Thermodynamics of formation of coffinite,USiO4

Coffinite, USiO₄, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO₂ (uraninite) and SiO₂ (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U₃O₈ and SiO₂ starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.In many countries with nuclear energy programs, spent nuclear fuel (SNF) and/or vitrified high-level radioactive waste will be disposed in an underground geological repository. Demonstrating the long-term (10⁶–10⁹ y) safety of such a repository system is a major challenge. The potential release of radionuclides into the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO₄, is expected to be an important alteration product of SNF in contact with silica-enriched groundwater under reducing conditions (2–8). It is also found, accompanied by thorium orthosilicate and uranothorite, in igneous and metamorphic rocks and ore minerals from uranium and thorium sedimentary deposits (2, 4, 5, 8–16). Under reducing conditions in the repository system, the uranium solubility (very low) in aqueous solutions is typically derived from the solubility product of UO₂. Stable U(IV) minerals, which could form as secondary phases, would impart lower uranium solubility to such systems. Thus, knowledge of coffinite thermodynamics is needed to constrain the solubility of U(IV) in natural environments and would be useful in repository assessment.In natural uranium deposits such as Oklo (Gabon) (4, 7, 11, 12, 14, 17, 18) and Cigar Lake (Canada) (5, 13, 15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4, 5, 7, 11, 13, 17, 19, 20) and transmission electron microscopy (TEM) (8, 15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dissolution and reprecipitation.The precipitation of USiO₄ as a secondary phase should be favored in contact with silica-rich groundwater (21) [silica concentration >10⁻⁴ mol/L (22, 23)]. Natural coffinite samples are often fine-grained (4, 5, 8, 11, 13, 15, 24), due to the long exposure to alpha-decay event irradiation (4, 6, 25, 26) and are associated with other minerals and organic matter (6, 8, 12, 18, 27, 28). Hence the determination of accurate thermodynamic data from natural samples is not straightforward. However, the synthesis of pure coffinite also has challenges. It appears not to form by reacting the oxides under dry high-temperature conditions (24, 29). Synthesis from aqueous solutions usually produces UO₂ and amorphous SiO₂ impurities, with coffinite sometimes being only a minor phase (24, 30–35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33). Thus, there are only a few reported estimates of thermodynamic properties of coffinite (22, 36–40) and some of them are inconsistent. To resolve these uncertainties, we directly investigated the energetics of synthetic coffinite by high-temperature oxide melt solution calorimetry to obtain a reliable enthalpy of formation and explored its thermal decomposition.     

Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic–vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active (E)-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air–water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S₂, is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm⁻¹. Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S₂ state to the lower excited state S₁. We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces.

Electronic and vibrational degrees of freedom are the most important physical quantities in molecular systems at interfaces and surfaces. Knowledge of interactions between electronic and vibrational motions, namely electronic–vibrational couplings, is essential to understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Many excited-states relaxation processes occur at interfaces and surfaces, including charge transfer, energy transfer, proton transfer, proton-coupled electron transfer, configurational dynamics, and so on (1–11). These relaxation processes are intimately related to the electronic–vibrational couplings at interfaces and surfaces. Strong electronic–vibrational couplings could promote nonadiabatic evolution of excited potential energy and thus, facilitate chemical reactions or intramolecular structural changes of interfacial molecules (10, 12, 13). Furthermore, these interactions of electronic and vibrational degrees of freedom are subject to solvent environments (e.g., interfaces/surfaces with a restricted environment of unique physical and chemical properties) (9, 14, 15). Despite the importance of interactions of electronic and vibrational motions, little is known about excited-state electronic–vibrational couplings at interfaces and surfaces.Interface-specific electronic and vibrational spectroscopies enable us to characterize the electronic and vibrational structures separately. As interface-specific tools, second-order electronic sum frequency generation (ESFG) and vibrational sum frequency generation (VSFG) spectroscopies have been utilized for investigating molecular structure, orientational configurations, chemical reactions, chirality, static potential, environmental issues, and biological systems at interfaces and surfaces (16–52). Recently, structural dynamics at interfaces and surfaces have been explored using time-resolved ESFG and time-resolved VSFG with a visible pump or an infrared (IR) pump thanks to the development of ultrafast lasers (6–9, 13–15, 49, 53–61). Doubly resonant sum frequency generation (SFG) has been demonstrated to probe both electronic and vibration transitions of interfacial molecular monolayer (15, 62–71). This frequency-domain two-dimensional (2D) interface/surface spectroscopy could provide information regarding electronic–vibrational coupling of interfacial molecules. However, contributions from excited states are too weak to be probed due to large damping rates of vibrational states in excited states (62, 63). As such, the frequency-domain doubly resonant SFG is used only for electronic–vibrational coupling of electronic ground states. Ultrafast interface-specific electronic–vibrational spectroscopy could allow us to gain insights into how specific nuclear motions drive the relaxation of electronic excited states. Therefore, development of interface-specific electronic–vibrational spectroscopy for excited states is needed.In this work, we integrate the specificity of interfaces and surfaces into the capabilities of ultrafast 2D spectroscopy for dynamical electronic–vibrational couplings in excited states of molecules; 2D interface-specific spectroscopies are analogous to those 2D spectra in bulk that spread the information contained in a pump−probe spectrum over two frequency axes. Thus, one can better interpret congested one-dimensional signals. Two-dimensional vibrational sum frequency generation (2D-VSFG) spectroscopy was demonstrated a few year ago (72–74). Furthermore, heterodyne 2D-VSFG spectroscopy using middle infrared (mid-IR) pulse shaping and noncollinear geometry 2D-VSFG experiments have also been developed to study vibrational structures and dynamics at interfaces (31, 75–78). Recently, two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy has also been demonstrated for surfaces and interfaces (79). On the other hand, bulk two-dimensional electronic–vibrational (2D-EV) spectroscopy has been extensively used to investigate the electronic relaxation and energy transfer dynamics of molecules, biological systems, and nanomaterials (80–90). The 2D-EV technique not only provides electronic and vibrational interactions between excitons or different excited electronic states of systems but also, identifies fast nonradiative transitions through nuclear motions in molecules, aggregations, and nanomaterials. However, an interface-specific technique for two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy has yet to be developed.Here, we present the development of 2D-EVSFG spectroscopy for the couplings of electronic and nucleic motions at interfaces and surfaces. The purpose of developing 2D-EVSFG spectroscopy is to bridge the gap between the visible and IR regions to reveal how structural dynamics for photoexcited electronic states are coupled with vibrations at interfaces and surfaces. As an example, we applied this 2D-EVSFG experimental method to time evolution of electronic–vibrational couplings at excited states of interface-active molecules at the air–water interface.     

Predictive Value of Brain Natriuretic Peptides in Patients on Peritoneal Dialysis: Results from the ADEMEX Trial

Background and objectives: Natriuretic peptides have been suggested to be of value in risk stratification in dialysis patients. Data in patients on peritoneal dialysis remain limited.Design, setting, participants, & measurements: Patients of the ADEMEX trial (ADEquacy of peritoneal dialysis in MEXico) were randomized to a control group [standard 4 × 2L continuous ambulatory peritoneal dialysis (CAPD); n = 484] and an intervention group (CAPD with a target creatinine clearance ≥60L/wk/1.73 m²; n = 481). Natriuretic peptides were measured at baseline and correlated with other parameters as well as evaluated for effects on patient outcomes.Results: Control group and intervention group were comparable at baseline with respect to all measured parameters. Baseline values of natriuretic peptides were elevated and correlated significantly with levels of residual renal function but not with body size or diabetes. Baseline values of N-terminal fragment of B-type natriuretic peptide (NT-proBNP) but not proANP(1–30), proANP(31–67), or proANP(1–98) were independently highly predictive of overall survival and cardiovascular mortality. Volume removal was also significantly correlated with patient survival.Conclusions. NT-proBNP have a significant predictive value for survival of CAPD patients and may be of value in guiding risk stratification and potentially targeted therapeutic interventions.Plasma levels of cardiac natriuretic peptides are elevated in patients with chronic kidney disease, owing to impairment of renal function, hypertension, hypervolemia, and/or concomitant heart disease (1–7). Atrial natriuretic peptide (ANP) and particularly brain natriuretic peptide (BNP) levels are linked independently to left ventricular mass (3–5,8–16) and function (3,6–17) and predict total and cardiovascular mortality (1,3,8,10,12,18) as well as cardiac events (12,19). ANP and BNP decrease significantly during hemodialysis treatment but increase again during the interdialytic interval (1,2,4,6,7,14,17,20–23). Levels in patients on peritoneal dialysis (PD) have been found to be lower than in patients on hemodialysis (11,24–26), but the correlations with left ventricular function and structure are maintained in both types of dialysis modalities (11,15,27,28).The high mortality of patients on peritoneal dialysis and the failure of dialytic interventions to alter this mortality (29,30) necessitate renewed attention into novel methods of stratification and identification of patients at highest risk to be targeted for specific interventions. Cardiac natriuretic peptides are increasingly considered to fulfill this role in nonrenal patients. Evaluations of cardiac natriuretic peptides in patients on PD have been limited by small numbers (3,9,11,12,15,24–26) and only one study examined correlations between natriuretic peptide levels and outcomes (12). The PD population enrolled in the ADEMEX trial offered us the opportunity to evaluate cardiac natriuretic peptides and their value in predicting outcomes in the largest clinical trial ever performed on PD (29,30). It is hoped that such an evaluation would identify patients at risk even in the absence of overt clinical disease and hence facilitate or encourage interventions with salutary outcomes.     

Symmetric activation and modulation of the human calcium-sensing receptor

The human extracellular calcium-sensing (CaS) receptor controls plasma Ca²⁺ levels and contributes to nutrient-dependent maintenance and metabolism of diverse organs. Allosteric modulation of the CaS receptor corrects disorders of calcium homeostasis. Here, we report the cryogenic-electron microscopy reconstructions of a near–full-length CaS receptor in the absence and presence of allosteric modulators. Activation of the homodimeric CaS receptor requires a break in the transmembrane 6 (TM6) helix of each subunit, which facilitates the formation of a TM6-mediated homodimer interface and expansion of homodimer interactions. This transformation in TM6 occurs without a positive allosteric modulator. Two modulators with opposite functional roles bind to overlapping sites within the transmembrane domain through common interactions, acting to stabilize distinct rotamer conformations of key residues on the TM6 helix. The positive modulator reinforces TM6 distortion and maximizes subunit contact to enhance receptor activity, while the negative modulator strengthens an intact TM6 to dampen receptor function. In both active and inactive states, the receptor displays symmetrical transmembrane conformations that are consistent with its homodimeric assembly.

Critical to the maintenance of Ca²⁺ homeostasis, the extracellular calcium-sensing (CaS) receptor was the first G protein–coupled receptor (GPCR) discovered to sense ions (1–3). The CaS receptor detects fluctuations in plasma Ca²⁺ at the parathyroid. In response to increases in Ca²⁺, it transmits signals to inhibit the release of parathyroid hormone, in turn preventing further rises in Ca²⁺ concentration (2, 3). In the cortical thick ascending limb of the renal nephron, the CaS receptor is also activated by surges in plasma Ca²⁺ and responds by inhibiting Ca²⁺ reabsorption. The excess urinary calcium excretion arising from CaS receptor activation lowers the plasma Ca²⁺ level. The CaS receptor is implicated in various pathologies associated with hypercalcemia and hypocalcemia (4). It has also been linked to the progression of diseases such as breast and colon cancer, in which the receptor modulates tumor growth (3, 5–7).The CaS receptor senses a diverse array of extracellular stimuli. During normal function, it activates multiple intracellular signaling pathways involving G_q/11, G_i/o, or G_12/13; in tumor cells, it is coupled to G_s (2, 3, 8, 9). In addition to the principal agonist Ca²⁺, the receptor is directly activated by aromatic l-amino acids (10, 11). Other CaS agonists include various divalent and trivalent cations (12), referred to as type I calcimimetics for mimicking the action of Ca²⁺ (13).The activity of the CaS receptor is also subject to allosteric modulation. Positive allosteric modulators (PAMs) are classified as type II calcimimetics for increasing the receptor sensitivity for Ca²⁺ (12–16). The prototypical PAM molecules share a phenylalkylamine structure, including cinacalcet and NPS R-568 (abbreviated as R-568). Cinacalcet was the first drug described to target a GPCR allosterically, and it is used clinically to treat hyperparathyroidism in patients with chronic kidney diseases (15). Negative allosteric modulators (NAMs) of the CaS receptor are referred to as calcilytics for suppressing the receptor response to Ca²⁺ (12–16). Synthetic calcilytics such as NPS-2143 and ronacaleret are also structurally related to phenylalkylamines. Recently, inorganic phosphate has been identified as an inhibitor of the receptor (11, 17).The CaS receptor rests within the class C family of GPCRs and functions as an obligate homodimer. Like other class C GPCRs, each CaS subunit contains a large extracellular domain (ECD) involved in orthosteric ligand binding, a seven-helix transmembrane (TM) domain responsible for G protein coupling, followed by an extended cytoplasmic tail (18–23). The conformations of the CaS ECDs in both the inactive and active states have been determined by X-ray crystallography (11, 24). The ECD structures also revealed how the receptor recognizes various extracellular ligands, including Ca²⁺, the amino acid l-Trp, and inorganic phosphate. Although the role of amino acids is still under debate (25), recent structural studies of full-length CaS receptor further confirmed that Ca²⁺ and amino acids cooperate to activate the receptor (26–28).The TM domain of the CaS receptor harbors the binding sites for PAM and NAM molecules according to previous mutagenesis studies (29–32). Recently reported modulator-bound CaS receptor structures revealed asymmetric TM configurations that are stabilized by PAM molecules binding in different poses within the separate subunits of the homodimer (33). We have determined PAM- and NAM-bound, as well as PAM-free, structures of a near–full-length CaS receptor using cryogenic-electron microscopy (cryo-EM) that display symmetric TM dimers and modulator poses, instead. This finding presents the possibility of receptor activation without requiring asymmetric conformational transition. Our structures also illustrate how distortion of TM6 provides the driving force for receptor activation. Furthermore, the presence of a PAM or NAM stabilizes distinct TM6 helix conformations to promote specific dimer arrangements and differentially modulate receptor function.     

Hierarchical self-assembly of polydisperse colloidal bananas into a two-dimensional vortex phase

Self-assembly of microscopic building blocks into highly ordered and functional structures is ubiquitous in nature and found at all length scales. Hierarchical structures formed by colloidal building blocks are typically assembled from monodisperse particles interacting via engineered directional interactions. Here, we show that polydisperse colloidal bananas self-assemble into a complex and hierarchical quasi–two-dimensional structure, called the vortex phase, only due to excluded volume interactions and polydispersity in the particle curvature. Using confocal microscopy, we uncover the remarkable formation mechanism of the vortex phase and characterize its exotic structure and dynamics at the single-particle level. These results demonstrate that hierarchical self-assembly of complex materials can be solely driven by entropy and shape polydispersity of the constituting particles.

Self-assembly of microscopic building blocks is a powerful route for preparing materials with predesigned structure and engineered properties (1–7). Nature provides a fascinating range of self-assembled architectures offering insight into how structural organization can emerge at different length scales (8–13). In the biological world, for instance, tobacco mosaic virus coat proteins self-organize into sophisticated capsids around viral RNA strands (11, 14). In molecular systems, lipid molecules, such as fatty acids, form a range of self-assembled structures as relevant as cell membranes and vesicles (15, 16). At the colloidal scale, a rich variety of crystals with remarkable optical properties, such as opal and other gemstones, also assembles from a range of colloidal constituents (12, 17–20). The structural complexity of self-assembled materials is typically dictated by the combination of the type of interactions between the constituent building blocks and their shape (2, 3, 5, 6). Colloids are ideal systems to independently study the role of these key parameters, as their shape and interactions can be systematically tuned and rationally designed (5, 18, 21–23).In colloidal systems interacting solely via excluded volume interactions, the shape of the particles can already lead to the assembly of complex structures (24–28). For instance, binary colloidal crystals (25) are obtained from spherical particles, complex dodecagonal quasicrystals are formed by tetrahedrons (26), and exotic banana-shaped liquid crystals are assembled from colloidal bananas (28). Introducing complex interactions between the colloidal building blocks—on the top of their shape—leads to their assembly into hierarchical materials with structural order at multiple length scales (3, 29–31). Examples include colloidal diamond structures assembled by patchy tetrahedrons functionalized with DNA strands (20) and superlattice structures formed by octapod-like particles functionalized with hydrophobic molecules (32). The successful hierarchical self-assembly of these structures relies not only on the directionality of the particle interactions but also, on the uniformity in size of the constituent building blocks, as polydispersity typically disrupts ordering via the formation of defects (33, 34).In this work, however, we show that a colloidal suspension of polydisperse banana-shaped particles interacting only via simple excluded volume interactions (28) self-assembles into remarkably ordered concentric structures, which we term colloidal vortices. At high packing fractions, these structures form a quasi–two-dimensional (quasi-2D) hierarchical material, which we term the vortex phase. Using confocal microscopy, we uncover the formation mechanism of this tightly packed phase and characterize its exotic structure and dynamics at the single-particle level.     

Distinct replay signatures for prospective decision-making and memory preservation

Theories of neural replay propose that it supports a range of functions, most prominently planning and memory consolidation. Here, we test the hypothesis that distinct signatures of replay in the same task are related to model-based decision-making (“planning”) and memory preservation. We designed a reward learning task wherein participants utilized structure knowledge for model-based evaluation, while at the same time had to maintain knowledge of two independent and randomly alternating task environments. Using magnetoencephalography and multivariate analysis, we first identified temporally compressed sequential reactivation, or replay, both prior to choice and following reward feedback. Before choice, prospective replay strength was enhanced for the current task-relevant environment when a model-based planning strategy was beneficial. Following reward receipt, and consistent with a memory preservation role, replay for the alternative distal task environment was enhanced as a function of decreasing recency of experience with that environment. Critically, these planning and memory preservation relationships were selective to pre-choice and post-feedback periods, respectively. Our results provide support for key theoretical proposals regarding the functional role of replay and demonstrate that the relative strength of planning and memory-related signals are modulated by ongoing computational and task demands.

Humans have a remarkable ability to process information that extends beyond the immediately perceptible, including simulation of prospective plans and retrieval of past memories. It has been hypothesized that hippocampal replay contributes to both these abilities (1–5). In rodents, replay is strongly linked to the hippocampus, where cells encoding distinct locations reactivate in a coordinated sequential manner, recapitulating past or simulating potential future trajectories (1–6). A similar phenomenon of sequential reactivation has been identified in humans using decoding techniques in conjunction with high temporal resolution magnetoencephalography (MEG) data (7–14).Prominent theories of neural replay propose that it is important for planning future actions (15–18) in addition to supporting memory preservation (19–25). One hypothesis is that task demands, operationalized as temporal proximity to action versus feedback, determine the contribution of replay to planning and memory, respectively (1, 26). However, the contribution of awake on-line replay to these two functions has largely been addressed in the context of separate experiments (1, 4, 6, 7, 26–33). Here, we directly address the contribution of replay to both these roles within a single task context.Replay of trajectories leading to a goal has been proposed to underpin decision-making that exploits structure knowledge of an environment, referred to as model-based decision-making (15, 16, 34). A number of rodent studies indicate a link between hippocampal neural sequences and subsequent path choice selection, consistent with a role for replay in planning (6, 31, 35–37). However, an inconsistency in such findings raises the possibility that any relationship between replay and subsequent choice might differ across evaluation strategies and reward environments (6, 13, 31, 33, 35, 36, 38, 39). Critically, and regardless of any relationship to choice identity, brain lesion studies highlight a necessary role for the hippocampus in model-based behavior (40, 41). This suggests that hippocampal replay may be enhanced when model-based decision-making (planning) is beneficial. Thus far, however, there is no clear evidence linking demands for model-based control and neural replay preceding choice (9–11, 13).Beyond planning, replay is considered critical for memory preservation, where replay of previous experiences might serve to strengthen memory and prevent interference from newer experiences (“catastrophic forgetting”) (19, 20, 22, 42, 43). Studies that have disrupted hippocampal activity support the idea that offline place cell reactivation is critical for learning, memory consolidation, or both (44–47). It has been conjectured that human rest-period replay subserves a similar function (8, 12, 48, 49). Studies of replay in rodents navigating a single environmental context provide initial, but inconclusive, evidence for a link between recent experience with an environment and replay (28, 39).Here, we address a role for replay in both planning and memory preservation in a context where participants needed to retain a memory of a “distal” environment while at the same time learning within a local one. To do this, we adapted a reward learning task originally designed to study model-based decision-making (41, 50–52), where distinct start states converge upon shared paths. Critically, to study memory, we included two independent randomly alternating environments. Both the early convergence on shared paths and the alternation of environments strongly favor online planning, and these features distinguish the current paradigm from a recent related report (11). Using recently developed MEG analytic methods (7–9, 11–13, 53), we first identify sequential neural reactivation and then ask whether replay strength varies as a function of task demands and recent experience. We hypothesized that replay would be boosted during pre-choice path planning when model-based decision-making was more beneficial (15–17, 40, 41, 50). By contrast, following receipt of choice feedback, we hypothesized that replay for an alternative environment would relate to the infrequency of recent experience, consistent with a role in supporting memory preservation (1).     

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Rapid developments of various modern information technologies such as big data transmission, cloud computing, artificial intelligence technology, and the internet of things have put forward higher requirements on network transmission speed and capacity, demanding higher-power and higher-speed electronic devices (1, 2) such as nitride-based high-electron-mobility transistors (3). Thermal management in such devices becomes crucial as the high output power density results in a strong Joule self-heating effect, which increases the channel temperature and severely degrades device performance (4, 5). Solutions for thermal management include searching for high-thermal-conductivity materials (6, 7) and increasing interface thermal conductance (ITC) via interface engineering (8–10). The latter approach becomes increasingly important when the size of the device approaches nanoscale as the ITC dominates the device’s thermal resistance (11, 12). However, it is challenging to obtain precise knowledge of ITC due to the atomic size and the buried nature of heterointerfaces. The common methods to characterize thermal conductivity, including the time-domain thermoreflectance (13), the frequency-domain thermoreflectance (14), the 3-ω method (15), and coherent optical thermometry (16), suffer from a poor spatial resolution that is insufficient to measure thermal properties at the nanoscale.In fact, the thermal properties of semiconductor and insulator interfaces are largely governed by the interface phonons, and interface phonons also dominate the ITC of metal/semiconductor interfaces because electron–phonon coupling has little effect on the ITC of metal/semiconductor interfaces and can be ignored (17, 18). Previous calculations indicate that the interface can bridge the phonons with different energies and thus boost the inelastic phonon transport (19). Recent methods such as modal analysis were used to correlate the interface phonons with interfacial heat flow (20–23). Specifically, the interface phonon modes can be classified into four classes: extended modes (EMs), partially extended modes (PEMs), isolated modes (IMs), and localized modes (LMs), based on how the vibrational energy is distributed in space (22). The atomic vibrations are delocalized into both sides of the interface for EMs and are localized on one side for PEMs. IMs are not present at the interface, while LMs are highly localized at the atomically thin interface.Of the four modes, EMs and LMs can act as phonon bridges to increase the chances of phonons crossing the interface by elastic/inelastic scattering, while PEMs and IMs only have a small transmission probability for phonon transport. The delocalized EMs exhibit strong correlation between phonons of both sides, thus effectively serving as phonon bridges to support the phonons of one side to cross the interface to the other side via both elastic and inelastic scattering (22, 24). LMs, arising from the few atomic layers at the interface, exhibit extremely strong correlation with phonon modes of both sides, thus effectively serving as phonon bridges to facilitate frequency up and down conversion. As a result, the phonons in one side dump their energy to LMs at the interface region and then transfer it to the phonons in the other side (22, 25). This process can bridge the phonons with significantly different energies through inelastic scattering. Indeed, LMs usually have the highest contribution to the ITC on a per-mode basis (19, 22, 26, 27). Very recently, the localized modes for MoS₂/WSe₂ (28), Ge/Si (29), SrTiO₃/CaTiO₃ superlattices (30), and cubic-BN/diamond system (31) have been experimentally observed. At a Ge/Si interface, calculations suggested a small number of LMs make substantial contributions to ITC, acting as bridges to connect the bulk modes of two sides, while IMs hardly contribute to ITC (25). However, the dominant type of interface phonons for ITC is likely different in different material systems depending on the interface bonding. It is thus useful to experimentally study nanoscale phonon behaviors at various interfaces and correlate them to thermal conductance across the interface.In this work, we study interface phonons at the AlN/Si and AlN/Al heterointerfaces. Due to the wide bandgap, high-breakdown electric field, and high carrier mobility, the III-V nitride semiconductors such as GaN, AlN, and their ternary compounds are considered promising in the next-generation high-power and high-frequency electronic devices (32), which, however, requires excellent thermal conductivity, especially high ITC (33). For these nitride heterostructures, it remains largely unknown what types of interface phonons exist, not to mention how they impact the ITC and which one dominates. By using vibrational electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), which has the ability to measure the phonon spectra (34–37) at atomic scale (28, 29, 38–42), we probe the interface phonons to gain insights into interface thermal properties.We observe multiple types of interface phonons at an AlN/Si interface and find that EMs that connect the bulk AlN phonon modes and bulk Si phonon modes, and LMs that promote a transverse acoustic (TA) mode of Si to penetrate into the AlN layer, act as bridges to mainly contribute to ITC. However, no such bridging effect is observed across the AlN/Al interface where PEMs dominate, while EMs and LMs only take up a small proportion in the total modes. The AlN/Si interface is therefore expected to have a much higher ITC than the AlN/Al interface since EMs and LMs contribute many times larger than PEMs to the ITC according to molecular dynamics (MD) simulations. Our results unveil the very different interface phonon modes at the heterointerfaces of AlN/Si and AlN/Al and find they have very different contributions to the ITC, providing insights into understanding and engineering the interface thermal properties.     

From the Cover: Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide

Inorganic nitrogen depletion restricts productivity in much of the low-latitude oceans, generating a selective advantage for diazotrophic organisms capable of fixing atmospheric dinitrogen (N₂). However, the abundance and activity of diazotrophs can in turn be controlled by the availability of other potentially limiting nutrients, including phosphorus (P) and iron (Fe). Here we present high-resolution data (∼0.3°) for dissolved iron, aluminum, and inorganic phosphorus that confirm the existence of a sharp north–south biogeochemical boundary in the surface nutrient concentrations of the (sub)tropical Atlantic Ocean. Combining satellite-based precipitation data with results from a previous study, we here demonstrate that wet deposition in the region of the intertropical convergence zone acts as the major dissolved iron source to surface waters. Moreover, corresponding observations of N₂ fixation and the distribution of diazotrophic Trichodesmium spp. indicate that movement in the region of elevated dissolved iron as a result of the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic phosphorus depletion. These conclusions are consistent with the results of an idealized numerical model of the system. The boundary between the distinct biogeochemical systems of the (sub)tropical Atlantic thus appears to be defined by the diazotrophic response to spatial–temporal variability in external Fe inputs. Consequently, in addition to demonstrating a unique seasonal cycle forced by atmospheric nutrient inputs, we suggest that the underlying biogeochemical mechanisms would likely characterize the response of oligotrophic systems to altered environmental forcing over longer timescales.Within the majority of the oligotrophic (sub)tropical regions of the surface ocean, bioavailable forms of fixed nitrogen (N) are severely depleted, restricting microbial biomass and productivity (1–3). Organisms capable of fixing atmospheric dinitrogen (N₂) thus potentially have a considerable selective advantage in these vast N-limited environments (2), with the combined activity of these diazotrophs subsequently being responsible for maintaining the oceanic fixed-N inventory and, hence, overall productivity (1, 2, 4). Although clearly not susceptible to N limitation, in common with all living organisms, oceanic diazotrophs have an absolute requirement for a wide range of other nutrient elements, including phosphorus (P) and iron (Fe) (5). Indeed, diazotrophs have an enhanced requirement for the micronutrient Fe (6–8) due to the absolute requirement for this element within nitrogenase, the catalyst responsible for N₂ fixation (1, 9, 10). It has consequently been suggested that Fe availability may play a major role in controlling oceanic N₂ fixation (1, 11).In contrast to the ubiquitous depletion of dissolved inorganic nitrogen (DIN), surface concentrations of dissolved Fe (DFe) and inorganic P (DIP) vary greatly among different regions of the oligotrophic low-latitude oceans (12–14). Marked regional differences in N₂ fixation are also observed (15) and, in combination, such observations have been argued to support the hypothesis that diazotrophy can be enhanced in regions of high-Fe inputs, resulting in subsequent drawdown of DIP (12, 13, 16). Although an ever-growing body of evidence supports the importance of Fe for diazotrophy (9, 12, 14, 17), artificial in situ tests comparable to those that unequivocally demonstrated the Fe limited status of high-nitrate low-chlorophyll (HNLC) regions (18) have yet to be performed. Indeed, the slow response timescales of some diazotrophs may ultimately render such experiments unfeasible (19, 20).The interactions among Fe, DIP, and diazotroph activity appear particularly evident in the low-latitude oligotrophic surface waters of the Atlantic Ocean (12, 17). Iron is highly insoluble and rapidly scavenged from oxic seawater (21). In consequence, the availability of this micronutrient in surface waters is often tightly coupled to external sources, including inputs of terrigenous materials via aeolian deposition (22–24). The (sub)tropical North Atlantic receives the highest deposition fluxes of aeolian dust in the global ocean (22) and is characterized by relatively high DFe (25, 26), with corresponding severe DIP depletion argued to result from significant diazotrophic activity either within or upstream of this system (12, 13). In contrast, the (sub)tropical South Atlantic gyre is characterized by relatively high DIP and very low DFe concentrations (12, 27, 28). The boundary region between these (sub)tropical gyre systems is typically characterized by high rates of N₂ fixation, which are dominated by the diazotrophic cyanobacterium Trichodesmium spp. (12, 29–31), and appear to be spatially correlated with some of the areas of highest DFe concentrations (12). Surface nutrient concentrations and diazotrophic activity thus appear to define a biogeochemical division of the Atlantic between a high DIP, low DFe (HPLFe) system in the south, and a low DIP, high DFe (LPHFe) system in the north (12).Although the biogeochemical division of the (sub)tropical Atlantic has previously been interpreted on the basis of the hypothesized Fe control of diazotrophy (12, 13), there remains the possibility that the observed spatial patterns could have other forcing factors (32). Moreover, the wider regional and global importance of Fe controls on diazotrophy in both the modern and paleo ocean remains a matter of active debate (12, 17, 33–35) with important implications for our understanding of, for example, glacial/interglacial nitrogen and carbon cycling (1, 36–38). More direct tests of the Fe–diazotrophy limitation hypothesis are thus desirable. In particular, predictable large-scale dynamic responses of marine diazotrophy and subsequent P cycling to seasonal variability in external Fe inputs has not previously been described.     

Morphological instability and roughening of growing 3D bacterial colonies

How do growing bacterial colonies get their shapes? While colony morphogenesis is well studied in two dimensions, many bacteria grow as large colonies in three-dimensional (3D) environments, such as gels and tissues in the body or subsurface soils and sediments. Here, we describe the morphodynamics of large colonies of bacteria growing in three dimensions. Using experiments in transparent 3D granular hydrogel matrices, we show that dense colonies of four different species of bacteria generically become morphologically unstable and roughen as they consume nutrients and grow beyond a critical size—eventually adopting a characteristic branched, broccoli-like morphology independent of variations in the cell type and environmental conditions. This behavior reflects a key difference between two-dimensional (2D) and 3D colonies; while a 2D colony may access the nutrients needed for growth from the third dimension, a 3D colony inevitably becomes nutrient limited in its interior, driving a transition to unstable growth at its surface. We elucidate the onset of the instability using linear stability analysis and numerical simulations of a continuum model that treats the colony as an “active fluid” whose dynamics are driven by nutrient-dependent cellular growth. We find that when all dimensions of the colony substantially exceed the nutrient penetration length, nutrient-limited growth drives a 3D morphological instability that recapitulates essential features of the experimental observations. Our work thus provides a framework to predict and control the organization of growing colonies—as well as other forms of growing active matter, such as tumors and engineered living materials—in 3D environments.

Bacteria are known to thrive in diverse ecosystems and habitats (1–4). In nature, bacteria can be found growing on surfaces, which in addition to the ease of visualization in two dimensions, have led laboratory studies to typically focus on colony growth on two-dimensional (2D) planar surfaces. However, in many cases in nature, bacteria grow in three-dimensional (3D) habitats, such as gels and tissues inside of hosts (5–7), soils and other subsurface media (8, 9), wastewater treatment devices, and naturally occurring bodies of water (10–12). Nonetheless, despite their prevalence, the morphodynamics of bacterial colonies growing in such 3D environments remains largely unknown. Here, we ask: what determines the shape of a bacterial colony growing in three dimensions? And are there general characteristics and universal principles that span across species and specific environmental conditions?Studies of bacteria growing on 2D planar surfaces have revealed a variety of growth patterns, ranging from circular-shaped colonies (13–19) to herringbone patterns (20) and ramified, rough interfaces (13–18, 21). Some of these patterns become 3D as the colony can grow and deform into the third dimension. These morphologies are now understood to arise from friction between the growing colony and the surface and differential access to nutrients, which may also be available from the third dimension (13–18, 20, 22–26). The emergent patterns have been rationalized by incorporating these key ingredients into reaction–diffusion equations (14–16, 18, 22, 27–36), active continuum theories (19–21, 28, 31, 37–53), and agent-based models (28, 31, 38, 40, 45, 46, 54–57). Moreover, it has been suggested that these growth patterns may, in turn, influence the global function and physiology of bacterial colonies (58–60), including resistance to antibiotics (61–63) and parasites (64), resilience to environmental changes (65, 66), and genetic diversity (56, 58, 59, 67–73). However, in stark contrast to the considerable scientific effort devoted to studying 2D growth, little is known about the collective processes, the resulting morphologies, or their functional consequences for bacteria growing in three dimensions.From a physics standpoint, growing in three dimensions is fundamentally different from growing in two dimensions in terms of both nutrient access and the ability to grow and deform into an additional dimension. Consequently, we expect colony morphodynamics—the way a colony’s overall shape changes over time—to also be different. Some recent studies hint that this is indeed the case, showing how specific mechanical interactions imposed by a 3D environment can influence the morphology of growing biofilms. For instance, external fluid flows are now known to trigger the formation of streamers that stem from an initially surface-attached colony (10, 74). Also, under quiescent conditions, small (at most tens of cells across) biofilm colonies constrained in cross-linked gels adopt internally ordered structures as they grow and push outward (75), mediated by elastic stresses arising at the interface between the colony and its stiff environment. However, the behavior of larger bacterial colonies growing freely in quiescent 3D environments remains underexplored, despite the fact that they represent a fundamental building block of more complex natural colonies.Here, we combine experiments, theoretical modeling, and numerical simulations to unravel the morphodynamics of large colonies growing in 3D environments. By performing experiments with four different species of bacteria growing in transparent and easily deformed granular hydrogel matrices, we find that dense colonies growing in three dimensions undergo a morphological instability and roughen when they become nutrient limited in their interior. Independent of variations in cell type and environmental conditions, growing colonies eventually adopt a generic highly branched, broccoli-like morphology with a characteristic roughness exponent and power spectrum. Employing a continuum “active fluid” model that incorporates the coupling between nutrient diffusion, consumption, and cell growth, we trace the origin of the instability to an interplay of competition for nutrients with growth pressure–driven colony expansion. In particular, we find that these dense 3D colonies inevitably become nutrient limited in their interior, which eventually drives the periphery of the colony to become unstable and roughen. Our results thus help establish a framework to predict and control the organization of growing colonies in 3D habitats. These principles could also extend to other morphogenesis processes driven by growth in 3D environments, such as developmental processes (76, 77), tumor growth (78–82), and the expansion of engineered soft living materials (83, 84).     

N terminus of ASPP2 binds to Ras and enhances Ras/Raf/MEK/ERK activation to promote oncogene-induced senescence

The ASPP2 (also known as 53BP2L) tumor suppressor is a proapoptotic member of a family of p53 binding proteins that functions in part by enhancing p53-dependent apoptosis via its C-terminal p53-binding domain. Mounting evidence also suggests that ASPP2 harbors important nonapoptotic p53-independent functions. Structural studies identify a small G protein Ras-association domain in the ASPP2 N terminus. Because Ras-induced senescence is a barrier to tumor formation in normal cells, we investigated whether ASPP2 could bind Ras and stimulate the protein kinase Raf/MEK/ERK signaling cascade. We now show that ASPP2 binds to Ras–GTP at the plasma membrane and stimulates Ras-induced signaling and pERK1/2 levels via promoting Ras–GTP loading, B-Raf/C-Raf dimerization, and C-Raf phosphorylation. These functions require the ASPP2 N terminus because BBP (also known as 53BP2S), an alternatively spliced ASPP2 isoform lacking the N terminus, was defective in binding Ras–GTP and stimulating Raf/MEK/ERK signaling. Decreased ASPP2 levels attenuated H-RasV12–induced senescence in normal human fibroblasts and neonatal human epidermal keratinocytes. Together, our results reveal a mechanism for ASPP2 tumor suppressor function via direct interaction with Ras–GTP to stimulate Ras-induced senescence in nontransformed human cells.ASPP2, also known as 53BP2L, is a tumor suppressor whose expression is altered in human cancers (1). Importantly, targeting of the ASPP2 allele in two different mouse models reveals that ASPP2 heterozygous mice are prone to spontaneous and γ-irradiation–induced tumors, which rigorously demonstrates the role of ASPP2 as a tumor suppressor (2, 3). ASPP2 binds p53 via the C-terminal ankyrin-repeat and SH3 domain (4–6), is damage-inducible, and can enhance damage-induced apoptosis in part through a p53-mediated pathway (1, 2, 7–10). However, it remains unclear what biologic pathways and mechanisms mediate ASPP2 tumor suppressor function (1). Indeed, accumulating evidence demonstrates that ASPP2 also mediates nonapoptotic p53-independent pathways (1, 3, 11–15).The induction of cellular senescence forms an important barrier to tumorigenesis in vivo (16–21). It is well known that oncogenic Ras signaling induces senescence in normal nontransformed cells to prevent tumor initiation and maintain complex growth arrest pathways (16, 18, 21–24). The level of oncogenic Ras activation influences its capacity to activate senescence; high levels of oncogenic H-RasV12 signaling leads to low grade tumors with senescence markers, which progress to invasive cancers upon senescence inactivation (25). Thus, tight control of Ras signaling is critical to ensure the proper biologic outcome in the correct cellular context (26–28).The ASPP2 C terminus is important for promoting p53-dependent apoptosis (7). The ASPP2 N terminus may also suppress cell growth (1, 7, 29–33). Alternative splicing can generate the ASPP2 N-terminal truncated protein BBP (also known as 53BP2S) that is less potent in suppressing cell growth (7, 34, 35). Although the ASPP2 C terminus mediates nuclear localization, full-length ASPP2 also localizes to the cytoplasm and plasma membrane to mediate extranuclear functions (7, 11, 12, 36). Structural studies of the ASPP2 N terminus reveal a β–Grasp ubiquitin-like fold as well as a potential Ras-binding (RB)/Ras-association (RA) domain (32). Moreover, ASPP2 can promote H-RasV12–induced senescence (13, 15). However, the molecular mechanism(s) of how ASPP2 directly promotes Ras signaling are complex and remain to be completely elucidated.Here, we explore the molecular mechanisms of how Ras-signaling is enhanced by ASPP2. We demonstrate that ASPP2: (i) binds Ras-GTP and stimulates Ras-induced ERK signaling via its N-terminal domain at the plasma membrane; (ii) enhances Ras-GTP loading and B-Raf/C-Raf dimerization and forms a ASPP2/Raf complex; (iii) stimulates Ras-induced C-Raf phosphorylation and activation; and (iv) potentiates H-RasV12–induced senescence in both primary human fibroblasts and neonatal human epidermal keratinocytes. These data provide mechanistic insight into ASPP2 function(s) and opens important avenues for investigation into its role as a tumor suppressor in human cancer.     

The spontaneous emergence of conventions: An experimental study of cultural evolution

How do shared conventions emerge in complex decentralized social systems? This question engages fields as diverse as linguistics, sociology, and cognitive science. Previous empirical attempts to solve this puzzle all presuppose that formal or informal institutions, such as incentives for global agreement, coordinated leadership, or aggregated information about the population, are needed to facilitate a solution. Evolutionary theories of social conventions, by contrast, hypothesize that such institutions are not necessary in order for social conventions to form. However, empirical tests of this hypothesis have been hindered by the difficulties of evaluating the real-time creation of new collective behaviors in large decentralized populations. Here, we present experimental results—replicated at several scales—that demonstrate the spontaneous creation of universally adopted social conventions and show how simple changes in a population’s network structure can direct the dynamics of norm formation, driving human populations with no ambition for large scale coordination to rapidly evolve shared social conventions.Social conventions are the foundation for social and economic life (1–7), However, it remains a central question in the social, behavioral, and cognitive sciences to understand how these patterns of collective behavior can emerge from seemingly arbitrary initial conditions (2–4, 8, 9). Large populations frequently manage to coordinate on shared conventions despite a continuously evolving stream of alternatives to choose from and no a priori differences in the expected value of the options (1, 3, 4, 10). For instance, populations are able to produce linguistic conventions on accepted names for children and pets (11), on common names for colors (12), and on popular terms for novel cultural artifacts, such as referring to junk email as “SPAM” (13, 14). Similarly, economic conventions, such as bartering systems (2), beliefs about fairness (3), and consensus regarding the exchangeability of goods and services (15), emerge with clear and widespread agreement within economic communities yet vary broadly across them (3, 16).Prominent theories of social conventions suggest that institutional mechanisms—such as centralized authority (14), incentives for collective agreement (15), social leadership (16), or aggregated information (17)—can explain global coordination. However, these theories do not explain whether, or how, it is possible for conventions to emerge when social institutions are not already in place to guide the process. A compelling alternative approach comes from theories of social evolution (2, 18–20). Social evolutionary theories maintain that networks of locally interacting individuals can spontaneously self-organize to produce global coordination (21, 22). Although there is widespread interest in this approach to social norms (6, 7, 14, 18, 23–26), the complexity of the social process has prevented systematic empirical insight into the thesis that these local dynamics are sufficient to explain universally adopted conventions (27, 28).Several difficulties have limited prior empirical research in this area. The most notable of these limitations is scale. Although compelling experiments have successfully shown the creation of new social conventions in dyadic and small group interactions (29–31), the results in small group settings can be qualitatively different from the dynamics in larger groups (Model), indicating that small group experiments are insufficient for demonstrating whether or how new conventions endogenously form in larger populations (32, 33). Important progress on this issue has been made using network-based laboratory experiments on larger groups (15, 24). However, this research has been restricted to studying coordination among players presented with two or three options with known payoffs. Natural convention formation, by contrast, is significantly complicated by the capacity of individuals to continuously innovate, which endogenously expands the “ecology” of alternatives under evaluation (23, 29, 31). Moreover, prior experimental studies have typically assumed the existence of either an explicit reward for universal coordination (15) or a mechanism that aggregates and reports the collective state of the population (17, 24), which has made it impossible to evaluate the hypothesis that global coordination is the result of purely local incentives.More recently, data science approaches to studying norms have addressed many of these issues by analyzing behavior change in large online networks (34). However, these observational studies are limited by familiar problems of identification that arise from the inability to eliminate the confounding influences of institutional mechanisms. As a result, previous empirical research has been unable to identify the collective dynamics through which social conventions can spontaneously emerge (8, 34–36).We addressed these issues by adopting a web-based experimental approach. We studied the effects of social network structure on the spontaneous evolution of social conventions in populations without any resources to facilitate global coordination (9, 37). Participants in our study were rewarded for coordinating locally, however they had neither incentives nor information for achieving large scale agreement. Further, to eliminate any preexisting bias in the evolutionary process, we studied the emergence of arbitrary linguistic conventions, in which none of the options had any a priori value or advantage over the others (3, 23). In particular, we considered the prototypical problem of whether purely local interactions can trigger the emergence of a universal naming convention (38, 39).     

Direct numerical simulations of aeolian sand ripples

Aeolian sand beds exhibit regular patterns of ripples resulting from the interaction between topography and sediment transport. Their characteristics have been so far related to reptation transport caused by the impacts on the ground of grains entrained by the wind into saltation. By means of direct numerical simulations of grains interacting with a wind flow, we show that the instability turns out to be driven by resonant grain trajectories, whose length is close to a ripple wavelength and whose splash leads to a mass displacement toward the ripple crests. The pattern selection results from a compromise between this destabilizing mechanism and a diffusive downslope transport which stabilizes small wavelengths. The initial wavelength is set by the ratio of the sediment flux and the erosion/deposition rate, a ratio which increases linearly with the wind velocity. We show that this scaling law, in agreement with experiments, originates from an interfacial layer separating the saltation zone from the static sand bed, where momentum transfers are dominated by midair collisions. Finally, we provide quantitative support for the use of the propagation of these ripples as a proxy for remote measurements of sediment transport.Observers have long recognized that wind ripples (1, 2) do not form via the same dynamical mechanism as dunes (3). Current explanations ascribe their emergence to a geometrical effect of solid angle acting on sediment transport. The motion of grains transported in saltation is composed of a series of asymmetric trajectories (4–7) during which they are accelerated by the wind. These grains, in turn, decelerate the airflow inside the transport layer (1, 7–12). On hitting the sand bed, they release a splash-like shower of ejected grains that make small hops from the point of impact (1, 13, 14). This process is called reptation. Previous wind ripple models assume that saltation is insensitive to the sand bed topography and forms a homogeneous rain of grains approaching the bed at a constant oblique angle (15–20). Upwind-sloping portions of the bed would then be submitted to a higher impacting flux than downslopes (1). With a number of ejecta proportional to the number of impacting grains, this effect would produce a screening instability with an emergent wavelength λ determined by the typical distance over which ejected grains are transported (15–17), a few grain diameters d. However, observed sand ripple wavelengths are about 1,000 times larger than d, on Earth. The discrepancy is even more pronounced on Mars, where regular ripples are 20–40 times larger than those on a typical Earth sand dune (21, 22). Moreover, the screening scenario predicts a wavelength independent of the wind shear velocity u_?, in contradiction with field and wind tunnel measurements that exhibit a linear dependence of λ with u_? (23–25).     

Fucoid brown algae inject fucoidan carbon into the ocean

Brown algae annually convert gigatons of carbon dioxide into carbohydrates, including the complex extracellular matrix polysaccharide fucoidan. Due to its persistence in the environment, fucoidan is potentially a pathway for marine carbon sequestration. Rates of fucoidan secretion by brown algae remain unknown due to the challenge of identifying and quantifying complex polysaccharides in seawater. We adapted the techniques of anion exchange chromatography, enzyme-linked immunosorbent assay, and biocatalytic enzyme-based assay for detection and quantification of fucoidan. We found the brown alga Fucus vesiculosus at the Baltic Sea coast of south-west Finland to secrete 0.3% of their biomass as fucoidan per day. Dissolved fucoidan concentrations in seawater adjacent to algae reached up to 0.48 mg L⁻¹. Fucoidan accumulated during incubations of F. vesiculosus, significantly more in light than in darkness. Maximum estimation by acid hydrolysis indicated fucoidan secretion at a rate of 28 to 40 mg C kg⁻¹ h⁻¹, accounting for 44 to 50% of all exuded dissolved organic carbon. Composed only of carbon, oxygen, hydrogen, and sulfur, fucoidan secretion does not consume nutrients enabling carbon sequestration independent of algal growth. Extrapolated over a year, the algae sequester more carbon into secreted fucoidan than their biomass. The global utility of fucoidan secretion is an alternative pathway for carbon dioxide removal by brown algae without the need to harvest or bury algal biomass.

Brown algae fix more carbon per area than terrestrial forests (1–4), and brown algal biomass is considered a potential carbon sink (5–9). However, algae release 14 to 35% of net primary production as dissolved molecules (10–15). Whether these exudates fuel microbial remineralization or contribute to carbon sequestration through recalcitrance or export to the deep sea remains under debate. Dissolved organic carbon concentrations decrease offshore with distance to kelp forests, but the high molecular weight polysaccharide fraction has shown considerable persistence (12, 16, 17). Remineralization rates of carbohydrates depend on the molecular structure (18). There is a need to constrain the molecular composition of brown algal exudates to understand their role in the ecosystem and in the global carbon cycle.Primary producers, including algae, fix carbon into carbohydrates (19), which serve a multitude of functions. Brown algae mainly synthesize the polysaccharides laminarin, cellulose, alginate, and fucoidan: laminarin and cellulose from the immediate photosynthesis product glucose, alginate from uronic acids, and fucoidan from fucose. Fucoidan can be decorated with additional monosaccharides, as well as acetyl and sulfate groups giving fucoidan a net negative charge (20–22). While laminarin stores energy within cells (21), cellulose and alginate fulfill structural functions as cell wall constituents (20). Fucoidan builds the extracellular matrix or mucilage in brown algae (23, 24), increases in response to salinity (24, 25), and acts as an antimicrobial agent (26).Fucoidan is delivered to the extracellular matrix by vesicle cells that secrete fucoidan into mucilage ducts (25, 27, 28). Thereby, brown algae maintain a mucilage barrier primarily consisting of fucoidan (20, 22, 27). Secretion of carbohydrates is a feature of epithelia and can mediate substantial carbon flux (29, 30). The high solubility of fucoidan and its location at the algae–seawater interface suggest that fucoidan has to be constantly resupplied.Fucoidan shows considerable recalcitrance and has been found to persist for centuries in sediments. Dissolved molecules can sequester carbon if they are not degraded due to recalcitrance or if they are exported to below 1,000-m depth (31, 32), for example after aggregation (33–35). Dissolved energy storage polysaccharides like laminarin are preferentially targeted by bacteria of different classes (36–38) that need only three enzymes for complete degradation within hours (37, 39–41). In contrast, fucoidan can only be degraded by specialized bacteria that require dozens of different enzymes (35, 42). Fucoidan-targeting antibodies revealed fucose-containing sulfated polysaccharides from diatoms assemble and form particles (34, 35) that transport carbon to depth. Consistent antibody signals along sediment cores indicate that fucoidan persists during transport through the water column and in sediments for hundreds of years (43, 44). Thus, mucus secretion by brown algae could mediate carbon sequestration through the recalcitrance and assembly of fucoidan.We hypothesized that brown algae secrete fucoidan into seawater based on reports of high carbohydrate content in dissolved organic carbon from brown algae (10, 12). Without methods for dissolved polysaccharide quantification, fucoidan concentrations in seawater remain unknown (35). We analyzed seawater from seagrass and brown algae incubations with approaches dedicated to specific and quantitative carbohydrate detection. Monosaccharide quantification, antibody binding, anion exchange chromatography, and enzymatic hydrolysis conclusively point toward the accumulation of dissolved fucoidan during algae incubations. Fucoidan carbon contributed substantially to the alga-released organic carbon. Our results suggest secreted fucoidan to sequester carbon at the same rate as carbon is stored in biomass.     

Enhancing ion transport in charged block copolymers by stabilizing low symmetry morphology: Electrostatic control of interfaces

Recently, the interest in charged polymers has been rapidly growing due to their uses in energy storage and transfer devices. Yet, polymer electrolyte-based devices are not on the immediate horizon because of the low ionic conductivity. In the present study, we developed a methodology to enhance the ionic conductivity of charged block copolymers comprising ionic liquids through the electrostatic control of the interfacial layers. Unprecedented reentrant phase transitions between lamellar and A15 structures were seen, which cannot be explained by well-established thermodynamic factors. X-ray scattering experiments and molecular dynamics simulations revealed the formation of fascinating, thin ionic shell layers composed of ionic complexes. The ionic liquid cations of these complexes predominantly presented near the micellar interfaces if they had strong binding affinity with the charged polymer chains. Therefore, the interfacial properties and concentration fluctuations of the A15 structures were crucially dependent on the type of tethered acid groups in the polymers. Overall, the stabilization energies of the A15 structures were greater when enriched, attractive electrostatic interactions were present at the micellar interfaces. Contrary to the conventional wisdom that block copolymer interfaces act as “dead zone” to significantly deteriorate ion transport, this study establishes a prospective avenue for advanced polymer electrolyte having tailor-made interfaces.

Block copolymers have attracted extensive attention over the past decades owing to their fascinating self-assembly into nanoscale, periodic structures (1, 2). Due to past industrial demands for polymer processing, early studies on block copolymers focused on the modulation of their rheological properties through the incorporation of selective or neutral solvents (3, 4). Such steadfast research efforts on block copolymer thermodynamics, with and without additives, have established the Flory–Huggins interaction parameter (χ) and the block volume fraction (f), which are universal factors used to determine phase diagrams (1, 5–7).Variable χ values of functionalized block copolymers were found to yield high-fidelity, nanoscale morphologies (8, 9). This was noted to promote advances in the controlled synthesis of block copolymers tethered with functional moieties (10, 11), specifically, the facile tuning of nanostructures (12, 13) and even the creation of hierarchical morphologies (14). The diverse functionality of tailor-made block copolymers enables their use in various applications, such as magnetic storage devices (15), nanoporous membranes (16), and nano-optics technologies (17). Particularly, the covalent and physical incorporation of ions into block copolymers has greatly reduced potential safety threats of energy storage and transfer devices (18, 19). Therefore, in recent years, this has emerged as a fundamental and imperative technology for developing electrochemical and electromechanical systems (20, 21).Notably, recent experimental (22–24) and theoretical studies (25–28) have revealed unique self-assembly behaviors of charged block copolymers. Examples of this include a series of experimental work of Park et al. (13, 24, 29–32) and theoretical investigation of Olvera de la Cruz et al. (26–28). These works demonstrated that if electrostatic interactions existed in one of the blocks, the phase boundaries between ordered morphologies radically shifted. Accordingly, even with minor ionic blocks (f_{ionic block} < 0.5), hexagonally packed cylindrical (HEX) structures with ionic matrices were formed (26, 29). The strength of the long-range interactions within these ionic blocks played a crucial role in modifying chain stretching and interfacial curvatures.Moreover, Park and coworkers have unveiled remarkably rich, three-dimensional morphologies, such as face-centered cubic (FCC), bicontinuous gyroid, and orthorhombic Fddd, from charged block copolymers through the fine control of intermolecular interactions (13, 31). These are noteworthy in two respects. First, although network morphologies are desirable for advanced polymer electrolytes with efficient ion conduction, they have not been easy to access, especially in the presence of electrostatic interactions. Second, all of the aforementioned morphologies possess major ionic phases, which greatly contribute to improving the ionic conductivity of block copolymer electrolytes. These are the first investigations seeking to boost the ion transport efficiency of block copolymer electrolytes, which namely focused on establishing thermodynamics and a morphology–transport relationship.Recent discoveries of topologically close-packed phases from a few block copolymers with high conformational asymmetry opened a new forum for polymer science, as implicated by Shi et al. (33, 34), Bates et al. (35–37), and Mahanthappa et al. (37). To introduce interfacial curvature into low symmetry morphologies, a universally applied approach includes varying the chemical structures of the repeating units of the block copolymers to ensure they have largely dissimilar statistical segment lengths (38, 39). Even though the stability window of such morphologies is known to be very narrow, block copolymer/homopolymer blends (40–42), asymmetric block copolymer/symmetric block copolymer blends (43, 44), and block oligomer/water mixtures (45) have been recently found to exhibit Frank–Kasper phases and fascinating phase transitions among them.Nevertheless, examples of low symmetry morphologies constructed from charged block copolymers remain scarce; therefore, the mechanism for stabilizing such polymorphs with prevailing long-range intermolecular interactions is unknown. Here, we report the stabilization of A15 structure, a type of the Frank–Kasper phases, of charged block copolymers through the control of electrostatic interactions. A methodology that enables prediction and easy access of A15 phases for charged block copolymers was established, which has not been reported in the literature on polymer electrolytes. By combining X-ray scattering experiments and molecular dynamics (MD) simulations, it was revealed that interfacial charge distribution and fluctuation played a key role in stabilizing the A15 structures of the charged block copolymers. Furthermore, when comparing the A15-forming samples with their lamellar-forming counterparts, the ionic conductivity of the former was found to be greater than that of the latter by one order of magnitude.     

Conformation-sensitive antibody reveals an altered cytosolic PAS/CNBh assembly during hERG channel gating

The human ERG (hERG) K⁺ channel has a crucial function in cardiac repolarization, and mutations or channel block can give rise to long QT syndrome and catastrophic ventricular arrhythmias. The cytosolic assembly formed by the Per-Arnt-Sim (PAS) and cyclic nucleotide binding homology (CNBh) domains is the defining structural feature of hERG and related KCNH channels. However, the molecular role of these two domains in channel gating remains unclear. We have previously shown that single-chain variable fragment (scFv) antibodies can modulate hERG function by binding to the PAS domain. Here, we mapped the scFv2.12 epitope to a site overlapping with the PAS/CNBh domain interface using NMR spectroscopy and mutagenesis and show that scFv binding in vitro and in the cell is incompatible with the PAS interaction with CNBh. By generating a fluorescently labeled scFv2.12, we demonstrate that association with the full-length hERG channel is state dependent. We detect Förster resonance energy transfer (FRET) with scFv2.12 when the channel gate is open but not when it is closed. In addition, state dependence of scFv2.12 FRET signal disappears when the R56Q mutation, known to destabilize the PAS–CNBh interaction, is introduced in the channel. Altogether, these data are consistent with an extensive structural alteration of the PAS/CNBh assembly when the cytosolic gate opens, likely favoring PAS domain dissociation from the CNBh domain.

Members of the KCNH superfamily of voltage-gated K⁺ channels contribute to neuronal excitability, cardiac repolarization, and cellular proliferation and are linked to human disease (1–12). In particular, the human ERG (hERG) channel has a crucial role in repolarization of the cardiac action potential; channel malfunction, either from genetic alterations or from unwanted pharmacological channel block, results in long QT syndrome (LQTS), a condition associated with cardiac arrhythmias and sudden death (2, 13, 14).KCNH channels are characterized by a “non-domain swapped” architecture, where the voltage-sensor domain interfaces with the pore domain in the same subunit (15). While the domain architecture is present in other channels (16), what distinguishes KCNH channels is the conserved cytosolic assembly formed by the N-terminal PAS (Per-Arnt-Sim) domain and the C-terminal cyclic nucleotide binding homology (CNBh) domain (17–24). PAS domains are widespread in nature, sensing light, redox potential, or small molecules and mediating protein–protein interactions (25–27). CNBh domains closely resemble CNB domains but lack the ability to bind nucleotides (28–32). Instead, the C-terminal tail of the CNBh domain acts as an intrinsic ligand, occupying the same position as a cyclic nucleotide in a bona fide CNB domain (30, 31).It is well established that PAS and CNBh domains interact with each other in the channel (15, 22, 23, 33, 34). Mutations that interfere with this interaction, disrupt the CNBh intrinsic ligand, or destabilize the fold of the domains give rise to changes in hERG gating. Many are associated with type 2 LQTS (LQT2) (29–31, 35–41). It is also clear that hERG function is modulated by variation in the number of PAS/CNBh assemblies present in individual channels, resulting from heteromers of two isoforms, one isoform with the PAS domain (hERG1a) and another without (hERG1b) (42–47).Further clues about the role of PAS and CNBh domains have been provided by ligands that target the cytosolic channel domains. We have demonstrated that allosteric modulation of hERG channel function through the PAS domain is possible by using scFv (single-chain variable fragment) antibodies (scFv2.10 and scFv2.12) that bind to PAS at distinct epitopes (48). scFv2.10 binds to residues R4 and R5 of hERG, in the PAS-cap region that spans the first 25 residues of the channel N terminus, just before the globular region of the PAS domain. In contrast, scFv2.12 binds to a region in the globular domain. In addition, small-molecule screening campaigns have identified ligands of PAS and CNBh domains that affect channel function (49–52). These data suggest that the assembly formed by the PAS and CNBh domains has an important role in the mechanism of gating of KCNH channels.Comparison of the cryoelectron microscopy (cryo-EM) structures of hERG, with an open cytosolic gate, and the calmodulin-inhibited rat EAG (another KCNH channel), with a closed gate, provides clear insights about the role of the PAS-cap in gating (15, 34). The PAS-cap engages the channel gating machinery when the gate opens, with its N terminus trapped between the C-linker, the S4-S5 linker, and the S2–S3 cytosolic loop. When the channel closes, the C-linker moves away, widening the PAS-cap binding site and releasing the N terminus. In contrast, the cryo-EM structure comparison shows that the PAS/CNBh assembly is not altered, even relative to the crystal structure of the isolated complex (33), undergoing only a simple rigid-body rotation (15, 34, 53). The overall view is that that role of the PAS/CNBh assembly is limited to correctly position the N-terminal PAS-cap for engagement with the gating machinery.Here, we propose a model in which the PAS/CNBh domain assembly is an active participant in the mechanism of hERG channel gating, undergoing a stabilization/destabilization cycle during hERG gating. This proposal results from the characterization of the molecular basis for the functional effect of scFv2.12 on the hERG channel—defining the antibody’s epitope, determining the impact of antibody binding on the PAS interaction with the CNBh domain in vitro and in the cell, and finally from monitoring the association of a fluorescent scFv2.12 antibody to the PAS domain during hERG gating.     

On the question of whether lubricants fluidize in stick–slip friction

Intermittent sliding (stick–slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick–slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4–0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.Intermittent sliding (stick–slip) of solids in contact is an everyday effect, such as in the squeak of hinges or the music of violins, when the bow slides past the strings, or, at a different scale, in earthquakes (where tectonic plates slide past each other). Such solid sliding is a major cause of frictional dissipation, and can persist even in the presence of lubricants (1). At a nanotribological level, surface force balance (SFB) measurements, supported by theory and computer simulations, have shown that when simple organic liquids are confined between atomically smooth, solid (mica) surfaces to films thinner than some six to eight molecular layers, they may become solid-like, and are often layered (2–14). Subsequent sliding of the surfaces across such films when they are subjected to shear may then take place via stick–slip motion (15, 16). During the stick part, the surfaces are in rigid contact until the shear force between them exceeds the static friction, at which point they slip rapidly past each other (relaxing the shear stress) and then stick again, in a repeating cycle. The issue of how the confined (lubricant) layer progressively yields and then becomes rigid again during such stick–slip sliding has been intensely studied over the past several decades, not least because a better understanding may result in improved lubrication approaches.The molecular basis of the stick–slip cycle in sheared solid-like lubricant films as described above is not well understood (17–28). This is at least in part because, experimentally, it is very challenging to capture what happens to the lubricant layer during the fleeting, individual slip events taking place in the nanometrically confined film. Even when measured under controlled conditions, as in the SFB, these slip events are not only of very short duration [ca. 20 ms (18)] but generally occupy only a tiny fraction of the stick–slip cycle, with the surfaces in nonsliding contact (stick) for almost the entire cycle period. For this reason, much of our understanding has been derived from theoretical modeling and computer simulation studies (17, 19–25, 27–29). Classically, these almost all suggest that the stick–slip motion involves periodic shear melting transitions and resolidification of the film as it undergoes transition between solid-like and liquid-like phases during sliding. Even where there is some disagreement in the model details [for example, on the precise mechanism by which the films solidify at the end of the slip (22, 25)], they maintain the essential idea of fluidization of the lubricant layers during the slip part of the stick–slip cycle. In the shear-induced solid to liquid transition (fluidization), a density change is also expected because the fluidized phase is less dense than the solid phase. This leads to a volumetric expansion and contraction cycle (corresponding respectively to slip and stick), with a dilation of the thin lubricant film during the slip event (17, 23, 25, 27). Some more recent simulations suggest that slip may occur at the wall–fluid interfaces or via interlayer slip within the film rather than via film melting (19, 27, 28), although the scenario of lubricant fluidization during slip is the generally accepted mechanism.There have been few experimental studies on individual slips during stick–slip sliding across lubricant films, and none where the film thickness in such fleeting events has been examined (15, 16, 18, 30–32). Clues may also be extracted from stick–slip motion of confined granular systems under shear, where numerical simulations (33, 34) and some experiments (35–37) suggest that fluidization and dilation may play a role in the stick–slip instability. While this is suggestive, differences between granular layers and lubricant films include not only five orders of magnitude between size of grains and of molecules but, in particular, the issue of molecular interactions, negligible in granular shear but all-important when shearing lubricants.In the present study, we examine directly the individual slip events during stick–slip sliding across thin lubricant films, and in particular the issue of film dilation during the fleeting slip motion itself. This is done to provide “smoking gun” evidence concerning the issue of film fluidization, where such dilation is expected to be a clear signature. We confine a thin (few nanometers) model liquid film between smooth solid surfaces in an SFB, shear it, and monitor the film thickness during stick–slip sliding via fast video microscopy. To overcome the major challenge presented by the shortness of the slip events, which occupy only some 1% of the stick–slip cycle over which a subnanometer dilation needs to be detected against a comparable level of noise, we analyze our data using tools from classical signal detection theory to correlate the slip events with the instantaneous value of the film thickness.     

Opacity and conductivity measurements in noble gases at conditions of planetary and stellar interiors

The noble gases are elements of broad importance across science and technology and are primary constituents of planetary and stellar atmospheres, where they segregate into droplets or layers that affect the thermal, chemical, and structural evolution of their host body. We have measured the optical properties of noble gases at relevant high pressures and temperatures in the laser-heated diamond anvil cell, observing insulator-to-conductor transformations in dense helium, neon, argon, and xenon at 4,000–15,000 K and pressures of 15–52 GPa. The thermal activation and frequency dependence of conduction reveal an optical character dominated by electrons of low mobility, as in an amorphous semiconductor or poor metal, rather than free electrons as is often assumed for such wide band gap insulators at high temperatures. White dwarf stars having helium outer atmospheres cool slower and may have different color than if atmospheric opacity were controlled by free electrons. Helium rain in Jupiter and Saturn becomes conducting at conditions well correlated with its increased solubility in metallic hydrogen, whereas a deep layer of insulating neon may inhibit core erosion in Saturn.Noble gases play important roles in the evolution and dynamics of planets and stars, especially where they appear in a condensed, purified state. In gas giant planets, helium and neon can precipitate as rain in metallic hydrogen envelopes, leading to planetary warming and specifically the anomalously slow cooling of Saturn (1–8). In white dwarf stars cooling can be especially fast due to the predicted low opacity of dense helium atmospheres, affecting the calibration of these objects as cosmological timekeepers (9–12). In these systems, the transformation of dense noble gases (particularly He) from optically transparent insulators to opaque electrical conductors is of special importance (2, 9, 11, 12).Dense noble gases are expected to show systematic similarities in their properties at extreme conditions (13–17); however, a general understanding of their insulator–conductor transformation remains to be established. Xe is observed to metallize near room temperature under pressures similar to those at Earth’s core–mantle boundary (18, 19). Ar and He are observed to conduct only at combined high pressure and temperature (12, 13, 17). Ne is predicted to have the highest metallization pressure of all known materials—10³ times that of Xe and 10 times that of He (14, 18, 20, 21)—and has never been documented outside of its insulating state. Experimental probes of extreme densities and temperatures in noble gases have previously relied on dynamic compression by shock waves (12, 13, 17, 22–24). However, in such adiabatic experiments, light and compressible noble gases heat up significantly and can ultimately reach density maxima (12, 13, 17, 21, 24, 25), so that conditions created often lie far from those deep within planets (7, 8) and stars (9).Here we report experiments in the laser-heated diamond anvil cell (15, 16, 26–29) on high-density and high-temperature states of the noble gases Xe, Ar, Ne, and He (Fig. 1). Rapid heating and cooling of compressed samples using pulsed laser heating (26, 27) is coupled with time domain spectroscopy of thermal emission (26) to determine sample temperature and transient absorption to establish corresponding sample optical properties (Figs. S1 and S2). A sequence of heat cycles to increasing temperature documents optical changes in these initially transparent insulators.Open in a separate windowFig. 1.Creating and probing extreme states of noble gases. (A) Configuration of laser heating and transient absorption probing of the diamond anvil cell, with probe beams transmitted through the cell into the detection system. (B) Microscopic view of the diamond cell cavity, which contains a noble gas sample and a metal foil (Ir) which converts laser radiation to heat and has small hole at the heated region through which probe beams are transmitted to test optical character of samples. (C) Finite element model (26) (Fig. S3) of the temperature distribution in heated Ar at 51 GPa (Fig. 2), with solid–melt (16) and insulator–conductor (α = 0.1 μm⁻¹) boundaries in the sample marked dashed and dotted, respectively. (D) Schematic of time domain probing during transient heating. Temperature is determined from thermal emission (red) and absorption from transmitted probe beams: a continuous laser (cw; green) and pulsed supercontinuum broadband (bb; blue).