposure ages within the knickzone-headed canyon indicate progressive upstream younging of strath terraces and a knickzone propagation rate of 2.5 cm/y over at least the past 33 ka. Results point to a potential diagnostic link between vertical amphitheater headwalls in basalt and rapid erosion during megaflooding due to the onset of block toppling, rather than previous interpretations of seepage erosion, with implications for quantifying the early hydrosphere of Mars.Landscapes adjust to perturbations in tectonics and base level through upstream propagation of steepened river reaches, or knickzones, thereby communicating environmental signals throughout a drainage basin (e.g., ref. 1). Nowhere are knickzones more important and apparent than in landscapes where canyon heads actively cut into plateaus, such as tributaries of the Grand Canyon, United States, and the basaltic plains of Mars (e.g., refs. 2–4). Here the stark topographic contrast between low-relief uplands and deeply incised canyons sharply delineates canyon rims and planform morphology. Canyon heads can have varied shapes from amphitheaters with vertical headwalls to more pointed planform shapes with lower gradients, and a prominent goal in geomorphology and planetary science is to link canyon morphology to formation processes (e.g., refs. 4–8), with implications for understanding the history of water on Mars.Amphitheater-headed canyons on Mars are most likely cut into layered basalt (9, 10), and canyon-formation interpretations have ranged widely from slow seepage erosion to catastrophic megafloods (4–6, 11, 12). Few studies have been conducted on the formation of amphitheater-headed canyons in basalt on Earth, however, and instead, terrestrial canyons in other substrates are often used as Martian analogs. For example, groundwater sapping is a key process in forming amphitheater-headed canyons in unconsolidated sand (e.g., refs. 8, 13, 14), but its importance is controversial in rock (5, 12, 15). Amphitheater-headed canyons are also common to plateaus with strong-over-weak sedimentary rocks (3, 16); however, here the tendency for undercutting is so strong that canyon-head morphology may bear little information about erosional processes, whether driven by groundwater or overland flow (e.g., refs. 3, 5, 17). Canyons in some basaltic landscapes lack strong-over-weak stratigraphy, contain large boulders that require transport, and show potential for headwall retreat by block toppling (18–21), all of which make extension of process–form relationships in sand and sedimentary rocks to basalt and Mars uncertain.To test the hypothesis of a link between canyon formation and canyon morphology in basalt, we need field measurements that can constrain formation processes for canyons with distinct morphologies, but carved into the same rock type. Here we report on the origin of Malad Gorge, a canyon complex eroded into columnar basalt with markedly different shaped canyon heads. Results point to a potential diagnostic link between canyon-head morphology and formative process by megaflood erosion in basalt.Malad Gorge is a tributary to the Snake River Canyon, Idaho, within the Snake River Plain, a broad depression filled by volcanic flows that erupted between ∼15 Ma and ∼2 ka (22, 23). The gorge sits at the northern extent of Hagerman Valley, a particularly wide (∼7 km) part of the Snake River Canyon (Fig. 1). Malad Gorge is eroded into the Gooding Butte Basalt [⁴⁰Ar/³⁹Ar eruption age: 373 ± 12 ka (25)] which is composed of stacked lava beds, each several meters thick with similar well-defined columns bounded by cooling joints and no apparent differences in strength between beds. The Wood (or Malad) River, a major drainage system from the Sawtooth Range to the north, drains through Malad Gorge before joining the Snake River. The Wood River is thought to have been diverted from an ancestral, now pillow lava-filled canyon into Malad Gorge by McKinney Butte basalt flows (24) [⁴⁰Ar/³⁹Ar eruption age: 52 ± 24 ka (25) (Fig. 1).Open in a separate windowFig. 1.Shaded relief map of the study region (50-m contour interval) showing basalt flows (23), their exposure age sample locations, and the path of the ancestral Wood River following Malde (24) (US Geological Survey).Malad Gorge contains three distinct canyon heads herein referred to as Woody’s Cove, Stubby Canyon, and Pointed Canyon (Fig. 2A). Woody’s Cove and Stubby have amphitheater heads with ∼50-m-high vertical headwalls (Fig. 2C), and talus accumulation at headwall bases indicates long-lived inactive fluvial transport (Fig. 3 A and B). Woody’s Cove, the shortest of the three canyons, lacks major spring flows and has minor, intermittent overland flow partially fed by irrigation runoff that spills over the canyon rim. Stubby has no modern-day overland flow entering the canyon, and springs emanate from a pool near its headwall (Fig. 3B). In contrast, Pointed Canyon is distinctly more acute in planform morphology, contains a 7% grade knickzone composed of multiple steps rather than a vertical headwall (Figs. 2C and ​and3C),3C), and extends the farthest upstream.Open in a separate windowFig. 2.Malad Gorge topography (10-m contour interval) and aerial orthophotography (US Geological Survey). (A) Overview map and (B) close-up for Stubby and Pointed canyons showing mapped bedrock scours (white arrows), exposure age sample locations (red circles) with age results, location of the uppermost active knickpoint (black circle), abandoned bedrock channels (blue dashed lines), and grain-size analysis sites (blue squares). The blue star shows the reconstructed location of the headwall of Pointed Canyon at 46 ka (see Discussion and Fig. 5). (C) Longitudinal profile along Stubby and Pointed canyons from their confluence (shown as white lines in B) with local slope, S, averaged over regions demarked by dashed lines (Fig. 4A shows close-up of profile in Stubby Canyon).Open in a separate windowFig. 3.Photographs of (A) headwall of Woody’s Cove (person for scale, circled), (B) ∼50-m-high headwall of Stubby Canyon, (C) downstream-most waterfall at Pointed Canyon knickzone (12-m-high waterfall with overcrossing highway for scale), (D) fluted and polished notch at the rim of Stubby Canyon (notch relief is 10 m), (E) upstream-most waterfall at Pointed Canyon knickzone (within the southern anabranch of Fig. S2), and (F) upstream-most abandoned channel in Fig. 2B and Fig. S2 (channel relief is ∼10 m). White coloring on the headwalls in A and B is likely residue from irrigation runoff.Early work attributed the amphitheater-headed canyons in this region—Malad Gorge, Box Canyon, located 18 km south of Malad Gorge (Fig. 1), and Blue Lakes Canyon located 42 km to the SE—to formation by seepage erosion because of no modern overland flow and the occurrence of some of the largest springs in the United States in this region (7). Because spring flows (e.g., ∼10 m³/s in Box Canyon; US Geological Survey gauge 13095500) are far deficient to move the boulders that line the canyon floors, Stearns (7) reasoned that the boulders must chemically erode in place. This explanation is improbable, however, given the young age of the Quaternary basalt (25), spring water saturated in dissolved solids (19), and no evidence of rapid chemical weathering (e.g., talus blocks are angular and have little to no weathering rinds). Instead of groundwater sapping, Box Canyon was likely carved by a large-scale flood event that occurred ∼45 ka based on ³He cosmogenic exposure age dating of the scoured rim of the canyon headwall (19, 26). In addition, Blue Lakes Canyon was formed during the Bonneville Flood [∼18–22 ka (27, 28)], one of the world’s largest outburst floods that occurred as a result of catastrophic draining of glacial lake Bonneville (21). In both cases, canyon formation was inferred to have occurred through upstream headwall propagation by waterfall erosion.Herein we aim to test whether the amphitheater-headed canyons at Malad Gorge also owe their origin to catastrophic flooding, whether Pointed Canyon has a different origin, and whether canyon morphology is diagnostic of formation process. To this end we present field observations, sediment-size measurements, hydraulic modeling, and cosmogenic exposure ages of water-scoured rock surfaces and basalt-flow surfaces (Methods and Tables S1 and S2).     

Universal features in the photoemission spectroscopy of high-temperature superconductors

The energy gap for electronic excitations is one of the most important characteristics of the superconducting state, as it directly reflects the pairing of electrons. In the copper–oxide high-temperature superconductors (HTSCs), a strongly anisotropic energy gap, which vanishes along high-symmetry directions, is a clear manifestation of the d-wave symmetry of the pairing. There is, however, a dramatic change in the form of the gap anisotropy with reduced carrier concentration (underdoping). Although the vanishing of the gap along the diagonal to the square Cu–O bond directions is robust, the doping dependence of the large gap along the Cu–O directions suggests that its origin might be different from pairing. It is thus tempting to associate the large gap with a second-order parameter distinct from superconductivity. We use angle-resolved photoemission spectroscopy to show that the two-gap behavior and the destruction of well-defined electronic excitations are not universal features of HTSCs, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped samples either show two-gap behavior or not. In contrast, many other characteristics of HTSCs, such as the dome-like dependence of on doping, long-lived excitations along the diagonals to the Cu–O bonds, and an energy gap at the Brillouin zone boundary that decreases monotonically with doping while persisting above (the pseudogap), are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high- superconductivity are relatively insensitive to differences in the electronic states along the Cu–O bond directions.Elucidating the mechanism of high-temperature superconductivity in the copper–oxide materials remains one of the most challenging open problems in physics. It has attracted the attention of scientists working in fields as diverse as materials science, condensed matter physics, cold atoms, and string theory. To clearly define the problem of high-temperature superconductors (HTSCs), it is essential to establish which of the plethora of observed features are universal, namely, qualitatively unaffected by material-specific details.An important early result concerns the universality of the symmetry of the order parameter for superconductivity. The order parameter was found to change sign under a 90° rotation (1, 2), which implies that the energy gap must vanish along the diagonal to the Cu–O bonds, i.e., the Brillouin zone diagonal. This sign change is consistent with early spectroscopic studies of near-optimally-doped samples (those with the highest in a given family), where a energy gap (3, 4) was observed (ϕ being the angle from the Cu–O bond direction), the simplest functional form consistent with d-wave pairing. More recently, there is considerable evidence (5–8) that, with underdoping, the anisotropy of the energy gap deviates markedly from the simple form. Although the gap node at is observed at all dopings, the gap near the antinode (near and 90°) is significantly larger than that expected from the simplest d-wave form. Further, the large gap continues to persist in underdoped (UD) materials as the normal-state pseudogap (9–11) above . This suggests that the small (near-nodal) and large (antinodal) gaps are of completely different origin, the former related to superconductivity and the latter to some other competing order parameter.This two-gap picture has attracted much attention (8), raising the possibility that multiple energy scales are involved in the HTSC problem. There is mounting evidence for additional broken symmetries (12–14) in UD cuprates, once superconductivity is weakened upon approaching the Mott insulating state. The central issue is the role of these additional order parameters in impacting the universal properties of high- superconductivity.In this paper we use angle-resolved photoemission (ARPES) to examine the universality of the two-gap scenario in HTSCs by addressing the following questions. To what extent are the observed deviations from a simple d-wave energy gap independent of material details? How does the observed gap anisotropy correlate, as a function of doping, with other spectroscopic features such as the size of the antinodal gap, and the spectral weights of the nodal and antinodal quasiparticle excitations?We systematically examine the electronic spectra of various families of cation-substituted Bi₂Sr₂CaCu₂O_8+δ single crystals as a function of carrier concentration to elucidate which properties are universal and which are not. We present ARPES data on four families of float-zone-grown Bi₂Sr₂CaCu₂O_8+δ single crystals, where was adjusted by both oxygen content and cation doping. As-grown samples, labeled Bi2212, have an optimal of 91 K. These crystals were UD to by varying the oxygen content. Ca-rich crystals (grown from material with a starting composition Bi_2.1Sr_1.4Ca_1.5Cu₂O_8+δ) with an optimal of 82 K are labeled Ca. Two Dy-doped families grown with starting compositions Bi_2.1Sr_1.9Ca_{1 − x}Dy_xCu₂O_8+δ with x = 0.1 and 0.3 are labeled Dy1 and Dy2, respectively. A full list of the samples used and their determined from magnetization measurements are shown in SI Text, where we also show high-resolution X-ray data that give evidence for the excellent structural quality of our samples.Our main result is that the Dy1 and Dy2 samples show clear evidence of a two-gap behavior in the UD regime , with loss of coherent quasiparticles in the antinodal region of k space where the gap deviates from a simple d-wave form. In marked contrast, the UD Bi2212 samples and the Ca samples show a simple d-wave gap in the superconducting state and sharp quasiparticles over the entire Fermi surface in a similar range of the UD regime. We conclude by discussing the implications of the nonuniversality of the two-gap behavior for the phenomenon of high superconductivity.We begin our comparison of the various families of samples by focusing in Fig. 1 on the superconducting state antinodal spectra as a function of underdoping. The antinode is the Fermi momentum k_F on the Brillouin zone boundary, where the energy gap is a maximum and, as we shall see, the differences between the various samples are the most striking. We show data at optimal doping, corresponding to the highest in each family, in Fig. 1A. Increasing Dy leads to a small suppression of the optimal compared with Bi2212, together with an increase in the antinodal gap and a significant reduction of the quasiparticle weight. This trend continues down to moderate underdoping, as seen in Fig. 1B, where we show UD Bi2212 and Dy2 samples with very similar . For more severely UD samples, with , spectral changes in the Dy-substituted samples are far more dramatic. In Fig. 1C, we see that quasiparticle peaks in the Dy samples are no longer visible, even well below , consistent with earlier work on Y-doped Bi2212 and also Bi2201 and La_1.85Sr_0.15CuO₄ (5, 15–18). In contrast, Bi2212 and Ca-doped samples with comparable continue to exhibit quasiparticle peaks. In this respect the latter two are similar to epitaxially grown thin-film samples that exhibit quasiparticle peaks all of the way down to the lowest (19).Open in a separate windowFig. 1.Superconducting state antinodal ARPES spectra. We use the label “Bi2212” for samples without cation doping, “Dy1” for 10% Dy, “Dy2” for 30% Dy, and “Ca” for Ca-doped samples. The temperature is indicated along with . OP denotes optimal doped, UD underdoped, and OD overdoped samples. (A) Antinodal spectra for OP samples of three different families: Bi2212 (blue), Dy1 (green), and Dy2 (red), showing an increase in gap and a decrease in quasiparticle weight with increasing Dy content. (B) Antinodal spectra for UD samples with similar (≃66 K) for Bi2212 (blue) and Dy2 (red). As in A, there is a larger gap and smaller coherent weight in the Dy-substituted sample. (C) Same as in B, but for four UD samples with near 55 K for Bi2212 (dark blue), Ca (light blue), Dy1 (green), and Dy2 (red). The Bi2212 and Ca spectra are very similar to each other and quite different from those of the Dy1 and Dy2 materials. (D) Doping evolution of the antinodal spectra of four Dy1 samples from OP to UD . (E) Doping evolution of the antinodal spectra of four Dy2 samples from OP to UD . We see in D and E the sudden loss of quasiparticle weight for below 60 K. (F) Doping evolution of the antinodal spectra of three Bi2212 samples and three Ca samples, showing well-defined quasiparticle peaks in all cases.A significant feature of the highly UD Dy samples in Fig. 1C is that, in addition to the strong suppression of the quasiparticle peak, there is severe loss of low-energy spectral weight. To clearly highlight this, we show the doping evolution of antinodal spectra for the Dy1 (Fig. 1D) and Dy2 (Fig. 1E) samples. These observations are in striking contrast with the Bi2212 and Ca-doped data in Fig. 1F, where we do see a systematic reduction of the quasiparticle peak with underdoping, but not a complete wipeout of the low-energy spectral weight. To the extent that the superconducting state peak–dip–hump line shape (20, 21) originates from one broad normal-state spectral peak, the changes in spectra of the Dy materials are not simply due to a loss of coherence, but more likely a loss of the entire spectral weight near the chemical potential.The doping evolution of the k-dependent gap is illustrated in Figs. 2 and​and 3. 3. In Fig. 2 we contrast the optimally doped Dy1 (T_c = 86 K) sample (Fig. 2 A and B) with a severely UD Dy1 (T_c = 38 K) sample (Fig. 2 C and D ), the spectra being particle–hole-symmetrized to better illustrate the gap. The OP 86 K sample shows a well-defined quasiparticle peak over the entire Fermi surface (Fig. 2A) with a simple d-wave gap of the form (blue curve in Fig. 2B). For the UD 38 K sample, we see in Fig. 2C well-defined quasiparticles near the node (red spectra), but not near the antinode (blue spectra). The near-nodal gaps (red triangles in Fig. 2D) are obtained from the energy of quasiparticle peaks and continue to follow a d-wave gap (blue curve in Fig. 2D). However, once the quasiparticle peak is lost closer to the antinode, one has to use some other definition of the gap scale. We identify a break in the slope of the spectrum, by locating the energy scale at which it deviates from the black straight lines (Fig. 2C), which leads to the gap estimates (blue squares) in Fig. 2D.Open in a separate windowFig. 2.Superconducting state spectra and energy gap for OD and highly UD Dy1 samples. (A) Symmetrized spectra at k_F, from the antinode (Upper) to the node (Lower) for an OP 86 K Dy1 sample. (B) Gap as a function of Fermi surface angle (0° is the antinode and 45° the node). The blue curve is a d-wave fit to the data. (C) Same as A for an UD 38 K Dy1 sample. Curves, near the node, with discernible quasiparticle peaks are shown in red; those near the antinode are shown in blue. (D) Gap along the Fermi surface from data of C.Open in a separate windowFig. 3.Energy gap anisotropies of various samples. (A) OD 79 K Ca (where ); (B) UD 54 K Ca; (C) OP 81 K Dy2; and (D) UD 59 K Dy2. The two near-optimal samples in A and C both show a simple d-wave gap. This behavior persists in the UD Ca sample of B, but the UD Dy2 sample of D has a two-gap behavior despite having a similar to the UD Ca sample.Despite the larger error bar associated with gap scale extraction in the absence of quasiparticles, it is nevertheless clear (Fig. 2D) that the UD 38 K Dy1 sample has an energy gap that deviates markedly from the simple d-wave form. This observation is called two-gap in the UD regime, in contrast with a single gap near optimality (Fig. 2B). It is easy to observe from Fig. 2 that the Fermi surface angle at which the energy gap starts to deviate from the form matches the one at which the spectral peak gets washed out. This is very similar to the two-gap behavior demonstrated in refs. 5, 15–18. From this, one might conclude that two-gap behavior is directly correlated with a loss of well-defined quasiparticle excitations in the antinodal region. However, we point to recent ARPES data on Y-doped Bi2212 (6, 7), where two-gap behavior has been observed despite the presence of small antinodal quasiparticle peaks.We next show that the two-gap behavior is not a universal feature of all UD samples. To make this point, we compare in Fig. 3 the gap anisotropies of the Ca-doped samples (Fig. 3 A and B) with the Dy2 samples (Fig. 3 C and D) with essentially identical , where both families have the same optimal . The near-optimal samples, OD 79 K Ca (Fig. 3A) and OP 81 K Dy2 (Fig. 3C) samples, both have a simple d-wave anisotropy (although different maximum gap values at the antinode). However, upon underdoping to similar values, the two have markedly different gap anisotropies. The UD 59 K Dy2 sample (Fig. 3D) shows two-gap behavior, and an absence of quasiparticles near the antinode (similar to the discussion in connection with Fig. 2 above). However, the UD 54 K Ca sample (Fig. 3B) continues to exhibit sharp spectral peaks and a single-gap, despite a very similar as the UD 59 K Dy2.Having established the qualitative differences in the gap anisotropies for various samples as a function of underdoping, we next summarize in Fig. 4 the doping evolution of various spectroscopic features. Instead of estimating the carrier concentration in our samples using an empirical equation (22) (that may or may not be valid for various cation substitutions), we prefer to use the measured to label the doping. In Fig. 4A we show the doping evolution of the antinodal energy gap, which is consistent with the known increase in the gap with underdoping.Open in a separate windowFig. 4.Antinodal gaps and quasiparticle weights. (A) Antinodal energy gap as a function of doping for various samples is seen to grow monotonically with underdoping. Here, and in B and C, the doping is characterized by the measured quantity , with UD samples shown to the left of and OD samples to the right. All results are at temperatures well below . (B) Coherent spectral weight for antinodal quasiparticles as a function of doping. Dy-doped samples exhibit a rapid suppression of this weight to zero for UD , whereas the Ca-doped samples show robust antinodal peaks even for . (C) Coherent spectral weight for nodal quasiparticles as a function of doping, which is seen to be much more robust than the antinodal one.The coherent spectral weight Z for antinodal quasiparticles is plotted in Fig. 4B (for details on the procedure used to estimate this weight, from a ratio of spectral areas, see SI Text). The Dy1 and Dy2 samples both show a sudden and complete loss of Z with underdoping (23), which coincides with the appearance of two-gap behavior. In marked contrast with the Dy samples, the Bi2212 and Ca samples that exhibit a single d-wave gap show a gradual drop in the antinodal Z. On the other hand, we find that the nodal excitations are much less sensitive to how the sample is UD compared with the antinodal ones. Similar sharp nodal excitations have been observed in Dy-doped Bi2212 samples in ref. 7 as well. The nodal quasiparticle weight Z in Fig. 4C decreases smoothly with underdoping for all families of samples, as expected for a doped Mott insulator (24).The two-gap behavior and the attendant loss of quasiparticle weight near the antinode imply a nodal–antinodal dichotomy, aspects of which have been recognized in k space (25–27) and in real space (28–30). Two possible, not mutually exclusive, causes of this behavior are disorder and competing orders.It is known that antinodal states are much more susceptible to impurity scattering, whereas near-nodal excitations are protected (31). However, it is not a priori clear why certain cation substitutions (Dy) should lead to more electronic disorder than others (Ca). As shown by our X-ray studies in SI Text, there is no difference in the structural disorder in Dy and Ca samples. One possibility is that Dy has a local moment, but there is no direct experimental evidence for this.The two-gap behavior in UD materials, with a large antinodal gap that persists above , is suggestive of an order parameter, distinct from d-wave superconductivity, which sets in at the pseudogap temperature . There are several experiments (12–14) that find evidence for a broken symmetry at . However, it is not understood how the observed small, and often subtle, order parameter(s) could lead to large antinodal gaps of , with a loss of spectral weight over a much larger energy range (Fig. 1 D and E).We now discuss the pertinence of competing order parameters based on our measurements. First, in our ARPES data, we have not found any direct evidence for density wave ordering (say, from zone folding). Second, our X-ray data did not provide any signature for additional diffraction peaks expected for long-range density wave ordering. However, none of these null results provide definitive evidence for the absence of a density wave ordering, particularly if it were short range. In contrast, in previously published work (5, 15–18), two-gap behavior has been conjectured to be a direct consequence of phase competition between d-wave superconductivity and some type of density wave ordering. As we have demonstrated, two-gap behavior in and of itself is a sample-specific issue and hence, even if we assume a linkage between competing order and two-gap behavior, it cannot be central to the question of superconductivity in HTSC systems.Whatever the mechanism leading to qualitatively different gap anisotropies for the UD Dy and Ca samples, it only produces relatively small, quantitative changes in key aspects of these materials, such as the dependence of on doping, the presence of sharp nodal quasiparticles, and the pseudogap. We thus conclude that antinodal states do not make a substantial contribution to the universal features of HTSCs. Clearly, two gaps are not necessary for high-temperature superconductivity.     

Plasmonic photonic crystals realized through DNA-programmable assembly

Three-dimensional dielectric photonic crystals have well-established enhanced light–matter interactions via high Q factors. Their plasmonic counterparts based on arrays of nanoparticles, however, have not been experimentally well explored owing to a lack of available synthetic routes for preparing them. However, such structures should facilitate these interactions based on the small mode volumes associated with plasmonic polarization. Herein we report strong light-plasmon interactions within 3D plasmonic photonic crystals that have lattice constants and nanoparticle diameters that can be independently controlled in the deep subwavelength size regime by using a DNA-programmable assembly technique. The strong coupling within such crystals is probed with backscattering spectra, and the mode splitting (0.10 and 0.24 eV) is defined based on dispersion diagrams. Numerical simulations predict that the crystal photonic modes (Fabry–Perot modes) can be enhanced by coating the crystals with a silver layer, achieving moderate Q factors (∼10²) over the visible and near-infrared spectrum.Enhancing light–matter interactions is essential in photonics, including areas such as nonlinear optics (1), quantum optics (2, 3), and high-Q lasing (4). In general, there are two ways of achieving this in optical cavities: (i) with long cavity lifetimes (high Q factors) and (ii) with strong photonic confinement (small mode volume, V) (2, 3). In particular, 3D dielectric photonic crystals, with symmetry-induced photonic band gaps (Bragg gaps), enhance light–matter interactions via high Q factors (4–6). However, the coupling strength between photons and electronic transitions within such systems is intrinsically weak owing to diffraction-limited photonic confinement (3, 7). Recently, it was suggested that a plasmonic counterpart of photonic crystals can prohibit light propagation and open a photonic band gap by strong coupling between surface plasmons and photonic modes (a polariton gap) if the crystal is in deep subwavelength size regime (8); these crystals have been referred to as polaritonic photonic crystals (PPCs) (9–12). This opens up the exciting possibility of combining plasmonics with 3D photonics in the strong coupling regime and optimizing the photonic crystals as small-mode-volume devices owing to the strong plasmonic mode confinement (13). However, such systems require control over the positioning of the plasmonic elements in the crystal on the nano- or deep subwavelength scale (8), and owing to this synthetic challenge such 3D PPCs have largely remained unexplored in the visible wavelength range.The recent discovery that DNA can be used to program the assembly of high-quality single crystals with well-defined crystal habits consisting of nanoparticles occupying sites in a preconceived lattice (14) opens up possibilities for fine tuning the interaction between light and highly organized collections of particles as a function of lattice constant and particle size. Here, we report that 3D plasmonic photonic crystals made by DNA-programmable assembly can be used to establish strong light–plasmon coupling with tunability based upon the DNA interconnects and the corresponding volume fraction of the plasmonic elements. The strong coupling is manifested in crystal backscattering spectra and mode splitting (0.10 and 0.24 eV) in dispersion diagrams. Simulation results that we also include show that, by coating the crystals with a silver layer, Fabry–Perot photonic modes of crystals can be enhanced, with moderate cavity Q factors (∼10²) over the visible and near-infrared (NIR) spectrum. In addition to being the first devices made by DNA-programmable colloidal crystallization, they illustrate the potential of the technique for making novel 3D crystals for photonic studies and applications.The plasmonic PPCs are synthesized from two batches of gold nanoparticles, each functionalized with oligonucleotide sequences that are hybridized to complementary linker sequences that induce the assembly of the particles into rhombic dodecahedra single crystals with a body-centered-cubic (BCC) arrangement of the particles (14) (Supporting Information, sections S1 and S2, Fig. S1, and Tables S1 and S2). The lattice constants and gold nanoparticle diameters of the three PPCs that we present (denoted PPC1, PPC2, and PPC3) are 27.2 and 5.6 nm, 32.2 and 9.0 nm, and 44.0 and 20.0 nm, respectively, resulting in substantially different gold volume fractions (PPC1 ∼0.91, PPC2 ∼2.3, and PPC3 ∼9.8%).PPCs can exhibit Fabry–Perot cavity modes (FPMs) owing to light interference induced by two parallel facets (15) in the microcavity geometry (Fig. 1 A and B) as long as the size of the PPCs is much larger than the wavelength of light (Supporting Information, section S3 and Fig. S2). FPMs can be detected via backscattering spectra (16) (Fig. 1 A and B) and allow one to probe the optical response of the PPCs. Importantly, within the PPCs the propagating photonic modes are expected to strongly couple to the gold nanoparticle surface plasmons (Fig. 1 B and C), forming a polariton band gap (8, 17). This is probed by optical experiments and theoretical calculations (Fig. 2, Supporting Information, sections S4–S6, and Figs. S3–S6). The backscattering spectra from the PPC center spots (Fig. 2 A, C, and E, Bottom) show Fabry–Perot interference patterns in the visible region (Fig. 2 B, D, and F, red lines). The agreement between a finite-difference time-domain (FDTD) simulation with a rhombic dodecahedron shape and an infinite slab model (Supporting Information, section S5 and Fig. S5) reveals the Fabry–Perot nature of these backscattering spectra, because FPMs are the only existing modes in the infinite slab geometry. Significantly, the Fabry–Perot oscillations are suppressed only around the surface plasmon resonance energy (∼530 nm; ∼2.3 eV) for PPC1 and PPC2, indicating the suppression of light propagation owing to coupling to surface plasmons. This behavior provides direct evidence for polariton band gap formation that is consistent with the theoretical predictions (8, 9, 18). These experimental results are in remarkably good agreement with two different infinite slab models, one with BCC crystal geometry and the other an effective medium theory (EMT) approximation that is based simply on the gold volume fraction without the effect of interparticle coupling (Fig. 2 B, D, and F; blue solid and dashed lines). For PPC3, FPMs are not observed below 500 nm (Fig. 2F) because of the strong absorption caused by the gold interband transition at relatively higher gold volume fraction. The discrepancy between the two models in FPM cutoff location (Fig. 2F, denoted by the two vertical lines) indicates that a considerable amount of interparticle coupling exists close to the surface plasmon resonance because EMT does not include interparticle coupling.Open in a separate windowFig. 1.A polaritonic photonic crystal made by DNA-programmable assembly. (A) Three-dimensional illustration of a plasmonic PPC, in the shape of a rhombic dodecahedron, assembled from DNA-modified gold nanoparticles. Red arrows indicate light rays normal to the underlying substrate, impinging on and backscattering through a top facet of the crystal (FPMs). The blue ones represent light rays entering through the slanted side facets and leaving the PPC through the opposite side, not contributing to the FPMs (Fig. S2). The top right inset shows the top view of the crystal with two sets of arrows defining two polarization bases at the top and side facets. The bottom right inset shows an SEM image of a representative single crystal corresponding to the orientation of the top right inset. (Scale bar, 1 µm.) (B) A 2D scheme showing the geometric optics approximation of backscattering consistent with the explanation in A. The hexagon outline is a vertical cross-section through the gray area in the top right inset of A parallel to its long edge. The box enclosed by a dashed line depicts the interaction between localized surface plasmons and photonic modes (red arrows; FPMs) with a typical near-field profile around gold nanoparticles. The contribution of backscattering through the side facets (blue arrows) to FPMs is negligible. (C) Scheme of plasmon polariton formation. The localized surface plasmons (yellow bar) strongly couple to the photonic modes (red bars; FPMs).Open in a separate windowFig. 2.Experimental and theoretical backscattering spectra of PPC1–3. (A) SEM image (Top) and optical bright field reflection mode image (Bottom) of PPC1 on a silicon substrate. (Scale bar, 1 µm.) (B) Measured backscattering spectrum (red solid line) of PPC1 from the center red spot in A, Bottom. Calculated backscattering spectra based on two infinite slab models with BCC crystal geometry (blue solid line) and EMT approximation (blue dashed line). FPMs are indicated by markers. (C–F) The same datasets for PPC2 and PPC3 as in A and B. PPC2 and PPC3 are on indium tin oxide (ITO)-coated glass slides. The optical images show bright spots at the center owing to backscattering from the top and bottom facets. Two vertical lines in F indicate spectral positions where FPMs are suppressed. (Scale bars, 1 µm.)Based on the spectral results, we examine the strong coupling behavior between the surface plasmons and FPMs in the PPCs with dispersion diagrams generated by FDTD photonic crystal analyses, including changes in the light–matter interactions by tailoring the lattice constant and gold nanoparticle size (Fig. 3 and Supporting Information, section S5). When the mode energies of PPC1 and PPC2 grow close to that of the localized surface plasmon resonance (LSPR), ?ω₀ (∼2.3 eV), the dispersion curves of the propagating modes form band gaps (Fig. 3 A and B). This is clearer in the absence of interband transition (insets of Fig. 3 A–C and Fig. S7 A–F). The origin of these band gaps is not the BCC translational symmetry of the crystals (Bragg gap) as in conventional dielectric photonic crystals (6). For Bragg gap formation in the visible, photonic crystals require a lattice constant an order of magnitude larger than those in this work (∼λ/2). Instead, the origin of the gaps is strong coupling between the surface plasmons and photonic modes owing to deep subwavelength lattice constants that define the separation of the polarizable particle components (high-density localized surface plasmons) (8, 9). In each crystal type (Fig. 3 A and B), coupling of this kind creates plasmon polaritons with anticrossing upper and lower branches in the dispersion diagrams forming a polariton band gap between the two branches, where propagating photonic modes are prohibited (8, 9, 17). The strength of the coupling is quantified by the mode splitting, ?Ω_R, (Supporting Information, section S7 and Fig. S7), which is the energy gap between the two branches at the resonant coupling point (17) (?Ω_R∼0.10 and 0.24 eV for PPC1 and PPC2, which are about ∼5 and ∼10% of ?ω₀; Fig. 3D). These mode splittings are comparable to a recently reported value based on 1D nanowire arrays on waveguide substrates (17). The EMT-generated curve without the effect of the interband transition (9) predicts a monotonically increasing mode splitting with the increase in gold volume fraction (1–10%; Fig. 3D), which agrees well with the FDTD photonic crystal analyses (Supporting Information, sections S6 and S7 and Fig. S7). This suggests the possibility of using metal volume fraction as a parameter to control coupling strength based on fine geometric tuning afforded by the DNA-programmable assembly technique (19). For PPC3, owing to the strong gold interband transition the upper branch in the dispersion diagram (Fig. 3C; <500 nm in Fig. 2F) is not clearly observable in the experiment, and therefore the mode splitting is not measurable. Based on a photonic crystal analysis without the presence of interband transitions, the upper branch of PPC3 is observed (Fig. 3C, Inset), and the mode splitting is ∼30% of ?ω₀ (Supporting Information, section S7 and Fig. S7). This large value arises due to the capability of the PPCs to coherently couple a large number of oscillators within a single microcavity.Open in a separate windowFig. 3.Calculated photonic mode dispersion, mode splitting, and effective mode index of PPC1–3. (A) The spectral density in the ΓN direction is presented for PPC1 (red is high, blue is low). Log10 scale is used. Red triangular markers are the FPMs in Fig. 2B (red markers). They are assigned to peak positions of the spectral densities and the mode number (N) is assigned on one FPM. (Inset) The same spectral density calculated based on the Drude model for gold (where there is no interband transition). (B and C) The same information as in A for PPC2 and PPC3. (D) The mode splitting to plasmonic mode energy ratio, ?Ω_R/?ω₀, is shown in terms of gold volume fraction. Blue dots are calculated based on EMT with the Drude model for gold. Squares are generated by a FDTD photonic crystal analysis with the Drude model for gold (red, green, and black: nanoparticle diameters 5.6, 9.0, and 20 nm; volume fraction of PPC3 indicated for 20 nm), and circles with experimentally measured gold permittivity (red and green: nanoparticle diameters 5.6 and 9.0 nm; PPC1 and PPC2). (E) EMT-based effective indices, Re[n_eff], for PPC1 (dotted line), PPC2 (dash-dot line), and PPC3 (dashed line). The index of the silica host medium (black solid line) is added as a reference. Red markers are Re[n_eff] based on the FPMs in A–C.Significantly, the strong coupling that we observe is further evidenced by quantifying the effective mode indices, Re[n_eff] (Fig. 3E). As the gold volume fraction increases to that of PPC3, the effective mode index drastically increases (Re[n_eff] ∼2) close to the LSPR frequency, indicating strong light coupling to surface plasmons and a large mode momentum gain (18, 20, 21). This is also apparent in the spectral profile, which shows an abrupt suppression of FPMs (two vertical lines in Fig. 2F) and a sharp increase in reflectance from 650 to 550 nm. This transition from Fabry–Perot to mirror-like behavior is due to an increase in both Re[n_eff] and Im[n_eff] close to the LSPR frequency (Fig. S6) that causes strong facet reflection and damping of the FPMs (18).The PPCs with lattice constants in the deep subwavelength regime can also behave as plasmonic cavity devices for studies such as cavity quantum electrodynamics (QED) (3, 22, 23). The plasmonic PPCs have, within a single structure, optical elements working on two different length scales: the plasmonic nanoparticles and the Fabry–Perot microcavity. Owing to localized surface plasmons, the gold nanoparticles exhibit extremely tight light confinement [a small mode volume, V <10⁻⁴(λ/n) (3), in the visible and NIR] around their metallic surfaces that augments light–matter interactions such as exciton–photon coupling (13, 24). These highly confined modes can be further enhanced (23, 25) if Q factors of Fabry–Perot modes are increased by coating the crystals with a silver layer (10–30 nm) (see Fig. S8 for the calculation approach and Figs. S9 and S10 for the experimental process). By simplifying the 3D shape of PPCs to a slab we can use the infinite slab model with a BCC crystal geometry to predict moderate Q factors (∼10²) of FPMs with varying silver layer thickness in the visible and NIR for PPC1–3 (Fig. 4). At ∼30-nm silver layer thickness, the Q factor saturates, and PPC1 exhibits the highest Q values owing to the lowest gold volume fraction. These numbers (∼10²; Fig. 4) are comparable to those of other plasmonic cavities in the literature (20, 23). This shows the possibility of tuning not just plasmonic modes by controlling BCC crystals but also enhancing the properties of photonic modes (FPMs) for further applications with excitonic materials such as dyes and quantum dots (26).Open in a separate windowFig. 4.Prediction of backscattering spectra and Q factor of silver-coated PPC1–3. (A) Backscattering spectra of PPC1–3 (from bottom to top: PPC1, PPC2, and PPC3) based on the infinite slab model with BCC crystal geometry. The thickness of the slabs is ∼1.3 µm, and that of silver coating layer is varied from 10 to 30 nm. As the coating thickness increases the line shape becomes sharper. The spectra of PPC1 and 2 are translated for comparison. (B) Q factors of each silver-coated slab are shown at FPMs (PPC1, red; PPC2, green; and PPC3, blue). The coating thickness is 30 nm.This work has shown how bioprogrammable colloidal crystallization can be used to access a new class of PPCs and related optical devices. Although the ability to create well-formed crystals via this technique is essential, it is the ability to tune light–plasmon coupling and plasmonic particle volume fraction that makes this approach so powerful from both fundamental science and potential device application standpoints. We anticipate that the studies herein and the single crystals realizable through the methodology will open the door to studying exciton–photon coupling in novel PPC plasmonic cavities and lead to new directions in cavity QED (22, 27), quantum optics (28–30), and quantum many-body dynamics (31, 32).     

Emerging single-phase state in small manganite nanodisks

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La_0.325Pr_0.3Ca_0.375MnO₃ (LPCMO) single crystalline disks to study the effect of spatial confinement on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.Owing to strong coupling between spin, charge, orbital, and lattice (1, 2), different electronic phases often coexist spatially in strongly correlated materials known as electronic phase separation (EPS) (3, 4). For colossal magnetoresistance (CMR) manganites, EPS has been observed to have strong influence on the global magnetic and transport properties (5, 6). Regarding the physical origin of EPS, it has been shown theoretically that quenched disorder can lead to inhomogeneous states in manganites (1, 3, 7). Once long-range effects such as coulombic forces (8), cooperative oxygen octahedral distortions (9), or strain effects (10) are included, calculations show infinitesimal disorder (8, 11) or even no explicit disorder (10) may lead to EPS. Within a phenomenological Ginzburg–Landau theory, it has been shown that EPS is intrinsic in complex systems as a thermodynamic equilibrium state (12).Although the details of the origin of the EPS remain as a matter of dispute, its very existence as a new form of electronic state has been well accepted. The length scale of the EPS has been observed to vary widely from nanometers to micrometers depending on many parameters that can affect the competition between different electronic phases (13–20). It is thus of great interest to examine whether the EPS state still exists as the system is scaled down, especially when the spatial dimension of the system is smaller than the length scale of the EPS domains.In this work, we use La_0.325Pr_0.3Ca_0.375MnO₃ (LPCMO) as a prototype system to show a spatial confinement-induced transition from the EPS state to a single ferromagnetic phase state. The LPCMO system is chosen because of its well-known large length scale of EPS domains (approximately a micrometer) (21), which allows us to conveniently fabricate LPCMO epitaxial thin films into disks with diameters that are smaller than the EPS domain size. In LPCMO bulk (21) and thin films (6, 22), the EPS state was observed to be the low-temperature ground state. Using magnetic force microscope, we observe that the EPS state remains to be the ground state in disks with the size of 800 nm in diameter or larger but vanishes in the 500-nm-diameter disks whose size is distinctly smaller than the characteristic length scale of the EPS domains. In the 500-nm disks, only the ferromagnetic phase can be observed at all temperatures below Curie temperature T_c, indicating that the system is in a single-phase state rather than a EPS state. Our results further indicate that the large length scale EPS in the LPCMO system does not cost extra Coulomb energy, which otherwise should lead to a scaling down of EPS with decreasing size of the LPCMO disks (23, 24).LPCMO films with 60-nm thickness were epitaxially grown on SrTiO₃(001) substrates by pulsed-laser deposition. The substrates were kept at 780 °C in oxygen atmosphere of 5 × 10⁻³ millibars during growth. Unit cell by unit cell growth was achieved as indicated by oscillations of intensity of reflection high-energy electron diffraction (RHEED). The films were postannealed to 950 °C for 3 h in flowing oxygen to reduce oxygen vacancy and make sure that the films have the same magnetic properties as the bulk. The LPCMO disks with diameters from 500 nm to 20 μm were fabricated from the epitaxial thin films by electron beam lithography with a negative tone resist (for details, see the sample fabrication method and Fig. S1 in the Supporting Information). Magnetic properties of the LPCMO disk arrays were carried out using superconducting quantum interference device (SQUID) and magnetic force microscope (MFM) measurements.Open in a separate windowFig. S1.(A and B) Schematics of LPCMO disks samples for magnetic property measurement (A) and MFM mapping (B). (C) Optical microscopic image of LPCMO disk array with specific diameter. (D) SEM image of LPCMO disk sample for MFM imaging.A distinct signature of the EPS state in the LPCMO system is the thermal hysteresis for temperature-dependent magnetic and transport properties. Fig. 1 A–D shows temperature dependent magnetic properties of LPCMO disks with different diameters. To enhance the measuring signal for SQUID, we fabricate disk arrays for each selected diameter (the optical microscopic image shown in Fig. 1B, Inset for the 1-μm disk array). Fig. 1 A–D shows temperature-dependent magnetization measured under 1,000 Oe in-plane field for 7-μm, 1-μm, 800-nm, and 500-nm disk arrays, respectively. Thermal hysteresis can be observed for disk arrays with size down to 800 nm, reflecting the fact that ferromagnetic metallic (FMM) and charge order insulating (COI) phases coexist during the first-order phase transition (7, 25). For the 500-nm disk array, however, no thermal hysteresis can be observed. This observation implies that the EPS state may no longer exist in the system (7, 26).Open in a separate windowFig. 1.Temperature dependence of magnetization (black lines for cooling and red lines for warming) under 1,000 Oe (A–D) and initial magnetization (red lines) and hysteresis loop (black lines) (E–H) at 5 K of arrays of LPCMO disks with sizes of 7 μm (A and E), 1 μm (B and F), 800 nm (C and G), and 500 nm (D and H) in diameter and an area of 3 mm × 3 mm. (B, Inset) The optical microscopic image of d = 1 μm array. (C and D, Insets) Zoomed-in M vs. T loop around the thermal hysteresis region.The lack of EPS state in the 500-nm disk array is supported by the field-dependent magnetization measurements. Fig. 1 E–H shows in-plane initial magnetization curves and magnetic hysteresis loops (M-H loops) for the disk arrays measured at 5 K after zero-field cooling. For 800-nm or larger disk arrays, there is a clear difference between the initial magnetization curves and the corresponding M-H loops due to the coexistence of FMM and COI phases. When the magnetic field is applied from the initial state, the magnetization of the FMM phase first quickly aligns along the field direction, leading to the low field fast rise of the initial magnetization curve. With increasing field, the COI phase is melted and transits into the FMM phase. Once transited, the FMM phase will mostly stay even if the field is reduced, giving rise to the difference between initial magnetization curve and the M-H loop. The difference, however, becomes smaller with decreasing size. For the 500-nm disk array, the initial magnetization curve and the M-H loop virtually superimpose each other, indicating no melting of COI phase occurs. Both the temperature- and field-dependent magnetization measurements show a transition from the EPS state to a single FMM state with decreasing size of the disk, and the critical size should be between 500 nm and 800 nm.The transition from the EPS state to a single FMM state can be seen in MFM images shown in Fig. 2 (for MFM imaging details, see micromagnetic mapping method in Supporting Information). Fig. 2A shows morphological appearance of LPCMO disks with different sizes acquired by atomic force microscope (AFM). Fig. 2 B–D shows the corresponding MFM images of the LPCMO disks acquired at different temperatures under a perpendicular magnetic field of 1T. Here, the perpendicular magnetic field is applied to yield some perpendicular magnetization components for MFM imaging because the easy magnetization axis is in the plane. In the present color scale, the contrast below zero (red or black) represents FMM phase, whereas the contrast above zero (green or blue) represents nonferromagnetic phase [i.e., COI phase based on previous knowledge of the LPCMO system (21, 22)]. Apparently, except the 500-nm disk, all other disks show distinct features of the EPS state (i.e., the coexistence of the FMM and COI phases). Although the portion of FMM phase increases noticeably with decreasing temperature, the system stays in the EPS state even at 10 K. The typical length scale of the EPS domains is around a micrometer, which is consistent with previous reports (21, 27).Open in a separate windowFig. 2.(A) AFM images of LPCMO disks with sizes of 500 nm, 1 μm, 2 μm, 3.8 μm, 5 μm, and 7 μm in diameter. (B–D) The MFM images of LPCMO disks under 1T field (external magnetic field direction is pointing perpendicularly to the sample surface plane) taken at 10 K (B), 100 K (C), and 180 K (D). The sizes of disks in MFM images are adjusted and corrected to have same scales for each size with the help of scanning electron microscope (SEM) images (shown in Fig. S2) and dash lines show the approximate physical boundary of disks. The negative value in MFM image indicates attractive force and positive value indicates repulsive force.In stark contrast to the larger disks, the 500-nm disk does not exhibit any features of EPS in Fig. 2. Instead, the whole disk is in a ferromagnetic phase with a magnetization profile peaking in the center. To ensure that the EPS state is not diminished by the magnetic field applied during MFM imaging, we took MFM images of the 500-nm disk at 10 K under different perpendicular magnetic fields from 0T to 1T, as shown in Fig. 3A. At 0T, signals with opposite sign can only be seen on two sides of the disk along the marked line (MFM images of 4 disks shown in Fig. S3). This pattern is a typical MFM image for an in-plane ferromagnetic single domain, because only the two ends of an in-plane magnetic dipole yield perpendicular field gradient (with opposite signs) for the MFM tip to detect. Once a perpendicular field of 0.15T is applied, the in-plane magnetization is driven out of plane, leading to a center peaked MFM contour. The MFM signal increases with increasing field, as shown in Fig. 3B by the marked line profiles extracted from Fig. 3A.Open in a separate windowFig. 3.(A) MFM images of 500-nm disks at 10 K under different magnetic field. The lines show the path the line profile extracted. (B) Line profiles extracted from MFM images partly shown in Fig. 3A. (C) Simulated results corresponding to line-profiles in B. (D and E) The simulated Z-component of stray field 100 nm above sample disks and magnetic structure of 500-nm disks under a different magnetic field. In E, the direction and size of arrows show the direction and relative value of in-plane component of magnetization, and the color of disk indicates the value of Z-component of magnetization presented by ratio of Z-component to total magnetization, as shown in the color bar.Open in a separate windowFig. S3.MFM images of 500-nm disks taken at 10 K under zero field after zero-field cooling.The field-dependent behavior of the MFM contrast of the 500-nm disk is in qualitative agreement with micromagnetic simulations. Based on the MFM observation, the 500-nm disk is in an in-plane, single-domain state. Using this model as input, we performed micromagnetic simulation and obtained the Z-component of magnetic stray field distribution at 100 nm above sample surface (Fig. 3D; for details, see the micromagnetic simulation method in the Supporting Information), which is virtually the signal detected by MFM tip. The corresponding magnetic structures under different magnetic fields are shown in Fig. 3E. The marked line profiles extracted from simulation (Fig. 3D) are shown in Fig. 3C alongside with the experimental MFM line profiles (Fig. 3B). The subtle differences between Fig. 3B and Fig. 3C are likely caused by the fact that experimental MFM images are convoluted from signals of both the LPCMO disks and the MFM tips (∼100 nm in size). The consistency of MFM images and simulation confirms that the 500-nm disk is in a ferromagnetic single-domain state with an in-plane easy magnetization axis.Finally, we show that the 500-nm disk is in a single FMM state at all temperatures. Fig. 4 shows MFM images of the 500-nm disk acquired every 20 K, from 20 K to 200 K under 1,000 Oe. Other than the center-peaked FMM phase, no traces of COI phase can be observed. The MFM signal decreases with increasing temperature, which is consistent with the behavior of the temperature-dependent magnetization shown in Fig. 1. Considering the fact that we have never observed pure COI phase in the 500-nm disks, we believe this phenomenon may be caused by the existence of the ferromagnetic metallic edge state in the LPCMO system (22), which assists the 500-nm disk to be in pure ferromagnetic state when a single state is energetically preferred in the 500-nm disk.Open in a separate windowFig. 4.MFM images of 500-nm disks taken every 20 K, from 20 K to 200 K.In summary, we discovered a spatial confinement-induced transition from a EPS state featuring coexistence of FMM and COI phases to a single FMM state in the LPCMO system. The critical size for the transition is between 500 nm and 800 nm, which is similar to the characteristic length scale of the EPS state in the LPCMO system. Combining the MFM data and the micromagnetic simulation, we conclude that the 500-nm LPCMO disk is in a single-domain ferromagnetic state at all temperatures below T_c. A similar conclusion can be reached for 300-nm LPCMO disks (shown in Fig. S4), although it needs to be studied further whether a new state would emerge if the disk size becomes a few tens of nanometers or smaller. Our work opens a way to control EPS without external field or introducing strain and disorder, which is potentially useful to design electronic and spintronic devices in complex oxides systems.Open in a separate windowFig. S4.MFM images of 300-nm disks taken at different temperatures and magnetic fields.     

Integration of high-content screening and untargeted metabolomics for comprehensive functional annotation of natural product libraries

Traditional natural products discovery using a combination of live/dead screening followed by iterative bioassay-guided fractionation affords no information about compound structure or mode of action until late in the discovery process. This leads to high rates of rediscovery and low probabilities of finding compounds with unique biological and/or chemical properties. By integrating image-based phenotypic screening in HeLa cells with high-resolution untargeted metabolomics analysis, we have developed a new platform, termed Compound Activity Mapping, that is capable of directly predicting the identities and modes of action of bioactive constituents for any complex natural product extract library. This new tool can be used to rapidly identify novel bioactive constituents and provide predictions of compound modes of action directly from primary screening data. This approach inverts the natural products discovery process from the existing ‟grind and find” model to a targeted, hypothesis-driven discovery model where the chemical features and biological function of bioactive metabolites are known early in the screening workflow, and lead compounds can be rationally selected based on biological and/or chemical novelty. We demonstrate the utility of the Compound Activity Mapping platform by combining 10,977 mass spectral features and 58,032 biological measurements from a library of 234 natural products extracts and integrating these two datasets to identify 13 clusters of fractions containing 11 known compound families and four new compounds. Using Compound Activity Mapping we discovered the quinocinnolinomycins, a new family of natural products with a unique carbon skeleton that cause endoplasmic reticulum stress.Notwithstanding the historical importance of natural products in drug discovery (1) the field continues to face a number of challenges that affect the relevance of natural products research in modern biomedical science (2). Among these are the increasing rates of rediscovery of known classes of natural products (3–6) and the high rates of attrition of bioactive natural products in secondary assays due to limited information about compound modes of action in primary whole-cell assays (7). Although pharmaceutical companies recognize that natural products are an important component of drug discovery programs because of the different pharmacologies of natural products and synthetic compounds (8), there is a reluctance to return to “grind and find” discovery methods (9). Therefore, there is a strong need for technologies that address these issues and provide new strategies for the prioritization of lead compounds with unique structural and/or biological properties (10).Natural product drug discovery is challenging in any assay system because extract libraries are typically complex mixtures of small molecules in varying titers, making it difficult to distinguish biological outcomes (11). This is compounded by issues of additive effects of multiple bioactive compounds and the presence of nuisance compounds that cause false positives in assay systems (12). To address these issues, our laboratory has recently developed several image-based screening platforms that are optimized for natural product discovery (13–16). The cytological profiling platform optimized by Schulze and coworkers characterizes the biological activities of extracts using untargeted phenotypic profiling. These phenotypic profiles are compared with natural products extracts and a training set of compounds with known modes of action to characterize the bioactivity landscape of the screening library (17, 18). This cytological profiling tool forms the basis of the biological characterization component of the Compound Activity Mapping platform, as described below.In the area of chemical characterization of natural product libraries, untargeted metabolomics is gaining attention as a method for evaluating chemical constitution (3, 19–22). Modern “genes-to-molecules” and untargeted metabolomics approaches taking advantage of principal component analysis and MS² spectral comparisons have also been developed to quickly dereplicate complex extracts and distinguish noise and nuisance compounds from new molecules (23–27). Unfortunately, although these techniques are well suited to the discovery of new chemical scaffolds, they are unable to describe the function or biological activities of the compounds they identify. Therefore, there is still a need for new approaches to systematically identify novel bioactive scaffolds from complex mixtures.To overcome some of these outstanding challenges we have developed the Compound Activity Mapping platform to integrate phenotypic screening information from the cytological profiling assay with untargeted metabolomics data from the extract library (Fig. 1). By correlating individual mass signals with specific phenotypes from the high-content cell-based screen (Fig. 2), Compound Activity Mapping allows the prediction of the identities and modes of action of biologically active molecules directly from complex mixtures, providing a mechanism for rational lead selection based on desirable biological and/or chemical properties. To evaluate this platform for natural products discovery we examined a 234-member extract library, from which we derived 58,032 biological measurements (Fig. 1C) and 10,977 mass spectral features (Fig. 1A). By integrating and visualizing these data we created a Compound Activity Map for this library composed of 13 clusters containing 16 compounds from 11 compound classes (Fig. 3). This integrated data network enabled the discovery of four new compounds, quinocinnolinomycins A–D (1–4, Fig. 4), which are the first examples to our knowledge of microbial natural products containing the unusual cinnoline core (Fig. 5). Clustering the cytological profiles of the quinocinnolinomycins with those of the Enzo library training set suggests that these compounds induce endoplasmic reticulum (ER) stress and the protein unfolding response.Open in a separate windowFig. 1.Overview of Compound Activity Mapping. (A) Representation of the chemical space in the tested extract library. The network displays extracts (light blue) connected by edges to all m/z features (red) observed from the metabolomics analysis, illustrating the chemical complexity of even small natural product libraries. (B) Histograms of activity and cluster scores for all m/z features with cutoffs indicated as red lines (for full-size histograms see SI Appendix, Fig. S5). (C) Compound Activity Map, with the network displaying only the m/z features predicted to be associated with consistent bioactivity, and their connectivity to extracts within the library. (D) Expansion of the staurosporine cluster (dotted box in C) with extract numbers and relevant m/z features labeled.Open in a separate windowFig. 2.Determination of synthetic fingerprints and cluster and activity scores. (A) Table of Pearson correlations for the cytological profiles between all extracts containing a specific m/z feature (m/z of 489.1896, rt of 1.59). In each cytological profile, yellow stripes correspond to positive perturbations in the observed cytological attribute and blue stripes correspond to negatively perturbed attributes. The cluster score is determined by calculating the average of the Pearson correlation scores for all relevant extracts. (B) Calculated synthetic fingerprint and activity score for feature (m/z of 489.1896, rt of 1.59). Synthetic fingerprints are calculated as the averages of the values for each cytological attribute to give a predicted cytological profile for each bioactive m/z feature in the screening set.Open in a separate windowFig. 3.Annotated Compound Activity Map. An expanded view of the Compound Activity Map from Fig. 1C, with the extracts and m/z features separated into subclusters and colored coded using the Gephi modularity function. Each bioactive subcluster is composed of extracts containing a family of compounds with a defined biological activity. The Compound Activity Map is annotated with a representative molecule from each of the families of compounds that have been independently confirmed by purification and chemical analysis.Open in a separate windowFig. 4.The prioritization, isolation, and confirmation of the quinocinnolinomycins A–D (1–4). (A) Bioactive m/z features plotted on a graph of activity score vs. cluster score. The color of the dot corresponds to the retention time of the m/z feature with the color bar and scale below in minutes. (B) Isolated cluster from Fig. 1C and Fig. 3 containing both the relevant extracts (blue) and bioactive m/z features (red). (C) HPLC trace of extract RLPA-2003E and the isolation of quinocinnolinomycins A–D (highlighted with blue boxes on HPLC trace). (D) Cell images of pure compounds screened as a twofold dilution series for quinocinnolinomycins A and B in both stain sets compared with images of vehicle (DMSO) wells. (E) Comparison of the synthetic and actual cytological fingerprints of the pure compounds is presented below the relevant images, demonstrating the relationship between experimental and calculated cytological profiles for these two metabolites.Open in a separate windowFig. 5.Structure elucidation of quinocinnolinomycins A–D (1–4). (A) Structures of quinocinnolinomycins A–D. (B) Key NMR correlations used in the structure elucidation of quinocinnolinomycin A. COSY correlations are indicated by bold lines. Heteronuclear multiple-bond correlations are indicated by curved arrows. (C) ∆δ^SR values for the Mosher’s α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) ester analysis of the secondary alcohol in quinocinnolinomycin A (1) to assign the absolute configuration at position C11.     

North American tree migration paced by climate in the West,lagging in the East

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.

Effective planning for the redistribution of habitats from climate change will depend on understanding demographic rates that control population spread at continental scales. Mobile species are moving, some migrating poleward (1, 2) and/or upward in elevation (3, 4). Species redistribution is also predicted for sessile, long-lived trees that provide the resource and structural foundation for global forest biodiversity (5–7), but their movement is harder to study. Contemporary range shifts are recognized primarily where contractions have followed extensive die-backs (8) or where local changes occur along compact climate gradients in steep terrain (9, 10). Whether migration capacity can pace habitat shifts of hundreds of kilometers on decade time scales depends on seed production and juvenile recruitment (Fig. 1A), which have not been fitted to data in ways that can be incorporated in models to anticipate biogeographic change (11–13). For example, do the regions of rapid warming coincide with locations where species can produce abundant seed (Fig. 1B)? If so, does seed production translate to juvenile recruitment? Here, we combine continent-wide fecundity estimates from the Masting Inference and Forecasting (MASTIF) network (13) with tree inventories to identify North American hotspots for recruitment and find that species are well-positioned to track warming in the West and North, but not in parts of the East.Open in a separate windowFig. 1.Transitions, hypothesized effects on spread, and sites. (A) Population spread from trees (BA) to new recruits is controlled by fecundity (seed mass per BA) followed by recruitment (recruits per seed mass). (B) The CTH that warming has stimulated fecundity ahead of the center of adult distributions, which reflect climate changes of recent decades. Arrows indicate how centroids from trees to fecundity to recruitment could be displaced poleward with warming climate. (C) The RSH that cold-sensitive fecundity is optimal where minimum temperatures are warmer than for adult trees and, thus, may slow northward migration. The two hypotheses are not mutually exclusive. B and C refer to the probability densities of the different life stages. (D) MASTIF sites are summarized in SI Appendix, Table S2.2 by eco-regions: mixed forest (greens), montane (blues), grass/shrub/desert (browns), and taiga (blue-green).Suitable habitats for many species are projected to shift hundreds of kilometers in a matter of decades (14, 15). While climate effects on tree mortality are increasingly apparent (16–19), advances into new habitats are not (20–23). For example, natural populations of Pinus taeda may be sustained only if the Northeast can be occupied as habitats are lost in the South (Fig. 2). Current estimates of tree migration inferred from geographic comparisons of juvenile and adult trees have been inconclusive (2, 7, 21, 24, 25). Ambiguous results are to be expected if fecundity and juvenile success do not respond to change in the same ways (20, 26–29). Moreover, seedling abundances (7, 30) do not provide estimates of recruitment rates because seedlings may reside in seedling banks for decades, or they may turn over annually (31–33). Another method based on geographic shifts in population centers calculated from tree inventories (3, 34) does not separate the effects of mortality from recruitment, i.e., the balance of losses in some regions against gains in others. The example in Fig. 2 is consistent with an emerging consensus that suitable habitats are moving fast (2, 14, 15), even if population frontiers are not, highlighting the need for methods that can identify recruitment limitation on population spread. Management for forest products and conservation goals under transient conditions can benefit from an understanding of recruitment limitation that comes from seed supply, as opposed to seedling survival (35).Open in a separate windowFig. 2.Suitable habitats redistribute with decade-scale climate change for P. taeda (BA units m² /ha). (Suitability is not a prediction of abundance, but rather, it is defined for climate and habitat variables included in a model, to be modified by management and disturbance [e.g., fire]. By providing habitat suitability in units of BA, it can be related it to the observation scale for the data.) Predictive distributions for suitability under current (A) and change expected from mid-21^st-century climate scenario Representative Concentration Pathway 4.5 (B) showing habitat declines in the Southwest and East. Specific climate changes important for this example include net increases in aridity in the southeast (especially summer) and western frontier and warming to the North. Occupation of improving habitats depends on fecundity in northern parts of the range and how it is responding. Obtained with Generalized Joint Attribute Modeling (see Materials and Methods for more information).We hypothesized two ways in which fecundity and recruitment could slow or accelerate population spread. Contemporary forests were established under climates that prevailed decades to centuries ago. These climate changes combine with habitat variables to affect seeds, seedlings, and adults in different ways (36, 37). The “climate-tracking hypothesis” (CTH) proposes that, after decades of warming and changing moisture availability (Fig. 3 A and B), seed production for many species has shifted toward the northern frontiers of the range, thus primed for poleward spread. “Fecundity,” the transition from tree basal area (BA) to seed density on the landscape (Fig. 1A), is taken on a mass basis (kg/m² BA) as a more accurate index of reproductive effort than seed number (38, 39). “Recruitment,” the transition from seed density to recruit density (recruits per kg seed), may have also shifted poleward, amplifying the impact of poleward shifts in fecundity on the capacity for poleward spread (Fig. 1B). Under CTH, the centers for adult abundance, fecundity, and recruitment are ordered from south to north in Fig. 1B as might be expected if each life-history stage leads the previous stage in a poleward migration.Open in a separate windowFig. 3.Climate change and tracking. (A) Mean annual temperatures since 1990 have increased rapidly in the Southwest and much of the North. (Zero-change contour line is in red.) (B) Moisture deficit index (monthly potential evapotranspiration minus P summed over 12 mo) has increased in much of the West. (Climate sources are listed in SI Appendix.) (C) Fecundity (kg seed per BA summed over species) is high in the Southeast. (D) Recruits per kg seed (square-root transformed) is highest in the Northeast. (E and F) Geographic displacement of 81 species show transitions in Fig. 1A, as arrows from centroids for adult BA to fecundity (E) and from fecundity to recruitment (F). Blue arrows point north; red arrows point south. Consistent with the RSH (Fig. 1B), most species centered in the East and Northwest have fecundity centroids south of adult distributions (red arrows in E). Consistent with the CTH, species of the interior West have fecundity centroids northwest of adults (blue arrows). Recruitment is shifted north of fecundity for most species (blue arrows in F). SI Appendix, Fig. S2 shows that uncertainty in vectors is low.The “reproductive-sensitivity hypothesis” (RSH) proposes that recruitment may limit population growth in cold parts of the range (Fig. 1C), where fecundity and/or seedling survival is already low. Cold-sensitive reproduction in plants includes late frost that can disrupt flowering, pollination, and/or seed development, suggesting that poleward population frontiers tend to be seed-limited (40–44). While climate warming could reduce the negative impacts of low temperatures, especially at northern frontiers, these regions still experience the lowest temperatures. The view of cold-sensitive fecundity as a continuing rate-limiting step, i.e., that has not responded to warming in Fig. 1C, is intended to contrast with the case where warming has alleviated temperature limitation in Fig. 1B. Lags can result if cold-sensitive recruitment naturally limits growth at high-latitude/high-altitude population frontiers (Fig. 1C). In this case, reproductive sensitivity may delay the pace of migration to an extent that depends on fecundity, recruitment, or both at poleward frontiers. The arrows in Fig. 1C depict a case where optimal fecundity is equator-ward of optimal growth and recruitment. The precise location of recruitment relative to fecundity in Fig. 1C will depend on all of the direct and indirect effects of climate, including through seed and seedling predators and disturbances like fire. Fig. 1C depicts one of many hypothetical examples to show that climate variables might have opposing effects on fecundity and recruitment.Both CTH and RSH can apply to both temperature and moisture; the latter is here quantified as cumulative moisture deficit between potential evapotranspiration and precipitation, D=∑m=112(PETm−Pm) for month m, derived from the widely used Standardized Precipitation Evapotranspiration Index (45). Whereas latitude dominates temperature gradients and longitude is important for moisture in the East, gradients are complicated by steep terrain in the West, with temperature tending to decline and moisture increase with elevation.We quantified the transitions that control population spread, from adult trees (BA) to fecundity (seeds per BA) to recruitment (recruits per kg seed) (Fig. 1A–C). Fecundity observations are needed to establish the link between trees and recruits in the migration process. They must be available at the tree scale across the continent because seed production depends on tree species and size, local habitat, and climate for all of the dominant species and size classes (13, 46). These estimates are not sufficient in themselves, because migration depends on seed production per area, not per tree. The per-area estimates come from individual seed production and dispersal from trees on inventory plots that monitor all trees that occupy a fixed sample area. Fecundity estimates were obtained in the MASTIF project (13) from 211,000 (211K) individual trees and 2.5 million (2.5M) tree-years from 81 species. We used a model that accommodates individual tree size, species, and environment and the codependence between trees and over time (Fig. 1C). In other words, it allows valid inference on fecundity, the quasisynchronous, quasiperiodic seed production typical of many species (47). The fitted model was then used to generate a predictive distribution of fecundity for each of 7.6M trees on 170K forest inventory plots across the United States and Canada. Because trees are modeled together, we obtain fecundity estimates per plot and, thus, per area. BA (m² /ha) of adult trees and new recruits into the smallest diameter class allowed us to determine fecundity as kg seed per m² BA and recruitment per kg seed, i.e., each of the transitions in Fig. 1A.Recruitment rates, rather than juvenile abundances, come from the transitions from seedlings to sapling stages. The lag between seed production and recruitment does not allow for comparisons on an annual basis; again, residence times in a seedling bank can span decades. Instead, we focus on geographic variation in mean rates of fecundity and recruitment.We summarized the geographic distributions for each transition as 1) the mean transition rates across all species and 2) the geographic centroids (central tendency) for each species as weighted-average locations, where weights are the demographic transitions (BA to fecundity, fecundity to recruitment, and BA to recruitment). We analyzed central tendency, or centroids (e.g., refs. 3 and 34) because range limits cannot be accurately identified on the basis of small inventory plots (21). If fecundity is not limiting poleward spread (CTH of Fig. 1B), then fecundity centroids are expected to be displaced poleward from the adult population. If reproductive sensitivity dominates population spread (RSH of Fig. 1C), then fecundity and/or recruitment centroids will be displaced equator-ward from adult BA. The same comparisons between fecundity and recruitment determine the contribution of recruitment to spread.     

Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution

As one of the earliest-known mammaliaforms, Haramiyavia clemmenseni from the Rhaetic (Late Triassic) of East Greenland has held an important place in understanding the timing of the earliest radiation of the group. Reanalysis of the type specimen using high-resolution computed tomography (CT) has revealed new details, such as the presence of the dentary condyle of the mammalian jaw hinge and the postdentary trough for mandibular attachment of the middle ear—a transitional condition of the predecessors to crown Mammalia. Our tests of competing phylogenetic hypotheses with these new data show that Late Triassic haramiyids are a separate clade from multituberculate mammals and are excluded from the Mammalia. Consequently, hypotheses of a Late Triassic diversification of the Mammalia that depend on multituberculate affinities of haramiyidans are rejected. Scanning electron microscopy study of tooth-wear facets and kinematic functional simulation of occlusion with virtual 3D models from CT scans confirm that Haramiyavia had a major orthal occlusion with the tallest lingual cusp of the lower molars occluding into the lingual embrasure of the upper molars, followed by a short palinal movement along the cusp rows alternating between upper and lower molars. This movement differs from the minimal orthal but extensive palinal occlusal movement of multituberculate mammals, which previously were regarded as relatives of haramiyidans. The disparity of tooth morphology and the diversity of dental functions of haramiyids and their contemporary mammaliaforms suggest that dietary diversification is a major factor in the earliest mammaliaform evolution.Haramiyidans are among the first mammaliaforms to appear during the Late Triassic in the evolutionary transition from premammalian cynodonts. Their fossils have a cosmopolitan distribution during the Late Triassic to the Jurassic (1–8), tentatively with the youngest record in the Late Cretaceous of India (9). Most of these occurrences are of isolated teeth. For this reason, Haramiyavia clemmenseni (1) holds a special place in mammaliaform phylogeny: It is the best-preserved Late Triassic haramiyid with intact molars, nearly complete mandibles, and also postcranial skeletal elements (Figs. 1 and ​and22 and SI Appendix, Figs. S1–S4) (1). By its stratigraphic provenance from the Tait Bjerg Beds of the Fleming Fjord Formation, East Greenland (Norian-Rhaetic Age) (7), Haramiyavia is also the oldest known haramiyid (5, 7). Haramiyids, morganucodonts, and kuehneotheriids are the three earliest mammaliaform groups that are distinctive from each other in dental morphology and masticatory functions (10–12).Open in a separate windowFig. 1.(A and B) Composite reconstruction of Haramiyavia clemmenseni right mandible in lateral (A) and medial (B) views. Dark red: original bone with intact periosteal surface; brown: broken surface of preserved bone or remnant of bone; light blue: morphologies preserved in mold outlines or clear impression. (C) Morganucodon mandible in medial view.Open in a separate windowFig. 2.Molar features of Haramiyavia. (A) Right M1–M3 in occlusal view (medio-lateral orientation by the zygomatic root and the palate). (B) Occlusal facets of upper molars. (C) Lingual view of M1–M3. (D) Root structures of upper molars (M1 and M2 show three partially divided anterior roots connected by dentine and two posterior roots connected by dentine; M3 has two anterior and two posterior roots connected respectively by dentine). These roots have separate root canals. (E) Buccal view of M1–M3. All roots are bent posteriorly, suggesting that crowns shifted mesially, relative to the roots, during the tooth eruption, also known as mesial drift of teeth (arrowhead), typical of successive eruption of multirooted postcanines. (F) SEM photograph of lower m3 in a posterior occlusal view. (G) Approximate extent of wear facets by orthal occlusion (a1 cusp in embrasure of upper molars) (blue) and palinal movement of b2–b4 cusps sliding across the median furrow of upper molars (green). (H) Lingual view of m3. There are no wear facets on lingual side of cusps a1–a4. (I) Buccal view of m3 showing wear facets on the buccal sides of cusps b1–b4 and on apices.Haramiyids are characterized by their complex molars with longitudinal rows of multiple cusps. The cusp rows occlude alternately between the upper and lower molars. Primarily because of similarities in molar morphology, haramiyids are considered to be related to poorly known theroteinids of the Late Triassic (5, 13) and eleutherodontids of the Middle to Late Jurassic (14–17). Collectively haramiyids and eleutherodontids are referred to as “haramiyidans” (10, 14, 15, 18, 19). Recent discoveries of diverse eleutherodontids or eleutherodontid-related forms with skeletons from the Tiaojishan Formation (Middle to Late Jurassic) of China (18–20) have greatly augmented the fossil record of haramiyidans, ranking them among the most diverse mammaliaform clades of the Late Triassic and Jurassic.Historically, it has been a contentious issue whether haramiyidans (later expanded to include theroteinids and eleutherodontids) are closely related to the more derived multituberculates from the Middle Jurassic to Eocene (13) or represent a stem clade of mammaliaforms excluded from crown mammals (21, 22). The conflicting placement of haramiyidans was attributable in part to the uncertainties in interpreting the isolated teeth of most Late Triassic haramiyids (21, 22). More recent phylogenetic disagreements have resulted from different interpretations of mandibular characters in Haramiyavia (17–20, 23–25), which has not been fully described (figure 2 in ref. 1).Here we present a detailed study of the mandibles and teeth of Haramiyavia from the exhaustive documentation during initial fossil preparation (Fig. 1 and SI Appendix, Figs. S1–S4), from scanning electron microscopy (SEM) images, and from computed tomography (CT) scans and 3D image analyses of the two fossil slabs with mandibles (MCZ7/95A and B), plus a referred specimen of upper molars in a maxilla (MCZ10/G95) (Figs. 2 and ​and3,3, SI Appendix, Figs. S5–S8 and Tables S2 and S3, and Movie S1). These new data are informative for testing alternative mammaliaform phylogenies (Fig. 4 and SI Appendix) and are useful for reconstructing evolutionary patterns of feeding function in the earliest mammaliaforms.Open in a separate windowFig. 3.Molar occlusion of haramiyids. (A) In Haramiyavia the upper and lower molars form an en echelon pattern, a series of parallel and step-like occlusal surfaces in lingual and buccal views (based on 3D scaled models from CT scans of MCZ7/G95 and MCZ10/G95). (B–E) In Haramiyavia are shown the occlusal paths of cusps a1–a4 of the lingual row (B), cusps b2–b4 of the buccal row (shown with the lingual half of the tooth cut away) (C), and tooth orientation and the cut-away plane (D). During the orthal occlusion phase, the tallest lingual cusp a1 occludes into the embrasure of the preceding and the opposing upper molars (B and E), and the tallest buccal cusp b2 occludes into the upper furrow and behind the A1–B1 saddle of the upper molar (C). During the palinal occlusion phase, cusps b1–b4 of the buccal row slide posteriorly in the upper furrow, and in the upper row cusps B5–B1 slide in the lower furrow (lower molars with blue and green shading, superpositioned by flipped upper molar in clear outlines) (E). (F) Extent of wear on molars during the orthal phase (blue) and the palinal movement (green) produced by OFA simulation (Movie S1). (G–I) In Thomasia, reconstruction of upper and lower molar series on the basis of wear surfaces and tooth crown morphology (revised from refs. 2, 3, and 10). (G) The en echelon occlusal surfaces in lingual view. (H) Orthal occlusion (blue) is followed by palinal occlusal movement (green). (I) Occlusal wear facets of molars. Facets worn by orthal occlusion are shown in blue, and facets worn by palinal occlusion are shown in gray hatching. Cusp and facet designations are after refs 3 and 6.Open in a separate windowFig. 4.Hypotheses concerning the phylogenetic relationship of Haramiyavia and timing estimates of the basal diversification of crown mammals. (A) Haramiyavia is a close relative of multituberculates, both nested in the crown Mammalia. This hypothesis (haramiyidan node position 1) was based on a misinterpretation of a previous illustration of a fragment of the mandible (17, 18). (B) Haramiyavia is a stem mammaliaform, as determined by incorporating the features preserved on both mandibles into phylogenetic estimates (haramiyidan node position 2). (C) Placement of Haramiyavia and other haramiyidans among mammaliaforms according to this study. Many mandibular features were treated as unknown by studies favoring a Late Triassic diversification of mammals (18, 23). A more complete sampling of informative features revealed by this study now has overturned the previous placement. Clades: crown Mammalia (node a); Mammaliaformes (node b); haramiyidans (node 1 or 2, alternative positions); Eleutherodontida (node c). The rescored datasets and analyses are presented in SI Appendix.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Pelvic girdle and fin of Tiktaalik roseae

A major challenge in understanding the origin of terrestrial vertebrates has been knowledge of the pelvis and hind appendage of their closest fish relatives. The pelvic girdle and appendage of tetrapods is dramatically larger and more robust than that of fish and contains a number of structures that provide greater musculoskeletal support for posture and locomotion. The discovery of pelvic material of the finned elpistostegalian, Tiktaalik roseae, bridges some of these differences. Multiple isolated pelves have been recovered, each of which has been prepared in three dimensions. Likewise, a complete pelvis and partial pelvic fin have been recovered in association with the type specimen. The pelves of Tiktaalik are paired and have broad iliac processes, flat and elongate pubes, and acetabulae that form a deep socket rimmed by a robust lip of bone. The pelvis is greatly enlarged relative to other finned tetrapodomorphs. Despite the enlargement and robusticity of the pelvis of Tiktaalik, it retains primitive features such as the lack of both an attachment for the sacral rib and an ischium. The pelvic fin of Tiktaalik (NUFV 108) is represented by fin rays and three endochondral elements: other elements are not preserved. The mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives.At first glance, the origin of tetrapods (limbed vertebrates) from finned precursors seems an almost insurmountable transition between life in water and life on land. If the basis of comparison were living taxa alone, then the anatomical and behavioral differences among finned and limbed vertebrates could appear vast: for example, fin structure and function differ dramatically from those of limbs. Fossil evidence, in particular vertebrates from the middle and late part of the Devonian period (393–359 Mya), offers intermediate conditions that bridge this gap (1). The fossils that provide the most informative anatomical intermediates are from the tetrapodomorph lineage (also known as stem tetrapods) and have been recovered from a variety of nonmarine and marginal marine deposits from around the globe (2–4). The creatures closest to the node containing the most basal limbed vertebrates—elpistostegalids, such as Panderichthys, Tiktaalik, and Elpistostege—are most enlightening in understanding the primitive conditions from which tetrapods arose. Although most work has focused on revealing homologies and function of the pectoral appendage of these forms (4–7), relatively little is known of the pelvic appendage beyond limited material of Panderichthys (8). Consequently, analyses of the pelvic fin have been given only sporadic attention over the past decades (4, 8–11) largely because they are often poorly preserved or not preserved at all. In most cases, it is thought that this poor preservation of the pelvic appendage is due to its putative small size and fragility (10).Pelves, and in some cases pelvic appendages, of taxa that span the fin-to-limb transition are known from Gooloogongia (Rhizodontida) (12), Eusthenopteron (Osteolepida) (10, 13), Panderichthys (Elpistostegalia) (7, 8), and Acanthostega and Ichthyostega (Tetrapoda) (14–17). Comparisons of these forms reveal large differences between the pelvic appendages of finned tetrapodomorphs and tetrapods (Fig. 1). Most noticeable is that, in finned taxa, the entire pelvic appendage is significantly smaller than the pectoral. In particular, the pelvic girdle of finned tetrapodomorphs is diminutive relative to the pectoral: the pelvis represents a small fraction of the length of the body (the maximum length of pelvis-to-body length is 1:20 in Eusthenopteron per ref. 10). In addition, there are major differences in the morphology of the pelvic girdles of finned and limbed taxa. The girdles of Eusthenopteron and Gooloogongia have posteriorly facing acetabulae and lack sacral ribs and ischial bones, among other features (10, 12). Unfortunately, the pelvic girdle of Panderichthys is not preserved in sufficient detail to understand the distribution of these morphological features in elpistostegalids (8). However, the best comparisons available from these data strongly supported the hypothesis that the closest finned relatives of tetrapods were “front wheel drive animals,” possessing enlarged pectoral fins, robust pectoral girdles, and relatively small pelvic appendages that were incapable of providing extensive degrees of body support and propulsion.Open in a separate windowFig. 1.Right pelves of Gooloogongia (A), Eusthenopteron (B), and Acanthostega (C) in lateral view. Gooloogongia is preserved as a natural cast in one orientation. Figures were modified from refs. 10, 12, and 14. Cranial is to the right.Previously undescribed material of the stem tetrapod, Tiktaalik roseae, can inform these issues. The type specimen (NUFV108), recovered in 2004 and described in 2006 (5, 6), has since been revealed to contain a partial pelvic appendage, including the right side of the pelvic girdle and an incomplete pelvic fin consisting of endochondral bones and lepidotrichia (Figs. 2 and ​and3).3). This specimen allows direct comparison of the relative size of the pelvic girdle and appendage with the rest of the body because the type consists of a relatively articulated skeleton from head to pelvis. In addition, work at the same site in Nunavut Territory during 2006, 2008, and 2013 has revealed additional isolated pelves of four other individuals. Together, these specimens offer the possibility to test the front wheel drive hypothesis and provide insights into the sequence in the acquisition of tetrapod pelvic appendage structure and locomotor function.Open in a separate windowFig. 2.Type specimen (NUFV108): ventral surface of cranial block (figured in ref. 6) aligned in preserved position with ventral view of the block containing the pelvic fin. (Inset) Line diagram of lepidotrichia and preserved portions of endochondral bones of pelvic fin. f, fin; i, intermedium?; l, lepidotrichia; r, radials.Open in a separate windowFig. 3.Tiktaalik roseae, stereopairs of the right pelvis from NUFV108 in (A) ventral (cranial is to the Top), (B) dorsal (cranial is to the Bottom), (C) caudal (lateral is to the Right), and (D) cranial views (lateral is to the Left). A, acetabulum; i, ilium; p, pubis; r, ossified ridge; u, unfinished bone.     

Drought,agricultural adaptation,and sociopolitical collapse in the Maya Lowlands

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800–950 C.E.). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands but does not explain more pronounced and earlier societal disruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ∼200 C.E. to 500 C.E., during the Terminal Preclassic and Early Classic periods. Plant wax carbon isotope records indicate a decline in C₄ plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water-conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.The decline of the lowland Classic Maya during the Terminal Classic period (800–900/1000 C.E.) is a preeminent example of societal collapse (1), but its causes have been vigorously debated (2–5). Paleoclimate inferences from lake sediment and cave deposits (6–11) indicate that the Terminal Classic was marked by a series of major droughts, suggesting that climate change destabilized lowland Maya society. Most evidence for drought during the Terminal Classic comes from the northern Maya Lowlands (Fig. 1) (6–8, 10), where societal disruption was less severe than in the southern Maya Lowlands (12, 13). There are fewer paleoclimate records from the southern Maya Lowlands, and they are equivocal with respect to the relative magnitude of drought impacts during the Terminal Classic (9, 11, 14). Further, the supposition that hydrological impacts were a primary cause for societal change is often challenged by archaeologists, who stress spatial variability in societal disruption across the region and the complexity of human responses to environmental change (2, 3, 12). The available paleoclimate data, however, do not constrain possible spatial variability in drought impacts (6–11). Arguments for drought as a principal cause for societal collapse have also not considered the potential resilience of the ancient Maya during earlier intervals of climate change (15).Open in a separate windowFig. 1.Map of the Maya Lowlands indicating the distribution of annual precipitation (64) and the location of paleoclimate archives discussed in the text. The locations of modern lake sediment and soil samples (Fig. 2) are indicated by diamonds.For this study, we analyzed coupled proxy records of climate change and ancient land use derived from stable hydrogen and carbon isotope analyses of higher-plant leaf wax lipids (long-chain n-alkanoic acids) in sediment cores from Lakes Chichancanab and Salpeten, in the northern and southern Maya Lowlands, respectively (Fig. 1). Hydrogen isotope compositions of n-alkanoic acids (δD_wax) are primarily influenced by the isotopic composition of precipitation and isotopic fractionation associated with evapotranspiration (16). In the modern Maya Lowlands, δD_wax is well correlated with precipitation amount and varies by 60‰ across an annual precipitation gradient of 2,500 mm (Fig. 2). This modern variability in δD_wax is strongly influenced by soil water evaporation (17), and it is possible that changes in potential evapotranspiration could also impact paleo records. Accordingly, we interpret δD_wax values as qualitative records of water availability influenced by both precipitation amount and potential evapotranspiration. These two effects are complementary, since less rainfall and increased evapotranspiration would lead to both increased δD_wax values and reduced water resources, and vice versa.Open in a separate windowFig. 2.Scatter plot showing the negative relationship between annual precipitation and δD_wax-corr measured in modern lake sediment and soil samples (Fig. 1). Results from Lake Chichancanab (CH) and Salpeten (SP) are indicated. The black line indicates a linear regression fit to these data, with regression statistics reported at the bottom of the plot. Large squares indicate mean values for each sampling region, with error bars indicating SEM in both δD_wax-corr and annual precipitation. The black error bar indicates the 1σ error for δD_wax-corr values (SI Text). Original δD_wax data from ref. 17. VSMOW, Vienna Standard Mean Ocean Water.Plant wax carbon isotope signatures (δ¹³C_wax) in sediments from low-elevation tropical environments, including the Maya lowlands, are primarily controlled by the relative abundance of C₃ and C₄ plants (18–20). Ancient Maya land use was the dominant influence on the relative abundance of C₃ and C₄ plants during the late Holocene, because Maya farmers cleared C₃ plant-dominated forests and promoted C₄ grasses, in particular, maize (21–24). Thus, we apply δ¹³C_wax records as an indicator of the relative abundance of C₄ and C₃ plants that reflects past land use change (SI Text). Physiological differences between plant groups also result in differing δD_wax values between C₃ trees and shrubs and C₄ grasses (16), and we use δ¹³C_wax records to correct for the influence of vegetation change on δD_wax values (25) (δD_wax-corr, SI Text and Fig. S1).Plant waxes have been shown to have long residence times in soils in the Maya Lowlands (26). Therefore, age−depth models for our plant wax isotope records are based on compound-specific radiocarbon ages (Fig. 3), which align our δD_wax records temporally with nearby hydroclimate records derived from other methodologies (26) (SI Text and Fig. S2). The mean 95% confidence range for the compound-specific age−depth models is 230 y at Lake Chichancanab and 250 y at Lake Salpeten. Given these age uncertainties, we focus our interpretation on centennial-scale variability (26). The temporal resolution of our plant wax isotope records is lower than speleothem-derived climate records (8, 9), but combining plant wax records from multiple sites allows comparisons of climate change and land use in the northern and southern Maya Lowlands, which would otherwise not be possible. In addition, plant wax isotope records extend to the Early Preclassic/Late Archaic period (1500–2000 B.C.E.), providing a longer perspective on climate change in the Maya Lowlands than most other regional records (6, 8–11).Open in a separate windowFig. 3.Plant wax (green; left) and terrigenous macrofossil (red; right) age−depth models for (A) Lake Chichancanab and (B) Lake Salpeten. The age probability density of individual radiocarbon analyses is shown. The black lines indicate the best age model based on the weighted mean of 1,000 age model iterations (62). Colored envelopes indicate 95% confidence intervals. Cal, calendar.     

Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal

A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations.Liquid crystals (LCs) are self-organized mesomorphic materials that exhibit various symmetries and structures (1). They are widely used in flat panel displays for their exceptional electrooptical properties and a combination of orientational elasticity and fluidity. For example, nematic LCs (NLCs) are distinguished by their long-range orientational order, which favors alignment of the molecules (mesogens) in a preferred direction denoted as the director n. An exceptional feature of NLCs is that, despite their fluidity, they exhibit anisotropic optical and mechanical properties, and thus can transmit mechanical torque because of directional elasticity (1). Such torque occurs in response to deformations away from a uniform equilibrium state.Such unique features of LCs can be exploited for designing smart multifunctional materials. Among these materials, colloidal dispersions of microparticles and nanoparticles in LCs have been actively studied in research on soft-matter physics (2–7). Tunable anisotropic interactions between microparticles dispersed in LCs give rise to self-assembled 1D and 2D colloidal structures (6, 8, 9). Such colloidal dispersions are interesting not only from a fundamental point of view but also from a technological one. A wide range of self-assembled structures of particles and topological defects stabilized by LC-mediated interactions find numerous applications in designing metamaterials (10), photonic devices (5, 11), sensors (12), and microrheology (5, 10–12). Here, we demonstrate a phenomenon that can be used in intelligent devices using colloidal dispersions: controlled light-driven translational and rotational motions of microrods in a NLC matrix.The orientation of LC molecules at an interface is governed by anchoring conditions, i.e., whether the director is perpendicular (homeotropic) or parallel (planar) to the interface. The orientation of the director at surfaces can be controlled through interfacial energy, anisotropy of the surface tension, and surface topography, by the pretreatment of the surfaces using surface agents, such as polymers and surfactants, together with mechanical or optical treatments. In most of the previous experiments, the solid interface was fixed. Here we use a so-called command surface (13) for LC alignment and its real-time control; the director manipulation is achieved by manipulating the anchoring condition through a light-induced isomerization of a photoactive azo-dendrimer adsorbed at the surface of microrods. As the light-induced isomerization takes place, the anchoring conditions provided by the cis isomer are different from those of the trans isomer. The initial equilibrium state of the director is lost. An important point here is that the bending direction in the cis form is not random but determined by the distorted local director near the surfaces. The torque on the particle exerted by the liquid crystal director reorientation results in specific particle motions toward a new equilibrium state. Such molecular-assisted manipulation of particles provides a tool for studying interfacial effects in their interplay with the topology of the nematic director field, which is a key concept for smart microdevices. Development of such devices requires better understanding of the photoisomerization. Some studies of azobenzenes bound to a dendrimer core have been conducted and reported in the literature (14, 15). The photoisomerization mechanism, the dependence of the quantum yield on the phenyl ring substituents, the solvent properties, and the irradiation wavelength, are still not fully understood (16). The molecular groups attached to the dendrimer core with highly regular branching do not entangle as in the case of conventional macromolecules and seem to express similar properties to the original azobenzene chromophores. Thus, this provides another way to study isomerization and photochemical properties of azobenzene molecules.Active steering of particles in LC hosts is usually achieved by convection mechanisms, such as electrophoresis, or using high-power laser radiation in optical tweezers (17). Manipulation of the mesogen orientations is another way to control colloidal particles in an LC host. This approach mimics molecular motors, which use conformational changes of molecules (18). Several types of artificial molecular motors and actuators were designed whereby the energy of light is transformed into the mechanical energy (19–21). In the case of LCs, a change of the anchoring conditions can be achieved by photosensitive functionalizations of the surface with mesogen-like moieties connected via light-sensitive azo linkages. Yamamoto et al. used a photosensitive surfactant and succeeded in manipulating the anchoring condition of colloidal particles in NLCs (22). There is, of course, a large number of papers dealing with light-driven motion and deformation in (soft) solids, e.g., liquid crystal elastomers that make use of cis−trans isomerization (for instance, refs. 23–29).Yonetake et al. synthesized a poly (propyleneimine) liquid crystalline dendrimer, which spontaneously adsorbs at LC−glass interfaces and favors homeotropic alignment of NLC molecules (30). This material was used to fabricate in-plane-switching-mode LC displays without pretreatment of substrate surfaces (31). Li et al. recently synthesized a photosensitive dendrimer with azo linkages (azo-dendrimer) in the mesogenic end chains (Fig. S1) and demonstrated the adsorption of the dendrimer at a glass interface (32). By UV irradiation, an orientational change of the mesogenic moieties occurs associated with the trans−cis photoisomerization, as illustrated in Fig. 1 A and B. Such asymmetric adsorption of dendrimers at surfaces has already been proved with a surface second-harmonic generation experiment (33). Hence, the director field was distorted under suitable illumination. The azo-dendrimer has already been used for controlling ordering transitions in mobile LC microdroplets in a polymer matrix (34) and defect structures in microparticles in LCs (35). In contrast, the situation is different in our study: Because the embedded particles are mobile and anisometric (rod-shaped), light irradiation brings about a dynamic motion of the enclosed colloids. We exploit the spontaneous adsorption of the photoactive dendrimer on various interfaces to functionalize the surfaces of rod-shaped microparticles suspended in a nematic host. As a result, we not only can control the molecular orientation at the immobile rods by light but can also mechanically rotate and translate them.Open in a separate windowFig. 1.Conformations of the photoactive dendrimer: (A) trans-conformation favoring homeotropic alignment of the mesogens and (B) cis-conformation favoring planar alignment obtained under UV light irradiation.Now we describe our present experimental results: All of the experiments were performed using 4′-n-pentyl-4-cyanobiphenyl liquid crystal (5CB) mixed with 0.1 wt% azo-dendrimer and glass rods of 10- to 20-µm length and 1.5-µm diameter. For details, the reader is referred to Experimental Procedures. First, we describe the photo-induced director field change around immobile microrods. The changes of the director configuration can be easily studied when the rods are immobile, i.e., attached to the glass substrate. The case of cells with homeotropic anchoring is described in Fig. S2. The situation is more complicated in cells with a planar anchoring condition. Several configurations are possible: rods aligned parallel (i), perpendicular (ii), and diagonal (iii) to the rubbing direction (Fig. S3). Experimentally, many rods are initially aligned at an angle Θ of 60°–70° to the director n, although normal anchoring conditions seem to favor Θ = 90°. This discrepancy may be attributed to an asymmetry of the director field at the ends of the rod and in the vertical dimension, and the different energies of the associated defect lines. Two kinds of director field configurations were found: dipolar with a single hyperbolic defect (Fig. 2A) in case i and quadrupolar with a disclination loop surrounding the rod (Fig. 2C) in case iii. These configurations have been established earlier in thin 2-μm cells by Tkalec et al. (6). In our case, to study mobile rods, thicker cells are preferred, which, however, makes the optical characterization difficult. Without UV irradiation, the director is normal to the surface of the rod (Fig. 2 A and C). This corresponds to a radial hedgehog configuration of the director field with the effective topological strength +1. To comply with the homogeneous far-field director outside the rod, a disclination loop is expected to encircle the rod in case iii. Evidence of that is shown in Fig. 2 E and F, and the structure of the disclination loop is schematically sketched in Fig. 2C. The loop is attracted to the diagonal edges of the rod to relive the mechanical strain on the director field.Open in a separate windowFig. 2.Immobilized microrods attached to a surface in a planarly aligned LC cell. The rubbing direction is the horizontal direction of the images. (A) A rod lying along the rubbing direction without UV irradiation. The director map with dipolar point defects is shown, together with a microscope image in the Inset. With the inserted λ wave plate diagonal to the crossed polarizers, different birefringence colors are seen at both sides along the rod, and they are consistent with the director map shown. (B) The same rod under UV irradiation. In the vicinity of the particle, each of the director fields in A and B is mirror symmetric in the cell plane and, in first approximation, axially symmetric about the rod axis. (C) A rod lying approximately diagonal to the rubbing direction without UV irradiation. The director map with quadrupolar point defects is shown, together with a microscope image in the Inset. (D) The same rod under UV irradiation. Both director fields in C and D have C₂ symmetry about the cell normal in the cell midplane. The disclination loop shown in C, that is present at normal anchoring, vanishes under UV irradiation when the director anchors tangentially, as shown in D. It leaves two point defects. The director orientation and its change by UV irradiation are clearly visible in the images with a wave plate (see Insets). (E) Image of a rod between crossed polarizers without UV irradiation. (F) A bright field image of the same rod without polarizers. The arrows indicate the position of topological defects at the ends of the rod. The cell thickness is 10 μm.Under UV irradiation, the director configuration changes: The normal configuration of the director transforms into a tangential one. This is evident from the changes of the interference colors to the complementary ones observed with a wave plate (see schematics in Fig. 2 A−D, and microscope images in Fig. 2 A−D, Insets). The loop collapses and transforms into two separated surface boojums with1/2 topological charge on the surface of the rod to satisfy planar anchoring conditions. This motion results in a pair of defects attached to the corners (see Fig. 2F). Disclination loops (Saturn ring) were confirmed by Tkalec et al. (6) for rods aligned perpendicular to the rubbing direction (case ii). Those loops were also tilted with respect to the rods axes.Free rods exhibit opto-mechanical responses: (i) rotation of rods in the cell plane, (ii) rotation in the vertical plane, and (iii) translation in the cell plane. In the first case, the rods initially appear at an angle of ∼70° to the nematic director (Fig. 3 A and B). Under applied UV irradiation, the rods align nearly along the director (Fig. 3 A and C). In the second case (Fig. S4), rotation occurs about an axis perpendicular to the rubbing direction and the cell normal. In both cases, the rotations are fully reversible. The rods return to the original state when UV irradiation is removed (Fig. 3A). The angular variation and the switching rate depend on the intensity of the UV irradiation (Fig. 3F). The rotational motion involves several stages. In the first stage, the reorientation of the dendrimer moieties takes place resulting in a change of the anchoring condition from nearly orthogonal to nearly planar. The next (fast, characteristic time scale is 0.01–0.05 s) stage is accompanied by a rearrangement of the topological defects and a continuous reorientation of the director field. This triggers the last (slow, characteristic time scale is 0.1–2 s) stage: motion of the rod. The time dependence of the rod reorientation is shown in Fig. 3F. The solid curve is a best fit to one of the experimental data based on theoretical consideration (SI Text). Even at low light intensities, the rotation occurs, but the rotation angle is reduced (Fig. 3G). Both angular variations and the switching rates show a saturation-type dependence on the UV intensity (Fig. 3 G and H), which is also explained theoretically (Fig. S5B).Open in a separate windowFig. 3.Dynamic motion of rods under the action of UV irradiation. (A) Time dependence of the angle between the rod and the rubbing direction. (B) Image of the initial state of a rod without UV, and (C) the final state under UV irradiation. The length of the rod is 20 μm. The rubbing direction is marked by a white arrow (parallel to n). B and C reveal the rotational motion. (D) Image of a rod without UV irradiation. (E) Image of a rod under UV irradiation at t_exp = 43 s. D and E reveal the translational motion. A white arrow marks the rubbing direction. A dashed line indicates the initial position of the rod. Images D and E were taken between polarizers parallel to the sides of the frame. The length of the rods in D and E was 15 μm. (F) Time dependence of the rod inclination at various intensities of the UV light. A solid line is the theoretical fit of the experimental data (see SI Text for details). (G and H) UV intensity dependences of the deflection angle and the switching rates, respectively. The saturation behaviors are theoretically explained (compare H and Fig. S5B).Translational motion was observed in some rods, and the direction was more or less parallel to the long axis of the rods. Fig. 3 D and E shows the initial and intermediate stages during UV irradiation. The translational movement was extended to displacements of the order of one rod length. We could observe a slight backlash motion when the UV irradiation was stopped.Since UV irradiation affects the conformational state of the dendrimer adsorbed at the rod surface, it is reasonable to consider that the director reorientation direction is directly related to the cis conformation. Straight (trans) to bent (cis) conformational change occurs upon photoisomerization and results in a change of the anchoring condition. The rotational motion of the rods originates from the torques exerted by the director. How do those torques develop? The initial state depicted in Fig. 3B is stabilized by the director deformation and the configuration of the topological defects around the rod as well as the anchoring on the rod surface. The torque by the director in the volume is counteracted by the (anchoring) torque at the rod/LC boundary. Under UV light irradiation, the orthogonal anchoring condition changes to the planar one and the torque in the volume is not in balance with the boundary any more. This leads to the rotation of the rod to the new equilibrium state. In the first approximation, one may describe this anchoring by a potential as W_asin²(θ), where θ is the polar anchoring angle with respect to the surface, and W_a is the strength of the anchoring energy (1). The anchoring energy is determined by the ratio of cis and trans isomers at a given light intensity and spectral composition. Planar anchoring corresponds to positive W_a, while the orthogonal anchoring is achieved with W_a < 0. Strong anchoring is given if the penetration length ξ = |K/W_a| is short respective to typical geometrical sizes of the experiment; for weak anchoring, ξ is comparable or larger than system sizes. Obviously, strong illumination (large fraction of cis molecules) will correspond to large positive W_a, while no illumination (mainly trans molecules) corresponds to large negative W_a. The experiments suggest a monotonous functional form of the transition between both states with increasing/decreasing illumination intensity. One can demonstrate that intermediate stationary W_a?situations can be reached at certain moderate UV intensities. Then, the director field is not influenced by the rod; the microscopy image has a uniform color around the inclusions. This corresponds to an infinite ξ. These arguments enable us to roughly estimate the switching rates and qualitatively describe their dependence on the light intensity, as described in SI Text. At strong anchoring, the torque acting on the director is proportional to KL, where L is a characteristic length (of the order of magnitude of the rod length) and K is the mean Frank elastic constant. The viscous torque is proportional to ηL³, where η is a mean viscosity of the liquid crystal. This results in a switching time τ = ηL²/K ≈ 0.5? s. In case of weak anchoring, the switching rate Γ depends on the light intensity through the penetration length?ξ : Γ = K/[ηL(L + ξ)]. Since short ξ correspond to high UV intensities, whereas low UV intensities yield long?ξ, the switching rate increases with UV intensity and is expected to reach some saturation Γ = 1/τ. A qualitative agreement of the estimated switching rate with the experiment can be found by comparing Fig. 3H and Fig. S5B.How is the reorientation direction chosen? For rods with a surrounding director field similar to that shown in Fig. 2A, the director has an opposite sense of bending on two sides of the rod, as recognized by different birefringence colors under a wave plate. UV light irradiation under this condition induces the opposite director rotation at both sides of the rod (Fig. 4A), leading to lateral translational motion. In contrast, for rods similar to those shown in Fig. 2C, the director bending direction is the same at both sides (Fig. 4B), as shown by the same birefringence color. Hence, the same director rotation at both sides of the rod upon UV light irradiation exerts torques with the same sign, causing the rod to reorient (Fig. 4).Open in a separate windowFig. 4.Schematic illustrations of the microrod motions and the acting torques. (A) Translational and (B) rotational motions. Azo-dendrimers and their bending direction are marked by brown lines and arrows, respectively. Red arrows show the director rotation from 1 (normal to the rod) to 4 (parallel to the rod) and the torque direction. Green arrows designate the rod motion.Interactions between microparticles dispersed in an LC matrix lead to a formation of complex ordered structures. Such agglomerates of particles may have a form of chains or even more intricate structures. In this case, optomechanical effect manifests in the motion of either the whole agglomerate or its separate parts. Different examples of such structures are shown in Fig. 5.Open in a separate windowFig. 5.Switchable agglomerates of particles stabilized by the director-mediated interactions: doublets of rods (crossed polarizers with a wave-plate) without UV (A) and under UV irradiation (B). There is no rotation of the doublet since the mechanical torques on two arms cancel out. The switching of the director is clearly seen from the interference colors. A dumbbell of a rod with two beads without UV (C) and under UV irradiation (D) in unpolarized light. The director orients along the vertical direction in the images.In conclusion, we demonstrated the optomechanical effect of light-induced rotation and translation of micrometer-sized rod particles in a nematic host. This system represents an optically driven molecular microactuator, which exploits molecular reorientation on a particle surface and transforms it into a mechanical torque.     

Late Miocene episodic lakes in the arid Tarim Basin,western China

The Tibetan Plateau uplift and Cenozoic global cooling are thought to induce enhanced aridification in the Asian interior. Although the onset of Asian desertification is proposed to have started in the earliest Miocene, prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources, is only a geologically recent phenomenon. Here we report episodic occurrences of lacustrine environments during the Late Miocene and investigate how the episodic lakes vanished in the basin. Our oxygen isotopic (δ¹⁸O) record demonstrates that before the prevailing desert environment, episodic changes frequently alternating between lacustrine and fluvial-eolian environments can be linked to orbital variations. Wetter lacustrine phases generally corresponded to periods of high eccentricity and possibly high obliquity, and vice versa, suggesting a temperature control on the regional moisture level on orbital timescales. Boron isotopic (δ¹¹B) and δ¹⁸O records, together with other geochemical indicators, consistently show that the episodic lakes finally dried up at ∼4.9 million years ago (Ma), permanently and irreversibly. Although the episodic occurrences of lakes appear to be linked to orbitally induced global climatic changes, the plateau (Tibetan, Pamir, and Tianshan) uplift was primarily responsible for the final vanishing of the episodic lakes in the Tarim Basin, occurring at a relatively warm, stable climate period.Once part of the Neo-Tethys Sea, indicated by the Paleogene littoral-neritic deposits with intercalated marine strata (1), the Tarim Basin in western China (Fig. 1) has experienced dramatic environmental and depositional changes during the Cenozoic. The eventual separation of the basin from the remnant sea probably occurred during the middle to late Eocene (1, 2), and since then, terrestrial sedimentation and environment have prevailed. Today, the basin has relatively flat topography with elevation ranging between 800 and 1,300 m above sea level (asl), surrounded by high mountain ranges with average elevation exceeding 4,000 m. Hyperarid climate prevails with mean annual precipitation <50 mm and evaporation ∼3,000 mm. Active sand dunes occupy 80% of the basin, forming the Taklimakan Desert, the largest desert in China and the second largest in the world (3). Thick desert deposits in the basin also provide a major source for dust storms occurring in East Asia (4).Open in a separate windowFig. 1.Digital elevation model of the Tarim Basin and surrounding mountain ranges. The studied 1,050-m sediment core is retrieved from Lop Nor (1), at a relatively low elevation (∼800 m asl) in the eastern basin. Also indicated are locations of previous studied sections, near Sanju (2), Kuqa (3), and Korla (4) in the basin.The Tibetan Plateau uplift, long-term global cooling, and the associated retreat of the remnant sea during the Cenozoic, through their complex interplay, may have all contributed to the enhanced aridification in the Asian interior and eventually the desert formation (2, 5–11). Although the onset of Asian desertification is proposed to have started in the earliest Miocene (12, 13), prevailing desert environment in the Tarim Basin, currently providing much of the Asian eolian dust sources (4), is only a geologically recent phenomenon (9). Previous lithological and pollen studies (9, 14, 15) suggest that the currently prevailing desert environment started probably at ∼5 Ma. However, what kind of environmental conditions prevailed before that and how the dramatic changes occurred largely remain elusive.To better decipher the aridification history, we used a 1,050-m-long, continuous sediment core retrieved from Lop Nor (39°46''0''''N, 88°23''19''''E) in the eastern basin (Fig. S1). The core site has a relatively low elevation (∼800 m asl) (Fig. 1) and thus effectively records basin environment. Previous study sites came from elevated basin margins (9, 14). The core mainly consists of lacustrine sediments with associated fluvial-eolian sands (Fig. 2). The core chronology was established previously based on paleomagnetic polarity (15). The total 706 remanence measurements on the continuous sediment profile allow straightforward correlation with the CK95 geomagnetic polarity timescale (16) and identification of 14 normal (N1–N14) and 13 reversal (R1–R13) polarity zones over the last 7.1 Ma (Fig. S1). The CK95 timescale is largely consistent (within a few ky) with that inferred from marine archives (17) over the last 5.23 Ma, and before 5.23 Ma a practical measure of chronological uncertainty was estimated to be within ∼100 ky (18), assuring that the terrestrial records can be directly compared with marine records and orbital changes at the timescale of >100 ky. The derived chronology yields an average sedimentation rate of ∼200 m/Ma at lower sections (>1.77 Ma), allowing high-resolution studies of the desertification history at the critical interval.Open in a separate windowFig. 2.Records of δ¹¹B, δ¹⁸O, TOC, and CaCO₃ changes in the Lop Nor profile. Representative photos show lithological changes mainly from lacustrine bluish gray argillaceous limestone to fluvial-eolian brown/red clayey silt, as shown in the lithological column with visual colors indicated (15). High levels and large fluctuations of proxy values occurred only before ∼4.9 Ma, and since then, those remain low and within a small range, largely similar to modern conditions. The transition from episodic occurrences of lacustrine phases to prevailing desert environments thus appears to be permanent and irreversible.Boron and oxygen isotopes from carbonates are important environmental indicators (19–25) and used here to infer the environmental evolution in the Tarim Basin. We also present total organic carbon (TOC), calcium carbonate (CaCO₃), ostracod, and grain size records to substantiate the isotopic evidence (SI Materials and Methods). Our δ¹¹B profile shows substantial, stepwise changes over the last ∼7.1 Ma (Fig. 2). Carbonate δ¹¹B values were −5.0 ± 1.6‰ (n = 3) before 6.0 Ma, increased rapidly to ∼11‰ at 4.9–6.0 Ma, and then stayed at roughly the same level (10.7 ± 2.2‰, n = 25) for the remaining 4.9 Ma. Higher-resolution δ¹⁸O, TOC, and CaCO₃ profiles generally confirm the pattern observed in the low-resolution δ¹¹B one (Fig. 2). The δ¹⁸O values remained low, ranging from −10‰ to −4‰ over the last 4.9 Ma. However, δ¹⁸O values frequently oscillated between −10‰ and 5‰ before that. Similarly, the TOC profile shows consistently low organic carbon content (0–0.2%) after 4.9 Ma and large fluctuations (0–1.0%) before then. The CaCO₃ profile also indicates consistently low values (0–25%) after 4.9 Ma and large fluctuations (0–50%) earlier (Fig. 2).The multiple proxy records strongly suggest that critical environmental changes must have occurred at ∼4.9 Ma. δ¹¹B values of carbonates from marine sources differ substantially from those of nonmarine carbonates (19–21). δ¹¹B values after 4.9 Ma are close to those from marine carbonates, but values before 6 Ma fall into the range of lacustrine carbonates (22). Positive δ¹⁸O values before 4.9 Ma also indicate lacustrine environments at that time. Carbonates from modern lakes in arid and semiarid regions of northwestern China show similar positive δ¹⁸O values (23), due to strong evaporation processes. High TOC and CaCO₃ contents (Fig. 2) further support that lacustrine environments existed in the basin before ∼4.9 Ma. δ¹⁸O values after 4.9 Ma are comparable to those in Cenozoic soil carbonates (24) and ancient marine carbonates in the Tarim Basin (25). However, the accompanying carbonate δ¹³C values throughout the record, ranging from −4‰ to 1‰ (Dataset S1), are significantly higher than those from Cenozoic soil carbonates reported (26), essentially ruling out the possibility of soil carbonate source. Using modern prevailing desert environment in the basin as an analog, the combined δ¹¹B and δ¹⁸O evidence thus suggests that the sediment deposits in the basin after 4.9 Ma must be eolian-fluvial in origin and their sources, at least carbonate grains, came from weathered ancient marine carbonates in nearby regions.Sedimentological and stratigraphic patterns in other exposed sections from different parts of the basin (9, 14) share great similarity with the Lop Nor core profile (Fig. S1). Episodic lacustrine mudstones and/or siltstones during the Late Miocene were present in all sections and were replaced by fluvial-eolian deposits later. Studies of ostracod assemblages (27) also suggest a shallow paleolake with brackish water environments in the northern basin during the Late Miocene. Changes in the depositional environment from our Lop Nor profile alone could be plausibly explained by a shift in basin center due to tectonic compressions, as evidenced from the slightly uplifted central basin (Fig. 1). However, similar temporal changes occurring basin-wide at ∼4.9 Ma argue against it. Instead, our results, together with previous studies (5, 14, 15, 27), suggest that paleolakes were widely present in the low lands of the basin during the Late Miocene, much different from currently prevailing desert environments with a few scattered small lakes. The existing evidence, although still limited (Fig. 1), would point to the occurrence of a possible megalake in the Tarim Basin during the Late Miocene.Three high-resolution records, δ¹⁸O, TOC, and CaCO₃, further suggest that lacustrine environments before ∼4.9 Ma were not permanent (Fig. 2). These large fluctuations indicate frequent switches between lacustrine and fluvial-eolian environments in the basin. High proxy values, δ¹⁸O in particular, appear to indicate lacustrine environments, whereas low values, similar to ones after 4.9 Ma, correspond to fluvial-eolian deposits. This is consistent with lithological features at this interval, showing argillaceous limestone intercalated with clayey layers (15), the occurrence of ostracod assemblages (Fig. 3) from lacustrine sediments, grain size changes (Fig. S2), and detrital carbonate grains identified in photomicrographs of fluvial-eolian deposits (Fig. S3).Open in a separate windowFig. 3.δ¹⁸O fluctuations linked to eccentricity and obliquity orbital variations at 4.5–7.1 Ma. Lacustrine phases (high δ¹⁸O) generally correspond to periods of high eccentricity and obliquity. Fluvial-eolian environment (low δ¹⁸O, highlighted with gray bars), developed more around 6.5, 6.1, 5.7, and 5.2 Ma, at a ∼400-ky eccentricity beat. The red bar indicates the last occurrence of lacustrine environment at ∼4.9 Ma. Occurrences of ostracod assemblages with total number >100 are also indicated.To further investigate such episodic changes, we performed spectral analysis on the δ¹⁸O record over the interval 4.5–7.1 Ma. Strong spectral power at orbital frequencies were identified, with periods of ∼400 ky throughout the interval, ∼41 ky particularly at 6–6.5 Ma, and ∼100 ky at 5–6 Ma and 6.5–7.1 Ma (Fig. S4). Precessional ∼20-ky power might also have existed but was relatively weak and discontinuous. Lacustrine phase as indicated by high δ¹⁸O values and ostracod assemblages generally occurred at periods of high eccentricity and obliquity (28) (Fig. 3). At 6–6.5 Ma, δ¹⁸O shows clear correspondence to orbital obliquity variation (Fig. 3A). Additionally, the number of low δ¹⁸O values (fluvial-eolian environment) occurred more around 6.5, 6.1, 5.6, and 5.2 Ma, at a ∼400-ky beat following orbital eccentricity variation (Fig. 3B). The cluster of high δ¹⁸O values (>0‰) at 4.9–5.0 Ma signals the last occurrence of lacustrine environments in the basin. Our orbital association thus allows us to precisely determine the timing of desert formation at ∼4.9 Ma, ∼400 ky (an eccentricity cycle) later than the age inferred from basin margins (9, 14) and yet all occurring at eccentricity minima (Fig. 3B). As high eccentricity and obliquity generally correspond to warm conditions at orbital timescales, lacustrine (wet) phase could be associated with warm periods, consistent with the notion that cooler conditions would reduce moisture in the atmosphere and enhance continental drying (10, 11). We recognize that the chronological uncertainty from the geomagnetic polarity timescale before 5.23 Ma, within ∼100 ky (18), could confound our association of wet phase with high obliquity, although it is unlikely affected at the 400-ky eccentricity beat. However, the opposite association, wet phase with low obliquity, and the combination with high eccentricity, would require a different, yet unknown mechanism that is inconsistent with the orbital theory of Pleistocene ice ages.Superimposed on the orbitally episodic changes, the δ¹⁸O record also shows a long-term trend of deteriorating lacustrine conditions at 4.9–7.1 Ma. As δ¹⁸O values indicate two depositional environments, lacustrine and fluvial-eolian, the range of δ¹⁸O changes (between −10‰ and 5‰) does not vary much over this period (Fig. 3). Rather, the duration of high δ¹⁸O vs. low values would reflect the long-term trend. The mean δ¹⁸O values over 40-ky and 400-ky intervals both show a decreasing trend, with dominant lacustrine phase before 6.1 Ma, more developed fluvial-eolian environment at 5.7–6.1 Ma, a return to slightly better lacustrine environment at 5.3–5.7 Ma, and lacustrine phase permanently vanished around 4.9 Ma (Fig. 4).Open in a separate windowFig. 4.Long-term δ¹⁸O changes compared with global climatic conditions and regional tectonic activities. Global benthic δ¹⁸O (29) and northwestern Pacific SST (30) records show minimal long-term changes at 4–7 Ma, whereas occurrences of detrital apatite fission track ages (35) in northern western Kunlun peaked, and the Tarim episodic lakes gradually vanished. The dark thick lines are their 400-ky running means, and the light green line on δ¹⁸O represents the 40-ky running mean.The gradual disappearance of the Tarim episodic lakes could be potentially explained by the two driving forces, plateau uplift (the Tibetan Plateau, Pamir Plateau, and Tianshan) and long-term global cooling. However, the long-term global climate was relatively warm and stable during this period (Fig. 4). The global benthic δ¹⁸O record (29) shows that much of the Miocene cooling occurred between 15 and 11 Ma, and the cooling between 8 and 5 Ma was minimal. Supporting this view, sea surface temperature records from the northwestern Pacific (30) show that almost no cooling occurred between 6 and 4 Ma (perhaps further to 3 Ma) (Fig. 4). Particularly, the mean global climate (29, 30) was even warmer at 4.1–4.5 Ma than at 5.7–6.1 Ma, whereas lacustrine phase permanently disappeared after ∼4.9 Ma (Fig. 4), indicating decoupling of the lake evolution from global climate. Therefore, long-term global cooling might have played a subordinate role in the lake disappearance.Instead, the growth of surrounding mountain ranges (Tibetan, Pamir, and Tianshan) may have blocked moisture from the west and south, changed air circulations, and eventually led to the permanent lake disappearance within the basin. Today, the Tarim Basin receives limited moisture from westerlies (31) through the Pamir and Tianshan Ranges (and perhaps from the Indian Ocean in summertime as well). Although the Indo–Eurasian convergence since the Late Eocene resulted in high elevations of the Tibetan Plateau and, to a lesser degree, surrounding mountains including Pamir and Tianshan by the mid-Miocene time (8), tectonic activities in broad areas around the Tarim Basin appear to be rejuvenated since the Late Miocene. Tectonic deformations during the Late Miocene–Early Pliocene inferred from growth strata, sedimentary facies changes, and low-temperature thermochronologic studies occurred in Tianshan to the north of the Tarim Basin, in the Kunlun Mountains to its south and the Pamir to its west (32–35). Syntectonic growth strata from the foreland basins of the Kunlun and Tianshan Ranges (32) show that strong crust shortening and potential mountain uplift initiated ∼6.5–5 Ma and lasted to the Early Pleistocene (Fig. S5), similarly reported in northern Pamir (33). Cenozoic sequences in the Pamir–Tianshan convergence zone, changing from an arid continental plain to an intermountain basin by ∼5 Ma, support surface uplift of the west margin of the Tarim Basin (34). Occurrence of detrital apatite fission track ages from West Kunlun Ranges (35) also peaked at ∼4.5 Ma (Fig. 4). Thus, the reactivated uplift of Pamir and West Kunlun Ranges and northward movement of Pamir during the Late Miocene–Pliocene would progressively block moisture into the basin and enhance regional aridity to a certain threshold to terminate lacustrine environments even during warm periods with favorable orbital configuration. Although inconsistencies indeed exist in linking regional climatic and environmental changes to tectonic events (8), the final vanishing of the Tarim episodic lakes is better explained by tectonic factors.Therefore, our multiple-proxy results consistently show that the Taklimakan sand sea began to form at ∼4.9 Ma and that the transition into prevailing desert environment was permanent and irreversible. The episodic occurrences of lacustrine environments at favorable climatic conditions (warm periods) during the Late Miocene suggest that uplifted mountain ranges then were not high enough to effectively block moisture from being transported into the basin. The transition from episodic lacustrine environments to prevailing desert deposits was gradual, from 7.1 Ma (or earlier beyond our record) to 4.9 Ma, for which we suggest that although long-term global cooling enhanced the overall aridification in the Asian interior during the Late Cenozoic (10, 11), plateau uplift played a more important role in finally drying up the episodic lakes within a relatively warm, stable climate period, thus decoupling regional climate temporally from a global trend. Our high-resolution records thus demonstrate that regional climate in the Tarim Basin reached a critical state in the Late Miocene, with the dual effects from global climate conditions and regional tectonic settings then. With global climate remaining relatively stable and warm entering the Pliocene, by ∼3–4 Ma (Fig. 4), drying up of episodic lakes at ∼4.9 Ma could thus be largely attributed to rejuvenated tectonic activities.     

From the Cover: Montsechia,an ancient aquatic angiosperm

The early diversification of angiosperms in diverse ecological niches is poorly understood. Some have proposed an origin in a darkened forest habitat and others an open aquatic or near aquatic habitat. The research presented here centers on Montsechia vidalii, first recovered from lithographic limestone deposits in the Pyrenees of Spain more than 100 y ago. This fossil material has been poorly understood and misinterpreted in the past. Now, based upon the study of more than 1,000 carefully prepared specimens, a detailed analysis of Montsechia is presented. The morphology and anatomy of the plant, including aspects of its reproduction, suggest that Montsechia is sister to Ceratophyllum (whenever cladistic analyses are made with or without a backbone). Montsechia was an aquatic angiosperm living and reproducing below the surface of the water, similar to Ceratophyllum. Montsechia is Barremian in age, raising questions about the very early divergence of the Ceratophyllum clade compared with its position as sister to eudicots in many cladistic analyses. Lower Cretaceous aquatic angiosperms, such as Archaefructus and Montsechia, open the possibility that aquatic plants were locally common at a very early stage of angiosperm evolution and that aquatic habitats may have played a major role in the diversification of some early angiosperm lineages.When did early angiosperms begin to diversify ecologically? This question is currently unanswered. Age estimates of the divergence of crown-group angiosperms using molecular clock data vary considerably, although it is in the range of (max. 210–) often accepted, 150–140 (min. 130) million years (1–7). Parsimony reconstruction of early angiosperm habit suggests that they may have been shrubs living in “damp, dark, and disturbed” habitats (8). In contrast, many living aquatic angiosperms are basal in angiosperm phylogenies [e.g., Nymphaeales in Amborella, Nymphaeales and Illiciales, Trimeniaceae-Austrobaileya (ANITA) or Ceratophyllales with the eudicots as commonly understood]. In the fossil record, we have found an aquatic angiosperm, Montsechia vidalii (Zeiller) Teixeira, which is an atypical plant fossil found in the Barremian (130–125 million years ago) freshwater limestone in the Pyrenees and Iberian Range in Spain. Montsechia (Fig. 1) lacks roots (no proximal or adventitious roots were found in more than 1,000 shoots examined) and shows flexible axes and two types of phyllotaxy and leaf morphology. The cuticle is very thin with rare stomata. The fruit is closed with a pore near the distal tip, indehiscent, and contains one unitegmic seed developed from an orthotropous and pendent ovule (Figs. 2 and ​and3).3). Cladistic analysis of these characters places Montsechia on the stem lineage basal to extant Ceratophyllum or a clade formed by Ceratophyllum and Chloranthaceae (Fig. 4) suggesting that mesangiosperms (non-ANITA angiosperms) existed 125 million years ago, as indicated by the tricolpate pollen record. Montsechia is well-adapted to a submerged aquatic habit. Montsechia is contemporaneous with another aquatic plant fossil, Archaefructus, indicating that some of the earliest angiosperms were fully aquatic very early in their ecological diversification.Open in a separate windowFig. 1.Long- and short-leaved forms of Montsechia vidalii. (A) The long-leaved specimen shows very flexuous branches and opposite, long leaves. LH02556. (Scale bar, 10 mm.) (B) The short-leaved specimen shows regularly developed lateral branches and tiny leaf rosettes. LH07198. (Scale bar, 10 mm.)Open in a separate windowFig. 2.Fruit and seed of Montsechia vidalii. The fruit shows a small apical pore (po). The funicle (f) of the single, upside-down seed (orthotropous pendent) is attached from the hilum (h) to the placenta (pl). (Scale bar, 500 µm.)Open in a separate windowFig. 3.Reconstructions of Montsechia vidalii. (A) The long-leaved form shows the opposite leaves and branches. (B) The short-leaved form shows the alternate phyllotaxy of leaves and branches bearing pairs of ascidiate, nonornamented fruits. (C and D) The fruit shows a small apical pore and a single seed developed from an orthotropous pendent ovule. The funicle arises from the placenta (near the micropyle) to the hilum (near the pollination pore). (C) Lateral view. (D) Front view. Diagram by O. Sanisidro, B.G., and V.D.-G.Open in a separate windowFig. 4.Most parsimonious position of Montsechia in a simplified tree derived from the matrix by Endress and Doyle (26) using the J & M backbone. Taxa in blue are considered ancestrally water-related (27). Diagram by C.C. and B.G.     

Anomalous scaling law of strength and toughness of cellulose nanopaper

The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. So far there exists only limited success of attaining both strength and toughness, which often needs material-specific, complicated, or expensive synthesis processes and thus can hardly be applicable to other materials. A general mechanism to address the conflict between strength and toughness still remains elusive. Here we report a first-of-its-kind study of the dependence of strength and toughness of cellulose nanopaper on the size of the constituent cellulose fibers. Surprisingly, we find that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher. Further fundamental mechanistic studies reveal that reduced intrinsic defect size and facile (re)formation of strong hydrogen bonding among cellulose molecular chains is the underlying key to this new scaling law of mechanical properties. These mechanistic findings are generally applicable to other material building blocks, and therefore open up abundant opportunities to use the fundamental bottom-up strategy to design a new class of functional materials that are both strong and tough.The need for engineering materials that are both strong and tough is ubiquitous. However, the design of strong and tough materials is often inevitably a compromise as these two properties generally contradict each other (1). Toughness requires a material’s ability of dissipating local high stress by enduring deformation. Consequently, hard materials tend to be brittle (less tough); lower-strength materials, which can deform more readily, tend to be tougher (2, 3). For example, the toughness of metals and alloys is usually inversely proportional to their strength (4). Acknowledging such a necessary compromise, one would expect that research on advanced material design would be focused on achieving an optimum combination of these two properties. Indeed much research effort is focused on pursuing higher strength, with rather limited corresponding regard for toughness (5–10). One example is the enthusiasm sparked by the discovery of carbon nanotubes (CNTs), which exhibit remarkably high strength. However, it still remains uncertain how such a strong material can be incorporated with bulk materials to benefit from its high strength without sacrificing toughness.There have been tremendous efforts recently to develop materials with higher strength using smaller material structures. For example, by decreasing the grain size of metals, dislocation motions (thus plasticity) are more restricted, leading to a higher strength (5–10). However, such treatments also minimize possible mechanisms (e.g., crack-tip blunting) to relieve local high stress, resulting in lower toughness. The atomic scale origins of high strength of a material, e.g., strong directional bonding and limited dislocation mobility, are also essentially the roots for brittleness of the material. In short, the well-recognized scaling law of “the smaller, the stronger” comes at a price of sacrificing toughness (Fig. 1).Open in a separate windowFig. 1.An anomalous but desirable scaling law of mechanical properties requires defeating the conventional conflict between strength and toughness.The prevailing toughening mechanisms can be categorized into two types: intrinsic and extrinsic. Intrinsic toughening operates ahead of a crack tip to suppress its propagation; it is primarily related to plasticity, and thus the primary source of fracture toughness in ductile materials. Recent progress involves introducing high-density nanotwin boundaries in metals to achieve high strength and toughness (11–15). Intrinsic toughening mechanisms are essentially ineffective with brittle materials, e.g., ceramics, which invariably must rely on extrinsic toughening (2). Extrinsic toughening acts mainly behind the crack tip to effectively reduce the crack-driving force by microstructural mechanisms, e.g., crack bridging and meandering and crack surface sliding (16–18). A counterintuitive but successful example is the development of bulk metallic glass (BMG)-based composites, in which a crystalline dendrite second phase is introduced into the BMG matrix to promote the formation of multiple shear bands, leading to a strong and also tough material (3, 9, 16, 19–21). Intrinsic and extrinsic toughening mechanisms are also found to be effective in natural materials (e.g., bones and nacres), which often involve the hierarchical structure and/or a “brick-and-mortar” hybrid microstructure of the material (22–26). Nature-inspired toughening mechanisms are also used to synthesize biomimetic structural materials. Nonetheless, so far, there exists only rather limited success in attaining both strength and toughness, which often involve material-specific, complicated (e.g., growing high density nanotwins), or expensive (e.g., BMG-dendrite composites) synthesis processes and thus are hardly applicable to other materials. A general and feasible mechanism to address the conflict between strength and toughness still remains elusive.Aiming to shed insight on the long-sought strategy addressing the conflict between strength and toughness, we rationally design cellulose-based nanopaper and investigate the dependence of their mechanical properties on constituent cellulose fiber size. Surprisingly, we find that both the strength and toughness of the nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose building blocks decreases (from a mean diameter of 27 µm to 11 nm). These stimulating results suggest the promising potential toward a new and highly desirable scaling law: the smaller, the stronger and the tougher (Fig. 1). Though the increasing strength as the diameter of cellulose fiber decreases can be attributed to reduced intrinsic defect size, and the dependence is well captured by a continuum fracture mechanics model, our atomistic simulations reveal that facile formation and reformation of strong hydrogen bonding among cellulose chains is the key to the simultaneously increasing toughness. These mechanistic findings that underpin the highly desirable scaling law of mechanical properties suggest a fundamental bottom-up material design strategy generally applicable to other material building blocks as well, and therefore open up abundant opportunities toward a novel class of engineering materials that are both strong and tough.Cellulose is the most abundant biopolymer on Earth and has long been used as the sustainable building block for conventional paper. Cellulose has appealing mechanical properties, with specific modulus [∼100 GPa/(g/cm³)] and specific strength [∼4 GPa/(g/cm³)] higher than most metals and composites, and many ceramics, making it as a promising building block for functional and structural materials (27). Wood fibers are the main natural source of cellulose and have an intrinsically hierarchical structure (Fig. 2). A 20- to ∼50-µm-thick native wood fiber comprises thousands of nanofibrillated cellulose (CNF) fibers (5–50 nm in diameter), each of which can be disintegrated into finer elementary fibrils consisting of cellulose molecular chains (27–36). Cellulose molecule is a linear chain of ringed glucose molecules, with a repeat unit (Fig. S1) comprising two anhydroglucose rings (C₆H₁₀O₅) linked through C–O–C covalent bond. Rich hydroxyl groups in cellulose molecule (six in each repeat unit) enable facile formation of hydrogen bonds, both intrachain and interchain (Fig. 2). Whereas the intrachain hydrogen bonding stabilizes the linkage and results in the linear configuration of the cellulose chain, interchain hydrogen bonding among neighboring cellulose molecules plays a pivotal role in the deformation and failure behaviors of cellulose-based materials.Open in a separate windowFig. 2.Hierarchical structure of wood fibers and the characteristic of cellulose fibrils. Note the rich interchain hydrogen bonds among neighboring cellulose molecular chains.Open in a separate windowFig. S1.Atomic structure of a cellulose chain repeat unit. Note the six hydroxyl groups (red circles) in each repeat unit.In this study, cellulose fibers of different mean diameters [27 μm (native fiber), 28 nm, and 11 nm, respectively] are isolated from wood cell walls using a top-down approach and characterized (SI Text and Figs. S2 and ​andS3).S3). Cellulose nanopaper is made of a highly entangled random network of CNF fibers (Fig. 3A; Materials and Methods). Regular paper made of 27-μm native cellulose fibers with the same mass per area as the nanopaper is also fabricated as the control. The mechanical properties of both the cellulose nanopaper and regular paper are measured according to ASTM Standard D638 (details in SI Text).Open in a separate windowFig. 3.An anomalous scaling law of strength and toughness of cellulose nanopaper. (A) Schematic of cellulose nanopaper, made of a random network of CNF fibers. (Inset) High-resolution transmission electron microscopy (HRTEM) image of an ∼11-nm CNF fiber. (B) Stress–strain curves of cellulose paper made of cellulose fibers of various mean diameters. As the cellulose fiber diameter decreases from micrometer scale to nanometer scale, both tensile strength and ductility of the cellulose paper increases significantly, leading to an anomalous scaling law (C): the smaller, the stronger and the tougher. (D) Reveals that the ultimate tensile strength scales inversely with the square root of cellulose fiber diameter.Open in a separate windowFig. S2.(A) Optical microscope image of native cellulose fiber with a mean diameter of 27 μm. (B) Size distribution histogram. (C) AFM image of cellulose fibers with mean diameters of 28 nm. (D) Size distribution histogram. (E) HRTEM crystalline lattice image of fiber with a mean diameter of 11 nm. (F) Size distribution histogram.Open in a separate windowFig. S3.(A) A picture of a transparent cellulose nanopaper (made of CNF fibers of a mean diameter of 11 nm) on the university logo (Left). A schematic of fibrous nanostructure of the nanopaper is also shown (Right). (B) Optical transmittance of transparent cellulose nanopaper in visible and near-infrared range. (C) AFM image of cellulose nanopaper made of CNF fibers of a mean diameter of 28 nm. (D) AFM image and height scan of cellulose nanopaper made of CNF fibers of a mean diameter of 11 nm, showing rms at 1 × 1-μm scan size is 1.5 nm.     

One-step catalytic asymmetric synthesis of all-syn deoxypropionate motif from propylene: Total synthesis of (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoic acid

In nature, many complex structures are assembled from simple molecules by a series of tailored enzyme-catalyzed reactions. One representative example is the deoxypropionate motif, an alternately methylated alkyl chain containing multiple stereogenic centers, which is biosynthesized by a series of enzymatic reactions from simple building blocks. In organic synthesis, however, the majority of the reported routes require the syntheses of complex building blocks. Furthermore, multistep reactions with individual purifications are required at each elongation. Here we show the construction of the deoxypropionate structure from propylene in a single step to achieve a three-step synthesis of (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoic acid, a major acid component of a preen-gland wax of the graylag goose. To realize this strategy, we focused on the coordinative chain transfer polymerization and optimized the reaction condition to afford a stereo-controlled oligomer, which is contrastive to the other synthetic strategies developed to date that require 3–6 steps per unit, with unavoidable byproduct generation. Furthermore, multiple oligomers with different number of deoxypropionate units were isolated from one batch, showing application to the construction of library. Our strategy opens the door for facile synthetic routes toward other natural products that share the deoxypropionate motif.The deoxypropionate motif, an alternately methylated alkyl chain containing multiple stereogenic centers, is a common substructure found in natural products synthesized by bacteria, fungi, and plants (Fig. 1) (1). Because of the range of biological activities and abundance of this motif in natural products, its synthesis has received a great amount of attention (2, 3).Open in a separate windowFig. 1.Selected examples of natural products containing the deoxypropionate motif.In nature, the deoxypropionate motif is synthesized by using propionyl-CoA (or methylmalonyl-CoA) as a C3 building block (Fig. 2A). The deoxypropionate chain propagates by Claisen condensation of propionyl-CoA and acyl-CoA moiety at the chain end. After consecutive reduction of the β-ketone, dehydration, and asymmetric reduction of the carbon–carbon double bond, the deoxypropionate motif is elongated. We predicted that if the preparation of the deoxypropionate motif were possible by the asymmetric oligomerization of propylene, which is one of the simplest C3 building blocks, we could construct the analog of biosynthetic pathway in an even simplified manner (Fig. 2B).Open in a separate windowFig. 2.Synthesis of the deoxypropionate motif. (A) Biosynthetic scheme. (B) Synthesis by asymmetric oligomerization of propylene (current study). (C) Synthesis by iterative asymmetric carboalumination (7). (D) Synthesis by iterative asymmetric conjugate addition (11).To demonstrate our strategy, we chose (2R,4R,6R,8R)-2,4,6,8-tetramethyldecanoic acid 1 as a synthetic target. This carboxylic acid is a natural product containing the deoxypropionate motif, and is a major acid component of preen-gland wax of the graylag goose (4). Its total synthesis has been reported by two groups, both involving the oxidation of (2R,4R,6R,8R)-2,4,6,8-tetramethyldecan-1-ol 2. By using our strategy, this intermediate 2 can be constructed in a single step, significantly shortening the overall synthetic route.Conventionally, the deoxypropionate motif has been synthesized mainly using iterative reactions of complex building blocks or stoichiometric amount of organometallic reagents with unavoidable byproduct generation (e.g., inorganic salts) at each step. Previously reported synthetic routes include enolate alkylation (5, 6), carboalumination (7), organocuprate displacement (8), homologation of boronic esters (9), and conjugate addition (10). In addition, due to their iterative nature, long reaction sequences of 3–6 steps per unit were required to yield the desired products. As for the synthesis of 2, Liang et al. used an asymmetric carboalumination (Fig. 2C) (7), whereas ter Horst et al. used an asymmetric conjugate addition of methylcopper species (Fig. 2D) (11). Due to the iterative nature of methyl-branched chiral center formation, these syntheses required a total of 8–17 steps. Recently, convergent strategies for the synthesis of the deoxypropionate motif have been reported to shorten the synthetic route but generation of byproducts remains unavoidable (12, 13).Notably, propylene polymerization catalysts have rarely been used in the stereoselective oligomerization for synthesis of short oligomers, despite the great effort that has been devoted to the development of both homogeneous and heterogeneous catalysts for the highly isoselective propylene polymerization (14, 15). In 1987, Pino et al. reported an asymmetric propylene oligomerization catalyzed by enantiomerically pure chiral zirconocene in the presence of dihydrogen to afford saturated isotactic oligopropylenes (16). Kaminsky et al. later reported the preparation of moderately stereoregular oligomers with unsaturated chain end, which can be converted to other functional groups (17). To achieve natural product synthesis by the asymmetric oligomerization of propylene, a combination of stereoselectivity, functionalizability, and control over initiating groups is required. We herein report the one-step diastereoselective and enantioselective construction of the deoxypropionate motif by coordination chain transfer polymerization using an alkylmetal species as a chain-transfer agent (CTA) (18, 19). Our objective is to achieve highly stereoselective propylene oligomerization using this method. In addition, it is expected that the resulting oligomers will be end-capped with metals, thus enabling further functionalization.     

Taxodione and arenarone inhibit farnesyl diphosphate synthase by binding to the isopentenyl diphosphate site

We used in silico methods to screen a library of 1,013 compounds for possible binding to the allosteric site in farnesyl diphosphate synthase (FPPS). Two of the 50 predicted hits had activity against either human FPPS (HsFPPS) or Trypanosoma brucei FPPS (TbFPPS), the most active being the quinone methide celastrol (IC₅₀ versus TbFPPS ∼20 µM). Two rounds of similarity searching and activity testing then resulted in three leads that were active against HsFPPS with IC₅₀ values in the range of ∼1–3 µM (as compared with ∼0.5 µM for the bisphosphonate inhibitor, zoledronate). The three leads were the quinone methides taxodone and taxodione and the quinone arenarone, compounds with known antibacterial and/or antitumor activity. We then obtained X-ray crystal structures of HsFPPS with taxodione+zoledronate, arenarone+zoledronate, and taxodione alone. In the zoledronate-containing structures, taxodione and arenarone bound solely to the homoallylic (isopentenyl diphosphate, IPP) site, not to the allosteric site, whereas zoledronate bound via Mg²⁺ to the same site as seen in other bisphosphonate-containing structures. In the taxodione-alone structure, one taxodione bound to the same site as seen in the taxodione+zoledronate structure, but the second located to a more surface-exposed site. In differential scanning calorimetry experiments, taxodione and arenarone broadened the native-to-unfolded thermal transition (T_m), quite different to the large increases in ΔT_m seen with biphosphonate inhibitors. The results identify new classes of FPPS inhibitors, diterpenoids and sesquiterpenoids, that bind to the IPP site and may be of interest as anticancer and antiinfective drug leads.Farnesyl diphosphate synthase (FPPS) catalyzes the condensation of isopentenyl diphosphate (IPP; compound 1 in Fig. 1) with dimethylallyl diphosphate (DMAPP; compound 2 in Fig. 1) to form the C₁₀ isoprenoid geranyl diphosphate (GPP; compound 3 in Fig. 1), which then condenses with a second IPP to form the C₁₅ isoprenoid, farnesyl diphosphate (FPP; compound 4 in Fig. 1). FPP then is used in a wide range of reactions including the formation of geranylgeranyl diphosphate (GGPP) (1), squalene (involved in cholesterol and ergosterol biosynthesis), dehydrosqualene (used in formation of the Staphylococcus aureus virulence factor staphyloxanthin) (2), undecaprenyl diphosphate (used in bacterial cell wall biosynthesis), and quinone and in heme a/o biosynthesis. FPP and GGPP also are used in protein (e.g., Ras, Rho, Rac) prenylation, and FPPS is an important target for the bisphosphonate class of drugs (used to treat bone resorption diseases) such as zoledronate (compound 5 in Fig. 1) (3). Bisphosphonates targeting FPPS have activity as antiparasitics (4), act as immunomodulators (activating γδ T cells containing the Vγ2Vδ2 T-cell receptor) (5), and switch macrophages from an M2 (tumor-promoting) to an M1 (tumor-killing) phenotype (6). They also kill tumor cells (7) and inhibit angiogenesis (8). However, the bisphosphonates in clinical use (zoledronate, alendronate, risedronate, ibandronate, etidronate, and clodronate) are very hydrophilic and bind avidly to bone mineral (9). Therefore, there is interest in developing less hydrophilic species (10) that might have better activity against tumors in soft tissues and better antibacterial (11) and antiparasitic activity.Open in a separate windowFig. 1.Chemical structures of FPPS substrates, products, and inhibitors.The structure of FPPS (from chickens) was first reported by Tarshis et al. (12) and revealed a highly α-helical fold. The structures of bacterial and Homo sapiens FPPS (HsFPPS) are very similar; HsFPPS structure (13, 14) is shown in Fig. 2A. There are two substrate-binding sites, called here “S1” and “S2.” S1 is the allylic (DMAPP, GPP) binding site to which bisphosphonates such as zoledronate bind via a [Mg²⁺]₃ cluster (15) (Fig. 2B). S2 is the homoallylic site to which IPP binds, Fig. 2B. Recently, Jahnke et al. (10) and Salcius et al. (16) discovered a third ligand-binding site called the “allosteric site” (hereafter the “A site”). A representative zoledronate+A-site inhibitor structure [Protein Data Bank (PDB) ID code 3N46] (Nov_980; compound 6 in Fig. 1) showing zoledronate in S1 and Nov_980 (compound 6) in the A site is shown in a stereo close-up view in Fig. 2B, superimposed on a zoledronate+IPP structure (PDB ID code 2F8Z) in S2. Whether the allosteric site serves a biological function (e.g., in feedback regulation) has not been reported. Nevertheless, highly potent inhibitors (IC₅₀ ∼80 nM) have been developed (10), and the best of these newly developed inhibitors are far more hydrophobic than are typical bisphosphonates (∼2.4–3.3 for cLogP vs. ∼−3.3 for zoledronate) and are expected to have better direct antitumor effects in soft tissues (10).Open in a separate windowFig. 2.Structures of human FPPS. (A) Structure of HsFPPS showing zoledronate (compound 5) and IPP (compound 1) bound to the S1 (allylic) and S2 (homoallylic) ligand-binding sites (PDB ID code 2F8Z). (B) Superposition of the IPP-zoledronate structure (PDB ID code 2F8Z) on the zoledronate-Nov_980 A-site inhibitor structure (PDB ID code 3N46). Zoledronate binds to the allylic site S1, IPP binds to the homoallylic site S2, and the allosteric site inhibitor binds to the A site. Active-site “DDXXD” residues are indicated, as are Mg²⁺ molecules (green and yellow spheres, respectively). The views are in stereo.In our group we also have developed more lipophilic compounds (e.g., compound 7 in Fig. 1) (17, 18) as antiparasitic (19) and anticancer drug leads (18) and, using computational methods, have discovered other novel nonbisphosphonate FPPS inhibitors (e.g., compound 8 in Fig. 1) that have micromolar activity against FPPS (20). In this study, we extended our computational work and tried to discover other FPPS inhibitors that target the A site. Such compounds would be of interest because they might potentiate the effects of zoledronate and other bisphosphonates, as reported for other FPPS inhibitors (21), and have better tissue distribution properties in general.