Electrochemical borylation of carboxylic acids

A simple electrochemically mediated method for the conversion of alkyl carboxylic acids to their borylated congeners is presented. This protocol features an undivided cell setup with inexpensive carbon-based electrodes and exhibits a broad substrate scope and scalability in both flow and batch reactors. The use of this method in challenging contexts is exemplified with a modular formal synthesis of jawsamycin, a natural product harboring five cyclopropane rings.

Boronic acids are among the most malleable functional groups in organic chemistry as they can be converted into almost any other functionality (1–3). Aside from these versatile interconversions, their use in the pharmaceutical industry is gaining traction, resulting in approved drugs such as Velcade, Ninlaro, and Vabomere (4). It has been shown that boronic acids can be rapidly installed from simple alkyl halides (5–19) or alkyl carboxylic acids through the intermediacy of redox-active esters (RAEs) (Fig. 1A) (20–24). Our laboratory has shown that both Ni (20) and Cu (21) can facilitate this reaction. Conversely, Aggarwal and coworkers (22) and Li and coworkers (23) demonstrated photochemical variations of the same transformation. While these state-of-the-art approaches provide complementary access to alkyl boronic acids, each one poses certain challenges. For example, the requirement of excess boron source and pyrophoric MeLi under Ni catalysis is not ideal. Although more cost-effective and operationally simple, Cu-catalyzed borylation conditions can be challenging on scale due to the heterogeneity resulting from the large excess of LiOH•H₂O required. In addition to its limited scope, Li and coworkers’ protocol requires 4 equivalence of B₂pin₂ and an expensive Ir photocatalyst. The simplicity of Aggarwal and coworkers’ approach is appealing in this regard and represents an important precedent for the current study.Open in a separate windowFig. 1.(A) Prior approaches to access alkyl boronic esters from activated acids. (B) Inspiration for initiating SET events electrochemically to achieve borylation. (C) Summary of this work.At the heart of each method described above, the underlying mechanism relies on a single electron transfer (SET) event to promote decarboxylation and form an alkyl radical species. In parallel, the related borylation of aryl halides via a highly reactive aryl radical can also be promoted by SET. While numerous methods have demonstrated that light can trigger this mechanism (Fig. 1B) (16, 25–31), simple electrochemical cathodic reduction can elicit the same outcome (32–35). It was postulated that similar electrochemically driven reactivity could be translated to alkyl RAEs. The development of such a transformation would be highly enabling, as synthetic organic electrochemistry allows the direct addition or removal of electrons to a reaction, representing an incredibly efficient way to forge new bonds (36–40). This disclosure reports a mild, scalable, and operationally simple electrochemical decarboxylative borylation (Fig. 1C) not reliant on transition metals or stoichiometric reductants. In addition to mechanistic studies of this interesting transformation, applications to a variety of alkyl RAEs, comparison to known decarboxylative borylation methods, and a formal synthesis of the polycyclopropane natural product jawsamycin [(–)-FR-900848] are presented.     

Abrupt increases in Amazonian tree mortality due to drought–fire interactions

Interactions between climate and land-use change may drive widespread degradation of Amazonian forests. High-intensity fires associated with extreme weather events could accelerate this degradation by abruptly increasing tree mortality, but this process remains poorly understood. Here we present, to our knowledge, the first field-based evidence of a tipping point in Amazon forests due to altered fire regimes. Based on results of a large-scale, long-term experiment with annual and triennial burn regimes (B1yr and B3yr, respectively) in the Amazon, we found abrupt increases in fire-induced tree mortality (226 and 462%) during a severe drought event, when fuel loads and air temperatures were substantially higher and relative humidity was lower than long-term averages. This threshold mortality response had a cascading effect, causing sharp declines in canopy cover (23 and 31%) and aboveground live biomass (12 and 30%) and favoring widespread invasion by flammable grasses across the forest edge area (80 and 63%), where fires were most intense (e.g., 220 and 820 kW⋅m⁻¹). During the droughts of 2007 and 2010, regional forest fires burned 12 and 5% of southeastern Amazon forests, respectively, compared with <1% in nondrought years. These results show that a few extreme drought events, coupled with forest fragmentation and anthropogenic ignition sources, are already causing widespread fire-induced tree mortality and forest degradation across southeastern Amazon forests. Future projections of vegetation responses to climate change across drier portions of the Amazon require more than simulation of global climate forcing alone and must also include interactions of extreme weather events, fire, and land-use change.Large areas of moist tropical forests are being altered by land-use practices and severe weather. People are clearing, thinning, and changing the composition of tropical forests (1, 2). Severe drought events superimposed upon these land-use activities increase forest susceptibility to fires (1–5). In the 2000s, for example, 15,000–26,000 km² of Amazonian forests burned during years of severe drought (6). Widespread forest fires may become even more common in the Amazon Basin if the frequency of extreme weather events increases, particularly in the southeastern Amazon (1, 7). However, most model simulations of future trajectories of Amazonian forests have relied on global and regional climate forcing that do not consider the effects of fire on vegetation dynamics and structure (8–10).Our ability to predict future fire regimes in moist tropical forests is constrained by a lack of understanding of what triggers and controls high-intensity fires (7, 11). In nondrought years, primary forests typically do not catch fire during the dry season because the fine fuel layer is too humid to carry a fire (12). This characteristic of primary forests helps explain why forest fires were less frequent in pre-Colombian times than today (13), although indigenous peoples of the Amazon have used fire as a management tool for hundreds or thousands of years (14). Current anthropogenic disturbances in moist tropical forests (e.g., logging, forest conversion for crops and livestock, and the resulting fragmentation of forests) tend to thin forest canopies (5, 11) and expose forest interiors to warm air flowing horizontally from neighboring clearings, allowing the forest floor to dry more rapidly during rainless periods. When forest fires do occur under average weather conditions, they typically move through the understories slowly (15–25 m⋅hour⁻¹), release little energy (50 kW⋅m⁻¹), and are of short duration (4, 5, 15), extinguishing at night when relative humidity increases. Despite their low intensity, understory fires still exert strong influences on forest dynamics and structure because many tropical tree species are thin-barked and vulnerable to fire damage (12, 16, 17).During years of severe drought, Amazon forest fires are atypically intense, killing up to 64% of the trees (18, 19). This happens because fuel (e.g., twigs, leaves, branches, etc.) not only becomes drier, but also tends to become more abundant due to drought-related leaf and branch fall (20). Thus, compared with low-intensity fires that occur in nondrought years, severe droughts can trigger high-intensity fires that kill more trees. Unfortunately, the role of extreme weather events in the fire dynamics of moist tropical forests is difficult to study because they are hard to predict. As a result, the relationships between fire-induced tree mortality and extreme weather remain poorly understood, restricted mostly to postfire observations of tree mortality.To fill this gap, in 2004 we established a large-scale, long-term prescribed forest fire experiment in a transitional forest (between Amazon forests and savannas) in the southeastern Amazon (Figs. 1 and ​and2)—a2)—a region that is highly vulnerable to changes in fire regime, climate change, and their interactions (2). The experimental area consists of three adjacent 50-ha (1.0 × 0.5 km; Fig. 1) plots burned annually (B1yr), every 3 y (B3yr), or not at all (control) to represent a range of possible future forest fire frequencies (details in ref. 21). We used within-plot variability between the forest edge (0–100 m into the forest from the adjacent agricultural area) (Fig. 1) and forest interior (100–1,000 m) and the temporal variability in weather between 2004 and 2010 to address two questions: (i) Are there weather- and fuel-related thresholds in fire behavior that are associated with high levels of fire-induced tree mortality across two different fire regimes? (ii) What are the effects of an intense fire event on forest structure, flammability, and aboveground live carbon stocks? We also conducted a regional analysis of weather and fire scars to assess the spatial-temporal dynamics of forest fires in the 87,000 km² of remaining forests in the Upper Xingu River Basin (Fig. 1).Open in a separate windowFig. 1.High-resolution image (i.e., 1.85 m) of the experimental area in 2011 captured with the sensor Worldview-2. The dashed line represents the border between the North–South forest edge (0–100 m) and the forest interior (100–1,000 m). The North–South edge of the plots is bordered by a road and open agricultural fields, and the other plot boundaries are in contiguous forest. The control represents an unburned area, and B1yr and B3yr areas that were burned annually and every 3 y, respectively, from 2004 to 2010 (with the exception of 2008).Open in a separate windowFig. 2.(Left) Annual MCWD between 2000 and 2010 for the Upper Xingu Basin (solid circles) and the experimental field site (Fazenda Tanguro, solid triangles). The shaded area represents the SD of the mean and accounts for the spatial variability in MCWD across the Upper Xingu Basin. (Right) Average dry-season length (i.e., number of months with precipitation ≤100 mm) and the locations of both the Upper Xingu Basin (in gray) and the fire experiment (triangle). MCWD and monthly precipitation were derived from the TRMM.     

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet–visible (UV–Vis) absorption bands. Bulky ligands on the metal suppress π–π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV–Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

Conjugated polymers are macromolecules usually featuring a backbone chain with alternating double and single bonds (1–3). These characteristics allow the overlapping p-orbitals to form a system with highly delocalized π-electrons, thereby giving rise to intriguing chemical and physical properties (4–6). They have exhibited many applications in organic light-emitting diodes, organic thin film transistors, organic photovoltaic cells, chemical sensors, bioimaging and therapies, photocatalysis, and other technologies (7–10). To facilitate the use of solar energy, tremendous efforts have been devoted in recent decades to developing previously unidentified conjugated polymers exhibiting broad and strong absorption bands (11–13). The common strategies for increasing absorption involve extending π-conjugation by incorporating conjugated cyclic moieties, especially fused rings; modulating the strength of intramolecular charge transfer between donor and acceptor units (D–A effect); increasing the coplanarity of π conjugation through weak intramolecular interactions (e.g., hydrogen bonds); and introducing heteroatoms or heavy atoms into the repeat units of conjugated polymers (11–16). Additionally, appropriate solubility is a prerequisite for processing and using polymers and is usually achieved with the aid of long alkyl or alkoxy side chains (12, 17).Aromatic rings are among the most important building blocks for conjugated polymers. In addition to aromatic hydrocarbons, a variety of aromatic heterocycles composed of main-group elements have been used as fundamental components. These heteroatom-containing conjugated polymers show unique optical and electronic properties (4–10). However, while metalla-aromatic systems bearing a transition metal have been known since 1979 due to the pioneering work by Thorn and Hoffmann (18), none of them have been used as building blocks for conjugated polymers. The HOMO–LUMO gaps (E_g) of metalla-aromatics are generally narrower (Fig. 1) than those of their organic counterparts (19–22). We reasoned that this feature should broaden the absorption window if polymers stemming from metalla-aromatics are achievable.Open in a separate windowFig. 1.Comparison of traditional organic skeletons with metalla-aromatic building blocks (the computed energies are in eV). (A) HOMO–LUMO gaps of classic aromatic skeletons. (B) Carbolong frameworks as potential building blocks for novel conjugated polymers with broad absorption bands and improved solubility.In recent years, we have reported a series of readily accessible metal-bridged bicyclic/polycyclic aromatics, namely carbolong complexes, which are stable in air and moisture (23–25). The addition of osmium carbynes (in carbolong complexes) and alkynes gave rise to an intriguing family of d_π–p_π conjugated systems, which function as excellent electron transport layer materials in organic solar cells (26, 27). These observations raised the following question: Can this efficient addition reaction be used to access metalla-aromatic conjugated polymers? It is noteworthy that incorporation of metalla-aromatic units into conjugated polymers is hitherto unknown. In this contribution, we disclose a polymerization reaction involving M≡C analogs of C≡C bonds, which involves a unique carbyne shuttling strategy (Fig. 2A). This led to examples of metalla-aromatic conjugated polymers (polycarbolongs) featuring metal carbyne units in the main chain. On the other hand, the development of polymerization reactions plays a crucial role in involving certain building blocks in conjugated polymers (28–32). These efficient, specific, and feasible polymerizations could open an avenue for the synthesis of conjugated polymers.Open in a separate windowFig. 2.Design of polymers and synthesis of monomers. (A) Schematic illustration of the polymerization strategy. (B) Preparation of carbolong monomers. Insert: X-ray molecular structure for the cations of complex 3. Ellipsoids are shown at the 50% probability level; phenyl groups in PPh₃ are omitted for clarity.     

Electrophilic aromatic substitution reactions of compounds with Craig-Möbius aromaticity

Electrophilic aromatic substitution (EAS) reactions are widely regarded as characteristic reactions of aromatic species, but no comparable reaction has been reported for molecules with Craig-Möbius aromaticity. Here, we demonstrate successful EAS reactions of Craig-Möbius aromatics, osmapentalenes, and fused osmapentalenes. The highly reactive nature of osmapentalene makes it susceptible to electrophilic attack by halogens, thus osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene can undergo typical EAS reactions. In addition, the selective formation of a series of halogen substituted metalla-aromatics via EAS reactions has revealed an unprecedented approach to otherwise elusive compounds such as the unsaturated cyclic chlorirenium ions. Density functional theory calculations were conducted to study the electronic effect on the regioselectivity of the EAS reactions.

Aromaticity, a core concept in chemistry, was initially introduced to account for the bonding, stability, reactivity, and other properties of many unsaturated organic compounds. There have been many elaborations and extensions of the concept of aromaticity (1, 2). The concepts of Hückel aromaticity and Möbius aromaticity are widely accepted (Fig. 1A). A π-aromatic molecule of the Hückel type is planar and has 4n + 2 conjugated π-electrons (n = 0 or an integer), whereas a Möbius aromatic molecule has one twist of the π-system, similar to that in a Möbius strip, and 4n π-electrons (3, 4). Since the discovery of naphthalene in 1821, aromatic chemistry has developed into a rich field and with a variety of subdisciplines over the course of its 200-y history, and the concept of aromaticity has been extended to other nontraditional structures with “cyclic delocalization of mobile electrons” (5). For example, benzene-like metallacycles—predicted by Hoffmann et al. as metallabenzenes—in which a metal replaces a C–H group in the benzene ring (6), have garnered extensive research interest from both experimentalists and theoreticians (7–12). As paradigms of the metalla-aromatic family, most complexes involving metallabenzene exhibit thermodynamic stability, kinetic persistence, and chemical reactivity associated with the classical aromaticity concept (13–15). Typically, like benzene, metallabenzene can undergo characteristic reactions of aromatics such as electrophilic aromatic substitution (EAS) reactions (16–18) (Fig. 1B, I) and nucleophilic aromatic substitution reactions (19–21).Open in a separate windowFig. 1.Schematic representations of aromaticity classification (A) and EAS reactions (B) of benzene, metallabenzene, and polycyclic metallacycles with Craig-Möbius aromaticity.The incorporation of transition metals has also led to an increase in the variety of the aromatic families (22–25). We have reported that stable and highly unusual bicyclic systems, metallapentalenes (osmapentalenes), benefit from Craig-Möbius aromaticity (26–30). In contrast to other reported Möbius aromatic compounds with twisted topologies, which are known as Heilbronner-Möbius aromatics (31–34), the involvement of transition metal d orbitals in π-conjugation switches the Hückel anti-aromaticity of pentalene into the planar Craig-Möbius aromaticity of metallapentalene (35–38) (Fig. 1A, III). Both the twisted topology and the planar Craig-Möbius aromaticity are well established and have been accepted as reasonable extensions of aromaticity (39–43). There has been no experimental evidence, however, as to whether these Möbius aromatic molecules can undergo classical aromatic substitution reactions, such as EAS reactions, instead of addition reactions. Given the key role of EAS in aromatic chemistry to obtain various derivatives, we sought to extend the understanding of the reactivity paradigm in the metalla-aromatic family.Our recent synthetic efforts associated with the metallapentalene system prompted us to investigate whether typical EAS reactions could proceed in these Craig-Möbius aromatics. If so, how could substitution be achieved in the same way that it is with traditional Hückel aromatics such as benzenes? In this paper, we present EAS reactions, mainly the halogenation of osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene, which follow the classic EAS mechanistic scheme (Fig. 1B). With the aid of density functional theory (DFT) calculations, we characterized the effects on EAS reactivity and regioselectivity.     

Chlorination of arenes via the degradation of toxic chlorophenols

Aryl chlorides are among the most versatile synthetic precursors, and yet inexpensive and benign chlorination techniques to produce them are underdeveloped. We propose a process to generate aryl chlorides by chloro-group transfer from chlorophenol pollutants to arenes during their mineralization, catalyzed by Cu(NO₃)₂/NaNO₃ under aerobic conditions. A wide range of arene substrates have been chlorinated using this process. Mechanistic studies show that the Cu catalyst acts in cooperation with NO_x species generated from the decomposition of NaNO₃ to regulate the formation of chlorine radicals that mediate the chlorination of arenes together with the mineralization of chlorophenol. The selective formation of aryl chlorides with the concomitant degradation of toxic chlorophenol pollutants represents a new approach in environmental pollutant detoxication. A reduction in the use of traditional chlorination reagents provides another (indirect) benefit of this procedure.

Chlorophenols are widely encountered moieties present in herbicides, drugs, and pesticides (1). Owing to the high dissociation energies of carbon‒chloride bonds, chlorophenols biodegrade very slowly, and their prolonged exposure leads to severe ecological and environmental problems (Fig. 1A) (2–4). Several well-established technologies have been developed for the treating of chlorophenols, including catalytic oxidation (5–11), biodegradation (12–15), solvent extraction (16, 17), and adsorption (18–20) Among these methods, adsorption is the most versatile and widely used method due to its high removal efficiency and simple operation, but the resulting products are of no value, and consequently, these processes are not viable.Open in a separate windowFig. 1.Background and reaction design. (A) Examples of chlorophenol pollutants. (B) Examples of aryl chlorides. (C) The chlorination process reported herein was based on chloro-group transfer from chlorophenol pollutants.With the extensive application of substitution reactions (21, 22), transfunctionalizations (23, 24), and cross-coupling reactions (25, 26), aryl chlorides are regarded as one of the most important building blocks widely used in the manufacture of polymers, pharmaceuticals, and other types of chemicals and materials (Fig. 1B) (27–31). Chlorination of arenes is usually carried out with toxic and corrosive reagents (32–34). Less toxic and more selective chlorination reagents tend to be expensive [e.g., chloroamides (35, 36)] and are not atom economic (37–39). Consequently, from the perspective of sustainability, the ability to transfer a chloro group from unwanted chlorophenols to other substrates would be advantageous.Catalytic isofunctional reactions, including transfer hydrogenation and alkene metathesis, have been widely exploited in organic synthesis. We hypothesized that chlorination of arenes also could be achieved by chloro-group transfer, and since stockpiles of chlorophenols tend to be destroyed by mineralization and chlorophenol pollutants may be concentrated by adsorption (18–20), they could be valorized as chlorination reagents via transfer of the chloro group to arene substrates during their mineralization, thereby adding value to the destruction process (Fig. 1C). Although chlorophenol pollutants are not benign, their application as chlorination reagents, with their concomitant destruction to harmless compounds, may be considered as not only meeting the criteria of green chemistry but also potentially surpassing it. Herein, we describe a robust strategy to realize chloro-group transfer from chlorophenol pollutants to arenes and afford a wide range of value-added aryl chlorides.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Chirality-matched catalyst-controlled macrocyclization reactions

Macrocycles, formally defined as compounds that contain a ring with 12 or more atoms, continue to attract great interest due to their important applications in physical, pharmacological, and environmental sciences. In syntheses of macrocyclic compounds, promoting intramolecular over intermolecular reactions in the ring-closing step is often a key challenge. Furthermore, syntheses of macrocycles with stereogenic elements confer an additional challenge, while access to such macrocycles are of great interest. Herein, we report the remarkable effect peptide-based catalysts can have in promoting efficient macrocyclization reactions. We show that the chirality of the catalyst is essential for promoting favorable, matched transition-state relationships that favor macrocyclization of substrates with preexisting stereogenic elements; curiously, the chirality of the catalyst is essential for successful reactions, even though no new static (i.e., not “dynamic”) stereogenic elements are created. Control experiments involving either achiral variants of the catalyst or the enantiomeric form of the catalyst fail to deliver the macrocycles in significant quantity in head-to-head comparisons. The generality of the phenomenon, demonstrated here with a number of substrates, stimulates analogies to enzymatic catalysts that produce naturally occurring macrocycles, presumably through related, catalyst-defined peripheral interactions with their acyclic substrates.

Macrocyclic compounds are known to perform a myriad of functions in the physical and biological sciences. From cyclodextrins that mediate analyte separations (1) to porphyrin cofactors that sit in enzyme active sites (2, 3) and to potent biologically active, macrocyclic natural products (4) and synthetic variants (5–7), these structures underpin a wide variety of molecular functions (Fig. 1A). In drug development, such compounds are highly coveted, as their conformationally restricted structures can lead to higher affinity for the desired target and often confer additional metabolic stability (8–13). Accordingly, there exists an entire synthetic chemistry enterprise focused on efficient formation and functionalization of macrocycles (14–18).Open in a separate windowFig. 1.(A) Examples of macrocyclic compounds with important applications. HCV, hepatitis C virus. (B) Use of chiral ligands in metal-catalyzed or mediated stereoselective macrocyclization reactions. (C) Remote desymmetrization using guanidinylated ligands via Ullmann coupling. (D) This work: use of copper/peptidyl complexes for macrocyclization and the exploration of matched and mismatched effect.In syntheses of macrocyclic compounds, the ring-closing step is often considered the most challenging step, as competing di- and oligomerization pathways must be overcome to favor the intramolecular reaction (14). High-dilution conditions are commonly employed to favor macrocyclization of linear precursors (19). Substrate preorganization can also play a key role in overcoming otherwise high entropic barriers associated with multiple conformational states that are not suited for ring formation. Such preorganization is most often achieved in synthetic chemistry through substrate design (14, 20–22). Catalyst or reagent controls that impose conformational benefits that favor ring formation are less well known. Yet, critical precedents include templating through metal-substrate complexation (23, 24), catalysis by foldamers (25) or enzymes (26–29), or, in rare instances, by small molecules (discussed below). Characterization of biosynthetic macrocyclization also points to related mechanistic issues and attributes for efficient macrocyclizations (30–34). Coupling macrocyclization reactions to the creation of stereogenic elements is also rare (35). Metal-mediated reactions have been applied toward stereoselective macrocyclizations wherein chiral ligands transmit stereochemical information to the products (Fig. 1B). For example, atroposelective ring closure via Heck coupling has been applied in the asymmetric total synthesis of isoplagiochin D by Speicher and coworkers (36–40). Similarly, atroposelective syntheses of (+)-galeon and other diarylether heptanoid natural products were achieved via Ullman coupling using N-methyl proline by Salih and Beaudry (41). Finally, Reddy and Corey reported the enantioselective syntheses of cyclic terpenes by In-catalyzed allylation utilizing a chiral prolinol-based ligand (42). While these examples collectively illustrate the utility of chiral ligands in stereoselective macrocyclizations, such examples remain limited.We envisioned a different role for chiral catalysts when addressing intrinsically disfavored macrocyclization reactions. When unfavorable macrocyclization reactions are confronted, we hypothesized that a catalyst–substrate interaction might provide transient conformational restriction that could promote macrocyclization. To address this question, we chose to explore whether or not a chiral catalyst-controlled macrocyclization might be possible with peptidyl copper complexes. In the context of the medicinally ubiquitous diarylmethane scaffold, we had previously demonstrated the capacity for remote asymmetric induction in a series of bimolecular desymmetrizations using bifunctional, tetramethylguanidinylated peptide ligands. For example, we showed that peptidyl copper complexes were able to differentiate between the two aryl bromides during C–C, C–O, and C–N cross-coupling reactions (Fig. 1C) (43–45). Moreover, in these intermolecular desymmetrizations, a correlation between enantioselectivity and conversion was observed, revealing the catalyst’s ability to perform not only enantiotopic group discrimination but also kinetic resolution on the monocoupled product as the reaction proceeds (44). This latter observation stimulated our speculation that if an internal nucleophile were present to undergo intramolecular cross-coupling to form a macrocycle, stereochemically sensitive interactions (so-called matched and mismatched effects) (46) could be observed (Fig. 1D). Ideally, we anticipated that transition state–stabilizing interactions might even prove decisive in matched cases, and the absence of catalyst–substrate stabilizing interactions might account for the absence of macrocyclization for these otherwise intrinsically unfavorable reactions. Herein, we disclose the explicit observation of these effects in chiral catalyst-controlled macrocyclization reactions.     

From the Cover: 1.36 million years of Mediterranean forest refugium dynamics in response to glacial–interglacial cycle strength

The sediment record from Lake Ohrid (Southwestern Balkans) represents the longest continuous lake archive in Europe, extending back to 1.36 Ma. We reconstruct the vegetation history based on pollen analysis of the DEEP core to reveal changes in vegetation cover and forest diversity during glacial–interglacial (G–IG) cycles and early basin development. The earliest lake phase saw a significantly different composition rich in relict tree taxa and few herbs. Subsequent establishment of a permanent steppic herb association around 1.2 Ma implies a threshold response to changes in moisture availability and temperature and gradual adjustment of the basin morphology. A change in the character of G–IG cycles during the Early–Middle Pleistocene Transition is reflected in the record by reorganization of the vegetation from obliquity- to eccentricity-paced cycles. Based on a quantitative analysis of tree taxa richness, the first large-scale decline in tree diversity occurred around 0.94 Ma. Subsequent variations in tree richness were largely driven by the amplitude and duration of G–IG cycles. Significant tree richness declines occurred in periods with abundant dry herb associations, pointing to aridity affecting tree population survival. Assessment of long-term legacy effects between global climate and regional vegetation change reveals a significant influence of cool interglacial conditions on subsequent glacial vegetation composition and diversity. This effect is contrary to observations at high latitudes, where glacial intensity is known to control subsequent interglacial vegetation, and the evidence demonstrates that the Lake Ohrid catchment functioned as a refugium for both thermophilous and temperate tree species.

Identification and protection of past forest refugia, supporting a relict population, has gained interest in light of projected forest responses to anthropogenic climate change (1–4). Understanding the past and present composition of Mediterranean forest refugia is central to the study of long-term survival of tree taxa and the systematic relation between forest dynamics and climate (5). The Quaternary vegetation history of Europe, studied for over a century, is characterized by successive loss of tree species (6–8). Species loss was originally explained by the repeated migration across east–west oriented mountain chains during glacial–interglacial (G–IG) cycles (9). Later views gave more importance to the survival of tree populations during warm and arid stages in southern refugia (10, 11). Tree survival likely depends on persistence of suitable climate and tolerable levels of climate variability, as well as niche differentiation and population size at the refugium (12), although the precise relation between regional extinctions, climate variability, and local edaphic factors is not well known (13). Mediterranean mountain regions are considered to serve as forest refugia over multiple glacial cycles and frequently coincide with present-day biodiversity hotspots (14). Across the Mediterranean, increases in aridity and fire occurrence have impacted past vegetation communities (15–18). Comprehensive review of available Quaternary Mediterranean records indicates that Early (2.58 to 0.77 Ma) and Middle Pleistocene (0.77 to 0.129 Ma) tree diversity was higher compared to the present (13, 19–21). Particularly drought intolerant, thermophilic taxa were more abundant and diverse (8) but with strong spatial and temporal variations in tree diversity across the region. Long-term relationships between refugia function and environmental change over multiple G–IG cycles are hard to quantify due to the rarity of long, uninterrupted records.The Early–Middle Pleistocene Transition (EMPT), between 1.4 and 0.4 Ma (22), is of particular importance for understanding the relation between past climate change, vegetation dynamics, and biodiversity in the Mediterranean region. The EMPT is characterized by a gradual transition of G–IG cycle duration from obliquity (41 thousand years; kyr) to eccentricity (100 kyr) scale with increasing amplitude of each G–IG cycle (e.g., refs. 23, 24). The EMPT was accompanied by long-term cooling of the deep and surface ocean and was likely caused by atmospheric CO₂ decline and ice-sheet feedbacks (25–30). In Europe, the EMPT is associated with pronounced vegetation changes and local extinction and isolation of small tree populations (31).Here, we document vegetation history of the last 1.36 Ma in the Lake Ohrid (LO) catchment, located at the Albanian/North Macedonian border at 693 m above sea level (m asl, Fig. 1), the longest continuous sedimentary lake record in Europe (32, 33). The chronology of the DEEP core (International Continental Scientific Drilling Program site 5045-1; 41°02’57’’ N, 20°42’54’’ E, Fig. 1) is based on tuning of biogeochemical proxy data to orbital parameters with independent tephrostratigraphic and paleomagnetic age control (32, 33). The Balkan Peninsula has long been considered an important glacial forest refugium for presently widespread taxa such as Abies, Picea, Carpinus, Corylus, Fagus Ostrya, Quercus, Tilia, and Ulmus (7, 34–36). More than 60% of the Balkans is currently located >1,000 m asl (36), providing steep latitudinal and elevational gradients to support refugia under both cold and warm conditions. Today, the LO catchment is dominated by (semi) deciduous oak (Quercus) and hornbeam (Carpinus/Ostrya) forests. Above 1,250 m elevation, mixed mesophyllous forest with montane elements occurs (Fagus and at higher elevations Abies), which above 1,800 m elevation develops into subalpine grassland with Juniperus shrubs (see ref. 37 for site details). Isolated populations of Pinus peuce and Pinus nigra currently grow in the area (37–40).Open in a separate windowFig. 1.(A) Location of LO and TP on the Balkan Peninsula. (B) Local setting around LO, bathymetry (81), and DEEP coring site (adapted from ref. 32).Previous analysis of pollen composition of the last 500 kyr at the DEEP site revealed that the LO has been an important refugium. Arboreal pollen (AP) is deposited continuously and changes in abundance on multimillennial timescales in association with G–IG cycles, whereas millennial-scale variability is tightly coupled to Mediterranean sea-surface temperature variations (37, 41–45). Subsequent studies confirm the refugial character of the site recording Early Pleistocene (1.365 to 1.165 Ma) high relict tree diversity and abundance—and significant hydrological changes, including an increase in lake size and depth (38). Here, we present a continuous palynological record from LO with millennial resolution (∼2 kyr) back to 1.36 Ma to assess the systematic relationships between tree pollen abundance, forest diversity, and G–IG climate variability.Our objective is as follows: 1) infer the impact of past climate variability on local vegetation across the EMPT, 2) estimate tree species diversity in the catchment, and 3) examine how the amplitude and duration of preceding G–IG intervals affected the vegetation development and plant species diversity in this refugial area.     

Diffusion of a disordered protein on its folded ligand

Intrinsically disordered proteins often form dynamic complexes with their ligands. Yet, the speed and amplitude of these motions are hidden in classical binding kinetics. Here, we directly measure the dynamics in an exceptionally mobile, high-affinity complex. We show that the disordered tail of the cell adhesion protein E-cadherin dynamically samples a large surface area of the protooncogene β-catenin. Single-molecule experiments and molecular simulations resolve these motions with high resolution in space and time. Contacts break and form within hundreds of microseconds without a dissociation of the complex. The energy landscape of this complex is rugged with many small barriers (3 to 4 k_BT) and reconciles specificity, high affinity, and extreme disorder. A few persistent contacts provide specificity, whereas unspecific interactions boost affinity.

Specific molecular interactions orchestrate a multitude of simultaneous cellular processes. The discovery of intrinsically disordered proteins (IDPs) (1, 2) has substantially aided our understanding of such interactions. More than two decades of research revealed a plethora of functions and mechanisms (2–6) that complemented the prevalent structure-based view on protein interactions. Even the idea that IDPs always ought to fold upon binding has largely been dismantled by recent discoveries of high-affine–disordered complexes (7, 8). Classical shape complementary is indeed superfluous in the complex between prothymosin-α and histone H1, in which charge complementary is the main driving force for binding (7). However, complexes between IDPs and folded proteins can also be highly dynamic [e.g., Sic1 and Cdc4 (9), the Na⁺/H⁺ exchanger tail and ERK2 (10), nucleoporin tails, and nuclear transport receptors (11)]. Yet timescales of motions and their spatial amplitudes are often elusive, such that it is unclear how precisely the surfaces of folded proteins alter the dynamics of bound IDPs. Answering this question is a key step in understanding how specificity, affinity, and flexibility can be simultaneously realized in such complexes.To address this question, we focused on the dynamics of the cell adhesion complex between E-cadherin (E-cad) and β-catenin (β-cat), which is involved in growth pathologies and cancer (12). E-cad is a transmembrane protein that mediates cell–cell adhesions by linking actin filaments of adjacent epithelial cells (Fig. 1A). Previous NMR results showed that the cytoplasmic tail of E-cad is intrinsically disordered (13). E-cad binds β-cat, which establishes a connection to the actin-associated protein α-catenin (14–16). β-cat, on the other hand, is a multifunctional repeat protein (17–20) that mediates cadherin-based cell adhesions (21) and governs cell fate decisions during embryogenesis (22). It contains three domains: an N-terminal domain (130 amino acids [aa]), a central repeat domain (550 aa), and a C-terminal domain (100 aa). Whereas the N- and C-terminal domains of β-cat are in large parts unstructured (17), with little effect on the affinity of the E-cad/β-cat complex (23), the 12 repeats of the central domain arrange in a superhelix (24). The X-ray structure showed that the E-cad wraps around this central domain of β-cat (24) (Fig. 1B). However, not only is half of the electron density of E-cad missing, the X-ray unit cell also comprises two structures with different resolved parts of E-cad (Fig. 1B). In fact, only 45% of all resolved E-cad residues are found in both structures (Fig. 1C). Although this ambiguity together with the large portion of missing residues (25) suggests that E-cad is highly dynamic in the complex with β-cat, the timescales and amplitudes of these dynamics are unknown.Open in a separate windowFig. 1.Complex between the cytoplasmic tail of E-cad and β-cat. (A) Schematics of cell–cell junctions mediated by E-cad and β-cat. (B) The two X-ray structures of the complex between the tail of E-cad (red) and the central repeat domain of β-cat (white) resolve different parts of E-cad (Protein Data Bank: 1i7x), indicating the flexibility of E-cad in the complex. (Bottom) Cartoon representation of the resolved E-cad parts. (C) Scheme showing the resolved parts of E-cad (red).Here, we integrated single-molecule Förster resonance energy transfer (smFRET) experiments with molecular simulations to directly measure the dynamics of E-cad on β-cat with high spatial and temporal resolution. In our bottom-up strategy, we first probed intramolecular interactions within E-cad using smFRET to parameterize a coarse-grained (CG) model. In a second step, we monitored E-cad on β-cat, integrated this information into the CG model, and obtained a dynamic picture of the complex. We found that all segments of E-cad diffuse on the surface of β-cat at submillisecond timescales and obtained a residue-resolved understanding of these motions: A small number of persistent interactions provide specificity, whereas many weak multivalent contacts boost affinity, which confirms the idea that regulatory enzymes access their recognition motifs on E-cad and β-cat without requiring the complex to dissociate (24).     

Inside-out regulation of E-cadherin conformation and adhesion

Cadherin cell–cell adhesion proteins play key roles in tissue morphogenesis and wound healing. Cadherin ectodomains bind in two conformations, X-dimers and strand-swap dimers, with different adhesive properties. However, the mechanisms by which cells regulate ectodomain conformation are unknown. Cadherin intracellular regions associate with several actin-binding proteins including vinculin, which are believed to tune cell–cell adhesion by remodeling the actin cytoskeleton. Here, we show at the single-molecule level, that vinculin association with the cadherin cytoplasmic region allosterically converts weak X-dimers into strong strand-swap dimers and that this process is mediated by myosin II–dependent changes in cytoskeletal tension. We also show that in epithelial cells, ∼70% of apical cadherins exist as strand-swap dimers while the remaining form X-dimers, providing two cadherin pools with different adhesive properties. Our results demonstrate the inside-out regulation of cadherin conformation and establish a mechanistic role for vinculin in this process.

E-cadherins (Ecads) are essential, calcium-dependent cell–cell adhesion proteins that play key roles in the formation of epithelial tissue and in the maintenance of tissue integrity. Ecad adhesion is highly plastic and carefully regulated to orchestrate complex movement of epithelial cells, and dysregulation of adhesion is a hallmark of numerous cancers (1). However, little is known about how cells dynamically regulate the biophysical properties of individual Ecads.The extracellular region of Ecads from opposing cells bind in two distinct trans orientations: strand-swap dimers and X-dimers (Fig. 1 A and B). Strand-swap dimers are the stronger cadherin adhesive conformation and are formed by the exchange of conserved tryptophan (Trp) residues between the outermost domains of opposing Ecads (2–4). In contrast, X-dimers, which are formed by extensive surface interactions between opposing Ecads, are a weaker adhesive structure and serve as an intermediate during the formation and rupture of strand-swap dimers (5–7). Using cell-free, single-molecule experiments we previously showed that X-dimers and strand-swap dimers can be distinguished based on their distinctly different response to mechanical force. When a strand-swap dimer is pulled, its lifetime decreases with increasing force, resulting in the formation of a slip bond (8, 9) (Fig. 1B). In contrast, an X-dimer responds to pulling force by forming a catch bond, where bond lifetime initially increases up to a threshold force and then subsequently decreases (8, 10) (Fig. 1B). It has also been shown that wild-type Ecad ectodomains in solution can interconvert between X-dimer and strand-swap dimer conformations (9, 11). However, the biophysical mechanisms by which Ecad conformations (and adhesion) are regulated on the cell surface are unknown.Open in a separate windowFig. 1.Overview of experiment. (A) The extracellular region of Ecad from opposing cells mediates adhesion. The cytoplasmic region of Ecad associates either directly or indirectly with p120 catenin, β-catenin, α-catenin, vinculin, and F-actin. (B) Strand-swap dimers form slip bonds (blue) and X-dimers form catch bonds (red). Ecads interconvert between these two dimer conformations. Structures were generated from the crystal structure of mouse Ecad (PDB ID code 3Q2V); the X-dimer was formed by alignment to an X-dimer crystal structure (PDB ID code 3LNH). (C) Graphics showing the cell lines used in experiments and Western blot analysis of corresponding cell lysates.The cytoplasmic region of Ecad associates with the catenin family of proteins, namely, p120-catenin, β-catenin, and α-catenin. The Ecad–catenin complex, in turn, links to filamentous actin (F-actin) either by the direct binding of α-catenin and F-actin or by the indirect association of α-catenin and F-actin via vinculin (12) (Fig. 1A). Adhesive forces transmitted across intercellular junctions by Ecad induce conformational changes in α-catenin (13, 14), strengthen F-actin binding (15), and recruit vinculin to the sites of force application (16, 17). However, vinculin and α-catenin do not merely serve as passive cytoskeletal linkers; they also dynamically modulate cytoskeletal rearrangement and recruit myosin to cell–cell junctions (13, 18–20). Studies show that α-catenin and vinculin play important roles in strengthening and stabilizing Ecad adhesion: bead-twisting experiments show force-induced stiffening of Ecad-based junctions and cell doublet stretching experiments demonstrate reinforcement of cell–cell adhesion in vinculin- and α-catenin–dependent manners (18, 19, 21).Currently, actin anchorage and cytoskeletal remodeling are assumed to be the exclusive mechanisms by which α-catenin and vinculin strengthen Ecad adhesion (22–24). Here, we directly map the allosteric effects of cytoplasmic proteins on Ecad ectodomain conformation and demonstrate, at the single-molecule level, that vinculin association with the Ecad cytoplasmic region switches X-dimers to strand-swap dimers. We show that cytoskeletal tension, due to vinculin-mediated recruitment of myosin II, regulates Ecad ectodomain structure and adhesion. Finally, we demonstrate that only ∼50% of Ecads are linked to the underlying cytoskeleton and that while about 70% of Ecads form strand-swap dimers the remaining form X-dimers, which provides cells with two Ecad pools with different adhesive properties.     

El Ni?o?Southern Oscillation frequency cascade

The El Niño−Southern Oscillation (ENSO) phenomenon, the most pronounced feature of internally generated climate variability, occurs on interannual timescales and impacts the global climate system through an interaction with the annual cycle. The tight coupling between ENSO and the annual cycle is particularly pronounced over the tropical Western Pacific. Here we show that this nonlinear interaction results in a frequency cascade in the atmospheric circulation, which is characterized by deterministic high-frequency variability on near-annual and subannual timescales. Through climate model experiments and observational analysis, it is documented that a substantial fraction of the anomalous Northwest Pacific anticyclone variability, which is the main atmospheric link between ENSO and the East Asian Monsoon system, can be explained by these interactions and is thus deterministic and potentially predictable.The El Niño−Southern Oscillation (ENSO) phenomenon is a coupled air−sea mode, and its irregular occurring extreme phases El Niño and La Niña alternate on timescales of several years (1–8). The global atmospheric response to the corresponding eastern tropical Pacific sea surface temperature (SST) anomalies (SSTA) causes large disruptions in weather, ecosystems, and human society (3, 5, 9).One of the main properties of ENSO is its synchronization with the annual cycle: El Niño events tend to grow during boreal summer and fall and terminate quite rapidly in late boreal winter (9–18). The underlying dynamics of this seasonal pacemaking can be understood in terms of the El Niño/annual cycle combination mode (C-mode) concept (19), which interprets the Western Pacific wind response during the growth and termination phase of El Niño events as a seasonally modulated interannual phenomenon. This response includes a weakening of the equatorial wind anomalies, which causes the rapid termination of El Niño events after boreal winter and thus contributes to the seasonal synchronization of ENSO (17). Mathematically, the modulation corresponds to a product between the interannual ENSO phenomenon (ENSO frequency: f_E) and the annual cycle (annual frequency: 1 y^-1), which generates near-annual frequencies at periods of  ～  10 mo (1 + f_E) and  ～  15 mo (1 − f_E) (19).In nature, a wide variety of nonlinear processes exist in the climate system. Atmospheric examples include convection and low-level moisture advection (19). An example for a quadratic nonlinearity is the dissipation of momentum in the planetary boundary layer, which includes a product between ENSO (E) and the annual cycle (A) due to the windspeed nonlinearity: v_E⋅ v_A (17, 19). In the frequency domain, this product results in the near-annual sum (1 + f_E) and difference (1 − f_E) tones (19). The commonly used Niño 3.4 (N3.4) SSTA index (details in SI Appendix, SI Materials and Methods) exhibits most power at interannual frequencies (Fig. 1A). In contrast, the near-annual combination tones (1 ± f_E) are the defining characteristic of the C-mode (Fig. 1B).Open in a separate windowFig. 1.Schematic for the ENSO (E) and combination mode (ExA) anomalous surface circulation pattern and corresponding spectral characteristics. (A) Power spectral density for the normalized N3.4 index of the Hadley Centre Sea Ice and Sea Surface Temperature data set version 1 (HadISST1) 1958–2013 SSTA using the Welch method. (B) As in A but for the theoretical quadratic combination mode (ExA). (C) Regression coefficient of the normalized N3.4 index and the anomalous JRA-55 surface stream function for the same period (ENSO response pattern). (D) Regression coefficient of the normalized combination mode (ExA) index and the anomalous JRA-55 surface stream function (combination mode response pattern). Areas where the anomalous circulation regression coefficient is significant above the 95% confidence level are nonstippled.Physically, the dominant near-annual combination mode comprises a meridionally antisymmetric circulation pattern (Fig. 1D). It features a strong cyclonic circulation in the South Pacific Convergence Zone, with a much weaker counterpart cyclone in the Northern Hemisphere Central Pacific. The most pronounced feature of the C-mode circulation pattern is the anomalous low-level Northwest Pacific anticyclone (NWP-AC). This important large-scale atmospheric feature links ENSO impacts to the Asian Monsoon systems (20–25) by shifting rainfall patterns (SI Appendix, Fig. S1B), and it drives sea level changes in the tropical Western Pacific that impact coastal systems (26). It has been demonstrated using spectral analysis methods and numerical model experiments that the C-mode is predominantly caused by nonlinear atmospheric interactions between ENSO and the warm pool annual cycle (19, 20). Local and remote thermodynamic air−sea coupling amplify the signal but are not the main drivers for the phase transition of the C-mode and its associated local phenomena (e.g., the NWP-AC) (20).Even though ENSO and the C-mode are not independent, their patterns and spectral characteristics are fundamentally different, which has important implications when assessing the amplitude and timing of their regional climate impacts (Fig. 1). Here we set out to study the role of nonlinear interactions between ENSO and the annual cycle (10) in the context of C-mode dynamics. Such nonlinearities can, in principle, generate a suite of higher-order combination modes, which would contribute to the high-frequency variability of the atmosphere—in a deterministic and predictable way.     

Coevolution can reverse predator–prey cycles

A hallmark of Lotka–Volterra models, and other ecological models of predator–prey interactions, is that in predator–prey cycles, peaks in prey abundance precede peaks in predator abundance. Such models typically assume that species life history traits are fixed over ecologically relevant time scales. However, the coevolution of predator and prey traits has been shown to alter the community dynamics of natural systems, leading to novel dynamics including antiphase and cryptic cycles. Here, using an eco-coevolutionary model, we show that predator–prey coevolution can also drive population cycles where the opposite of canonical Lotka–Volterra oscillations occurs: predator peaks precede prey peaks. These reversed cycles arise when selection favors extreme phenotypes, predator offense is costly, and prey defense is effective against low-offense predators. We present multiple datasets from phage–cholera, mink–muskrat, and gyrfalcon–rock ptarmigan systems that exhibit reversed-peak ordering. Our results suggest that such cycles are a potential signature of predator–prey coevolution and reveal unique ways in which predator–prey coevolution can shape, and possibly reverse, community dynamics.Population cycles, e.g., predator–prey cycles, and their ecological drivers have been of interest for the last 90 y (1–4). Classical models of predator–prey systems, developed first by Lotka (5) and Volterra (6), share a common prediction: Prey oscillations precede predator oscillations by up to a quarter of the cycle period (7). When plotted in the predator–prey phase plane, these cycles have a counterclockwise orientation (4). These cycles are driven by density-dependent interactions between the populations. When predators are scarce, prey increase in abundance. As their food source increases, predators increase in abundance. When the predators reach sufficiently high densities, the prey population is driven down to low numbers. With a scarcity of food, the predator population crashes and the cycle repeats.While many cycles, like the classic lynx–hare cycles (Fig. 1A) (3), exhibit the above characteristics, predator–prey cycles with different characteristics have also been observed. For example, antiphase cycles where predator oscillations lag behind prey oscillations by half of the cycle period (Fig. 1B) (8) and cryptic cycles where the predator population oscillates while the prey population remains effectively constant (Fig. 1C) (9) have been observed in experimental systems. This diversity of cycle types motivates the question, “Why do cycle characteristics differ across systems?”Open in a separate windowFig. 1.Examples of different kinds of predator–prey cycles. (A) Counterclockwise lynx–hare cycles (3). (B) Antiphase rotifer–algal cycles (8). (C) Cryptic phage-bacteria cycles (9). In all time series, red and blue correspond to predator and prey, respectively. See SI Text, section C for data sources.In Lotka–Volterra and other ecological models, predator and prey life history traits are assumed to be fixed. However, empirical studies across taxa have shown that prey (9–16) and predators (17–20) can evolve over ecological time scales. That is, changes in allele frequencies (and associated phenotypes) can occur at the same rate as changes in population densities or spatial distributions and alter the ecological processes driving the changes in population densities or distributions; this phenomenon has been termed “eco-evolutionary dynamics” (21, 22). Furthermore, predator–prey coevolution is important for driving community composition and dynamics (16, 19, 20, 23–26). This body of work suggests that the interaction between ecological and evolution processes has the potential to alter the ecological dynamics of communities.Experimental (8, 9, 13, 14) and theoretical studies (13, 27, 28) have shown that prey or predator evolution alone can alter the characteristics of predator–prey cycles and drive antiphase (Fig. 1B) and cryptic (Fig. 1C) cycles. Additional theoretical work has shown that predator–prey coevolution can also drive antiphase and cryptic cycles (29). Thus, evolution in one or both species is one mechanism through which antiphase or cryptic predator–prey cycles can arise. However, it is unclear if coevolution can drive additional kinds of cycles with characteristics different from those in Fig. 1.The main contribution of this study is to show that predator–prey coevolution can drive unique cycles where peaks in predator abundance precede peaks in prey abundance, the opposite of what is predicted by classical ecological models. We refer to these reversed cycles as “clockwise cycles.” The theoretical and empirical finding of clockwise cycles represents an example of how evolution over ecological time scales can alter community-level dynamics.     

In situ structure of intestinal apical surface reveals nanobristles on microvilli

Microvilli are actin-bundle-supported membrane protrusions essential for absorption, secretion, and sensation. Microvilli defects cause gastrointestinal disorders; however, mechanisms controlling microvilli formation and organization remain unresolved. Here, we study microvilli by vitrifying the Caenorhabditis elegans larvae and mouse intestinal tissues with high-pressure freezing, thinning them with cryo-focused ion-beam milling, followed by cryo-electron tomography and subtomogram averaging. We find that many radial nanometer bristles referred to as nanobristles project from the lateral surface of nematode and mouse microvilli. The C. elegans nanobristles are 37.5 nm long and 4.5 nm wide. Nanobristle formation requires a protocadherin family protein, CDH-8, in C. elegans. The loss of nanobristles in cdh-8 mutants slows down animal growth and ectopically increases the number of Y-shaped microvilli, the putative intermediate structures if microvilli split from tips. Our results reveal a potential role of nanobristles in separating microvilli and suggest that microvilli division may help generate nascent microvilli with uniformity.

Microvilli are membrane-bound cell-surface protrusions that contain a core bundle of actin filaments enveloped in the plasma membrane (1–3). Many epithelial cells develop microvilli above their apical surface to enhance functional capacity for a range of physiological tasks, including nutrient absorption in the intestine (4), solute uptake in the renal tubules (5), mechanosensation in sensory stereocilia of the inner ear (6), and chemosensation in the gut, lung, and urogenital tracts (7–9). Abnormal microvillar structure and function lead to human disorders, such as life-threatening nutrient malabsorption, osmotic imbalances, and inherited deafness in Usher syndrome (1, 3, 4, 10).An intestinal absorptive cell enterocyte develops up to 1,000 densely packed microvilli in an array known as the brush border. These fingerlike outward projections enhance the functional surface area for nutrient absorption and provide the barrier for host defense against pathogens and toxins (1, 3, 4). Because the gut epithelium undergoes constant regenerative renewal, microvillus assembly is a process that continues throughout our lifetime (1, 3, 4, 11). The long-standing questions regarding microvillus formation are how microvilli are formed with striking uniformity in sizes and how these protrusions are maximally packed in a hexagonal pattern.The tip of microvilli is known to be decorated by additional filamentous structures. The glycoprotein-rich glycocalyx localizes between the apical tip of microvilli and the luminal space (Fig. 1A), provides a barrier for pathogens, and serves as the interface for nutrient digestion (3, 4). The protocadherin-based adhesion tip links localize between adjacent microvilli (Fig. 1A) (12). The mammalian cadherin superfamily members, including CDHR2 and CDHR5, play essential roles in packing microvilli, increasing surface density, and controlling microvilli length (13). Other cadherins, specifically CDH23 and PCDH15, have been implicated in organizing the exaggerated microvilli found on inner-ear hair cells (14, 15).Open in a separate windowFig. 1.In situ cryo-ET of the C. elegans intestinal brush border reveals nanobristles on the lateral surface of microvilli. (A) A schematic diagram of an intestinal epithelial cell (Left) and two microvilli (Right) from the dotted box in Left. The glycocalyx and the protocadherin tip link are the characterized cell-surface structure at microvillar tips. This work shows that numerous nanobristles (magenta) decorate the lateral surface of microvilli. (B, Left and Center) Representative cryo-SEM images of the C. elegans L1 larvae before and after FIB milling. (Scale bars, 10 μm.) (B, Right) Representative FIB image of the ∼200-nm-thick cryo-lamella. (Scale bar, 5 μm.) (C) A 3D rendering of the C. elegans intestinal brush border showing various macromolecules and structures. Magenta, nanobristles; cyan, membrane; yellow, actin; beige, ribosome; green, mitochondria; orange, ER) Nanobristles and ribosomes were mapped back in the tomogram with the computed location and orientation. (D) A selected microvillus from E magnified for visualization. (E and F) Cryo-ET tomogram slices of microvilli (E, top view; F, side view). (Scale bars in C–F, 50 nm.)An individual microvillus is only 0.1 μm in diameter and 1 to ∼2 μm in height (Fig. 1A), the tiny dimension of which, along with their high density and lumen localization, becomes a technical hurdle for in situ structural investigations at high resolution (1–3). Despite our understanding of microvillar tip decorations, it is unclear whether any structure projects from the lateral surface of microvilli. Recent methodology advance of cryo-electron tomography (cryo-ET) makes the platform well suited to address the challenges of studying microvillus structure in animals (16, 17). Here, we used cryo-ET to reveal a previously unrecognized nanobristle structure on the lateral surface of microvilli. We provide evidence that nanobristle formation depends on a protocadherin family protein, CDH-8, and that nanobristles regulate microvilli separation.     

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.     

From the Cover: Mammalian face as an evolutionary novelty

The anterior end of the mammalian face is characteristically composed of a semimotile nose, not the upper jaw as in other tetrapods. Thus, the therian nose is covered ventrolaterally by the “premaxilla,” and the osteocranium possesses only a single nasal aperture because of the absence of medial bony elements. This stands in contrast to those in other tetrapods in whom the premaxilla covers the rostral terminus of the snout, providing a key to understanding the evolution of the mammalian face. Here, we show that the premaxilla in therian mammals (placentals and marsupials) is not entirely homologous to those in other amniotes; the therian premaxilla is a composite of the septomaxilla and the palatine remnant of the premaxilla of nontherian amniotes (including monotremes). By comparing topographical relationships of craniofacial primordia and nerve supplies in various tetrapod embryos, we found that the therian premaxilla is predominantly of the maxillary prominence origin and associated with mandibular arch. The rostral-most part of the upper jaw in nonmammalian tetrapods corresponds to the motile nose in therian mammals. During development, experimental inhibition of primordial growth demonstrated that the entire mammalian upper jaw mostly originates from the maxillary prominence, unlike other amniotes. Consistently, cell lineage tracing in transgenic mice revealed a mammalian-specific rostral growth of the maxillary prominence. We conclude that the mammalian-specific face, the muzzle, is an evolutionary novelty obtained by overriding ancestral developmental constraints to establish a novel topographical framework in craniofacial mesenchyme.

In the movie For Whom the Bell Tolls (1943, Paramount), a girl says, “I do not know how to kiss, or I would kiss you. Where do the noses go?” (1) Nothing could reveal more vividly the curious morphological fact that it is the nose, not the tip of the upper jaw, that is the most protruding part of the mammalian face. Therian mammals are thus characterized by a protruding nose, representing a morphologically and functionally semi-independent module for tactile sensory detection and for mammalian olfactory function (Fig. 1A) (2–7). The topographical relationship between the nose and cranial bones also shows an exceptional pattern in mammals: the rostral-most bone of the upper jaw, or premaxilla, is found on the ventrolateral sides of the external nostrils in therian mammals, unlike in other amniotes in whom the premaxilla covers the rostromedial tip of the snout (Fig. 1 A and B) (2–4, 7, 8). However, the evolutionary origin of this therian-specific face (the so-called muzzle) and homology of the therian premaxilla (also known as the incisive bone) have not been examined for a long time (2–4, 7–9).Open in a separate windowFig. 1.Murine “premaxilla” develops differently from premaxillae of other tetrapods. (A) The anatomy of the therian mammal’s face. (B) General scheme of craniofacial development in amniotes (10, 11, 15). (C) Three-dimensional models of tetrapod embryos. The murine premaxilla ossifies in the same topographical position as the septomaxilla (orange) of other species. The infraorbital branch (nerve branch for vibrissae) of V2 was removed in 13.5 dpc mouse. The summary is shown in D. sn, solum nasi; nld, nasolacrimal duct; V1, ophthalmic nerve; V2, maxillary nerve. (Not to scale.)During vertebrate embryogenesis, the upper jaw is primarily formed by growth of the maxillary prominence of the mandibular arch, except for the premaxilla, the rostral midline part of the upper jaw, which develops by the convergence of the premandibular ectomesenchyme (frontonasal prominence) that initially develops rostral to the mandibular arch ectomesenchyme (Fig. 1B) (4, 10–12). This topographical configuration is recognized even in some placoderms; that is, the basic pattern of jaw morphology is thought to be constrained among the jawed vertebrates (12–14). However, the topographical position of the therian premaxilla suggests that this highly conserved pattern is disrupted in mammals in association with the evolution of the mammalian muzzle. Specifically, the innervation pattern of the homonymous “premaxilla” is significantly different in mammals (15), which is also suggestive of fundamental embryological changes.In the present study, we conducted comparative experimental embryological analyses and cell lineage tracing of the facial primordia to investigate the origin of the mammalian face.     

Biotic homogenization can decrease landscape-scale forest multifunctionality

Many experiments have shown that local biodiversity loss impairs the ability of ecosystems to maintain multiple ecosystem functions at high levels (multifunctionality). In contrast, the role of biodiversity in driving ecosystem multifunctionality at landscape scales remains unresolved. We used a comprehensive pan-European dataset, including 16 ecosystem functions measured in 209 forest plots across six European countries, and performed simulations to investigate how local plot-scale richness of tree species (α-diversity) and their turnover between plots (β-diversity) are related to landscape-scale multifunctionality. After accounting for variation in environmental conditions, we found that relationships between α-diversity and landscape-scale multifunctionality varied from positive to negative depending on the multifunctionality metric used. In contrast, when significant, relationships between β-diversity and landscape-scale multifunctionality were always positive, because a high spatial turnover in species composition was closely related to a high spatial turnover in functions that were supported at high levels. Our findings have major implications for forest management and indicate that biotic homogenization can have previously unrecognized and negative consequences for large-scale ecosystem multifunctionality.It is widely established that high local-scale biodiversity increases levels of individual ecosystem functions in experimental ecosystems (1–4), and that biodiversity is even more important for the simultaneous maintenance of multiple functions at high levels (i.e., ecosystem multifunctionality) (5–8). Because the capacity of natural ecosystems to maintain multiple functions and services is crucial for human well-being (9), the positive diversity–multifunctionality relationship is often used as an argument to promote biodiversity conservation (6, 10). However, although society seeks to maximize the delivery of potentially conflicting ecosystem services, such as food production, bioenergy generation, and carbon storage at the landscape scale (11–13), research into the relationship between biodiversity and ecosystem multifunctionality has been largely limited to local-scale studies, where diversity is manipulated in experimental plant communities. Although some studies have focused on more natural communities distributed over larger spatial extents (e.g., 14–16), they examined relationships between local-scale biodiversity and local-scale multifunctionality. The only previous study to investigate multifunctionality at larger scales (17) simulated artificial landscapes using data from experimental grassland communities. It showed that although different aspects of biodiversity affected multifunctionality, local-scale (α-) diversity was a much stronger driver than the turnover of species between sites (β-diversity). However, whether those findings can be extrapolated to real-world (i.e., natural, seminatural) ecosystems, such as forests, is unknown. As a result, we have a poor understanding of how multifunctionality relates to biodiversity at the larger spatial scales that are most relevant to ecosystem managers. This question is of particular concern, given recent findings suggesting that human-driven homogenization of communities [loss of β-diversity (18–21)] may be just as widespread as α-diversity declines (22, 23).Multifunctionality can be measured by a variety of methods, and the most appropriate means of doing so remains unresolved (24–27), particularly at larger scales, where the desired distribution of ecosystem function across the landscape has not been quantified. At local scales, one can quantify ecosystem multifunctionality as the number of ecosystem functions that exceed a given threshold value, where the threshold equals a certain percentage of the maximum observed value of each function (10, 24) (hereafter “threshold-based multifunctionality”; Fig. 1B). This threshold reflects the minimum value of ecosystem functioning that is deemed satisfactory. Because trade-offs between ecosystem functions or services are commonplace (5, 7, 28, 29), it is often impossible to maximize all of the desired functions in a local community (6). However, when different species provide different functions (5, 7) at larger spatial scales, a high spatial turnover in community composition (i.e., a high β-diversity) across the landscape can cause different parts of the landscape to provide different functions at high levels (defined as high threshold-based β-multifunctionality; Fig. 1B). Therefore, high β-diversity might cause all desired ecosystem functions to be provided at high levels in at least one patch within a landscape [and hence promote threshold-based landscape-scale or γ-multifunctionality (30)] (Fig. 1B), but only if (i) species differ in the functions they support and (ii) there is no “superspecies” that supports the majority of functions. This threshold-based γ-multifunctionality may be relevant for cases where forest landscapes are managed for many different services (e.g., timber production, limitation of nutrient runoff, ecotourism), but where each of these services only needs to be provided at high levels in a part of the landscape, not everywhere (31). Alternatively, a manager may seek to promote the total delivery of many summed individual ecosystem functions across a landscape. We define this total delivery as sum-based γ-multifunctionality (Fig. 1B). This metric may be a more appropriate measure of multifunctionality in cases where the benefits of ecosystem services are manifested at large scales, such as carbon sequestration or water purification (32). In this case, β-diversity might only promote sum-based γ-multifunctionality if nonadditive diversity effects, such as resource partitioning, species-environment matching, or spillover effects, operate at relatively large spatial scales (33, 34). It is therefore likely that the importance of β-diversity for γ-multifunctionality varies depending on the desired pattern of ecosystem service provision.Open in a separate windowFig. 1.Quantifying biodiversity and multifunctionality across spatial scales. The light yellow areas represent hypothetical landscapes, consisting of (white) local communities. In these communities, some species are present (colored icons in A and C), whereas others are absent (gray icons). Similarly, some functions are performing above a hypothetical threshold of 0.5 (colored icons in B), whereas others are not (gray icons). Diversity and threshold-based multifunctionality are quantified at (i) the local plot (α-) scale as the number of species present (two and three in A) or functions performing above a given threshold (two and three in B), (ii) the β-scale as the turnover in species composition [ = 1 ? log((a + b + 2c)/(a + b + c)) = 1 ? log((1 + 2 + 2)/(1 + 2 + 1)) = 0.90 in A (49)] or functions [ = 1 ? log((a + b + 2c)/(a + b + c)) = 1 ? log((1 + 2 + 2)/(1 + 2 + 1)) = 0.90 in B (49)] across plots, and (iii) the landscape (γ-) scale as the number of functions (four in B) present in at least one plot. Sum-based γ-multifunctionality is defined as the sum of all standardized ecosystem values in a landscape (= 0.8 + 0.2 + 0.7 + 0.4 + 0.9 + 1.0 + 0.1 + 0.6 = 4.7). In contrast to threshold-based multifunctionality, sum-based multifunctionality is not analogous to biodiversity (where species are either present or absent), and can therefore not be partitioned into α- or β-components. (C) This framework allows investigation of whether γ-multifunctionality is promoted by α- and/or β-diversity.Forests provide many ecosystem services, including wood production, regulation of water quality and climate, and recreation (35, 36). Most present-day European forests and almost all forest plantations worldwide are dominated by only one or a few tree species (15, 37), although their diversity could be promoted relatively easily by planting more species or by encouraging natural regeneration. This fact makes the understanding of diversity–multifunctionality relationships in these ecosystems highly relevant for forest management.We therefore assessed the importance of α- and β-diversity of tree species in driving γ-multifunctionality in mature European forests. To do so, we used data taken from a pan-European forest dataset consisting of 209 forest plots, specifically selected to investigate relationships between tree diversity and ecosystem functioning by maximizing variation in dominant “target” species richness and minimizing (i) variation in other potential drivers of ecosystem function (e.g., soil and climatic conditions) and (ii) covariation between tree α-diversity, species composition, and environmental variables as much as possible (38). Our plot selection therefore aimed to mimic biodiversity experiments to investigate relationships between biodiversity and ecosystem functioning in mature forests, which are difficult to undertake with manipulative approaches due to the longevity of tree species. The plots were widely distributed across six European countries, spanning boreal to Mediterranean zones and representing six major European forest types (38). In each plot, 16 ecosystem processes, functions, or properties (termed “functions” hereafter) were measured. These functions represented a wide range of supporting, provisioning, regulating, and cultural ecosystem services (sensu 9) (SI Appendix, Table S3). Next, we created simulated landscapes by randomly drawing plots from a country to generate a “landscape” of five plots, from which γ-multifunctionality was calculated. We then explored relationships between α- and β-diversity and different measures of γ-multifunctionality: threshold-based γ-multifunctionality, quantified as the number of functions with levels above a threshold [a certain percentage of maximum functioning observed across all plots (10)] in at least one plot of the landscape (quantification is shown in Fig. 1B), and sum-based γ-multifunctionality, quantified as the sum of scaled values of all functions across all plots within a landscape (quantification is shown in Fig. 1B). To demonstrate how α- and β-diversity can promote threshold-based γ-multifunctionality, we also measured the relationships between both α- and β-diversity and threshold-based α- and β-multifunctionality (quantification is shown in Fig. 1B).     

Enabling multiple intercavity polariton coherences by adding quantum confinement to cavity molecular polaritons

In this study, the “particle in a box” idea, which was broadly developed in semiconductor quantum dot research, was extended into mid-infrared (IR) cavity modes by applying lateral confinement in an optical cavity. The discrete cavity modes hybridized with molecular vibrational modes, resulting in a quartet of polariton states that can support multiple coherence states in the IR regime. We applied tailored pump pulse sequences to selectively prepare these coherences and verified the multi-coherence existence. The simulation based on Lindblad equation showed that because the quartet of polariton states resided in the same cavity, they were specifically robust toward decoherence caused by fluctuations in space. The multiple robust coherences paved the way for entangled states and coherent interactions between cavity polaritons, which would be critical for advancing polariton-based quantum information technology.

Molecular vibrational polaritons (1–18) are half-matter, half-light quasiparticles that possess unique abilities to change chemical reactions (3, 10, 12, 15, 17, 19–22), modify energy transfer pathways (7, 14, 23, 24), and have the potential to be an alternative platform for quantum simulation (9, 25–32). When the collective dipole coupling between cavity photon modes and molecular vibrational modes is so strong that the two modes exchange energy at a rate faster than the lifetimes of either mode, the upper and lower polaritons (UP and LP) are formed and the systems reach the so-called vibrational strong coupling (VSC) regime (31–33). Up to now, the majority of molecular vibrational polaritons are formed in Fabry-Perot (FP) cavity, which has one corresponding cavity photon mode for each specific in-plane momentum. These modes at different in-plane momentum form a continuous parabolic dispersion curve. As such, an FP cavity can support only one pair of UP and LP at a specific in-plane momentum, and thereby, have one coherence (30) (i.e., off-diagonal density matrix elements), namely |UP〉〈LP| (or its complex conjugate, Fig. 1A). Thus, UP and LP can be treated as one polariton qubit system at ambient conditions (28, 29, 34).Open in a separate windowFig. 1.Challenges of creating multiple coherences in cavity polaritons. (A) In an FP cavity composed by two flat mirrors, one pair of UP and LP is supported and thereby can only form one coherence |UP〉〈LP| and its conjugate. (B) In the dual cavity, two cavity modes are supported due to the longitudinal cavity thickness difference along the lateral dimension. This cavity can support two pairs of UP and LP and enrich the varieties of coherences. However, coherences such as |UP_B〉〈LP_A| cannot survive the fluctuations between cavities. (C) In this work, we demonstrated the confined cavity by implementing the “particle in a box” concept. In this way, two cavity modes and two pairs of polariton modes are supported in the same spatial location, enabling the creation of coherences among any pairs of polaritons. To clearly show the confinement in the illustration, the vertical dimension was exaggerated. (D) A close view of the confined-cavity pattern obtained by optical profilometer. The lateral dimension of the cavity (the short side of the trench) is 25 μm. The depth of the trench is 1 μm. (E) The linear transmission spectra obtained by focusing IR beams center at the trenched area on the sample. Two peaks at 1,971 cm^–1 and 1,995 cm^–1 are from the confined cavity, whereas the peak at 2,099 cm^–1 is from the unconfined region. The dashed line is the simulation result.The molecular vibrational polariton-based qubits is a potential platform for quantum simulation with several advantages, such as operating at ambient temperature, ease of tunability of cavities, intrinsic systems for quantum light molecular spectroscopy, and the customizable “designer” vibrational chromophores (35–37). Although similar efforts have been made on exciton polaritons, the single qubit property of the FP cavity has limited the scalability of molecular vibrational polaritons for advancing quantum simulation (34). One way to overcome the limitations is to form multi-qubit systems, also called qudits, using multi-cavity polariton systems. Early work from our group extended the FP cavity into two pairs of polaritons in spatially neighboring cavities (9, 26), which we termed as dual-cavity system herein (Fig. 1B). However, the high-frequency coherences composed of polaritons from different cavity modes (referred as intercavity coherences) cannot survive due to decoherence, because polaritons reside in different spatial locations. To address this limitation, a novel cavity structure is needed to multiplex polariton coherences for simulating complex quantum phenomena.In this work, we overcome the FP cavity limitations and create two cavity modes with distinct energies by applying an orthogonal confinement in FP cavity system. This confinement effect is similar to “particle in a box,” which is widely applied in semiconductor materials (38–45), including quantum dots and wells. Simply put, when the dimensions of a system are close to the wavelength of the particles, only certain wave functions can survive the boundary condition of the spatial confinement, leading to distinct quantum states and tunable energy gaps. However, compared to the confinement effect in semiconductor materials, this phenomenon has not been heavily explored in the IR regime. Here, we implemented confinement to IR cavities to create two photonic modes at a specific in-plane momentum, and we showed that the confined cavity had a discrete dispersion relation with respect to in-plane momentum. We further demonstrated that under VSC conditions, a quadruplet of polaritons (polaritonic qudits) was created which formed coherences between any pairs of polaritons (Fig. 1C). Thus, introducing confinement in a single cavity created a foundation for generating qudits with complex coherence states or even entanglements in the future (46–48). This advance could create a potential platform for quantum light spectroscopy and other quantum science and technology (49, 50). Therefore, this was a valuable step for molecular polaritonic quantum information technology.