Kinetic Signatures Research Articles

How Salt and Temperature Drive Reentrant Condensation of Aβ40.

Within the framework of liquid-liquid phase separation (LLPS), biomolecular condensation orchestrates vital cellular processes, and its dysregulation is implicated in severe pathological conditions. Recent studies highlight the role of intrinsically disordered proteins (IDPs) in LLPS, yet the influence of microenvironmental factors has remained a puzzling factor. Here, via computational simulation of the impact of solution conditions on LLPS behavior of neurologically pathogenic IDP Aβ40, we chanced upon a salt-driven reentrant condensation phenomenon, wherein Aβ40 aggregation increases with low salt concentrations (25-50 mM), followed by a decline with further salt increments. An exploration of the thermodynamic and kinetic signatures of reentrant condensation unveils a nuanced interplay between protein electrostatics and ionic strength as potential drivers. Notably, the charged residues of the N-terminus exhibit a nonmonotonic response to salt screening, intricately linked to the recurrence of reentrant behavior in hydrophobic core-induced condensation. Intriguingly, our findings also unveil the reappearance of similar reentrant condensation phenomena under varying temperature conditions. Collectively, our study illuminates the profoundly context-dependent nature of Aβ40s liquid-liquid phase separation behavior, extending beyond its intrinsic molecular framework, where microenvironmental cues wield significant influence over its aberrant functionality.

Solar Orbiter Observations of Proton and Alpha Particle Kinetic Signatures Related to the Presence of Switchbacks in the Inner Heliosphere: A Case Study

We investigate how ions, namely protons and alpha particles, kinetically react to the presence of strong deflections in the magnetic field, the so-called switchbacks, in the first stream of slow Alfvénic wind observed by Solar Orbiter at the heliocentric distance of 0.64 au. We focus on an isolated, large-scale switchback, and we study in detail ion kinetic properties. Beyond the expected correlation between the magnetic deflection and ion velocity related to the Alfvénic nature of the switchbacks, we find that, within the switchback, proton and alpha particle densities increase, suggesting ongoing wave activity. Very interestingly, we observe a clear correlation between the magnetic deflection and alpha particle temperature, while no correlation has been found with proton temperature. This is an indication of a possible role played by switchbacks in preferentially heating heavy ions. Our results suggest that the presence of switchbacks can induce a deformation of the proton velocity distribution function, while the preferential heating of alpha particles could be due to a denser secondary beam and a smaller relative drift speed between the beam and core.

Reconstitution of the alternative pathway of the complement system enables rapid delineation of the mechanism of action of novel inhibitors

The complement system plays a critical role in the innate immune response, acting as a first line of defense against invading pathogens. However, dysregulation of the complement system is implicated in the pathogenesis of numerous diseases, ranging from Alzheimer's to age-related macular degeneration and rare blood disorders. As such, complement inhibitors have enormous potential to alleviate disease burden. While a few complement inhibitors are in clinical use, there is still a significant unmet medical need for the discovery and development of novel inhibitors to treat patients suffering from disorders of the complement system. A key hurdle in the development of complement inhibitors has been the determination of their mechanism of action. Progression along the complement cascade involves the formation of numerous multimeric protein complexes, creating the potential for inhibitors to act at multiple nodes in the pathway. This is especially true for molecules that target the central component C3 and its fragment C3b, which serve a dual role as a substrate for the C3 convertases and as a scaffolding protein in both the C3 and C5 convertases. Here, we report a step-by-step invitro reconstitution of the complement alternative pathway using bio-layer interferometry. By physically uncoupling each step in the pathway, we were able to determine the kinetic signature of inhibitors that act at single steps in the pathway and delineate the full mechanism of action of known and novel C3 inhibitors. The method could have utility in drug discovery and further elucidating the biochemistry of the complement system.

Switch-2 determines Mg2+ADP-release kinetics and fine-tunes the duty ratio of Dictyostelium class-1 myosins.

Though myosins share a structurally conserved motor domain, single amino acid variations of active site elements, including the P-loop, switch-1 and switch-2, which act as nucleotide sensors, can substantially determine the kinetic signature of a myosin, i.e., to either perform fast movement or enable long-range transport and tension generation. Switch-2 essentially contributes to the ATP hydrolysis reaction and determines product release. With few exceptions, class-1 myosin harbor a tyrosine in the switch-2 consensus sequence DIYGFE, at a position where class-2 myosins and a selection of myosins from other classes have a substitution. Here, we addressed the role of the tyrosine in switch-2 of class-1 myosins as potential determinant of the duty ratio. We generated constitutively active motor domain constructs of two class-1 myosins from the social amoeba Dictyostelium discoideum, namely, Myo1E, a high duty ratio myosin and Myo1B, a low duty ratio myosin. In Myo1E we introduced mutation Y388F and in Myo1B mutation F387Y. The detailed functional characterization by steady-state and transient kinetic experiments, combined with in vitro motility and landing assays revealed an almost reciprocal relationship of a number of critical kinetic parameters and equilibrium constants between wild-type and mutants that dictate the lifetime of the strongly actin-attached states of myosin. The Y-to-F mutation increased the duty ratio of Moy1B by almost one order of magnitude, while the introduction of the phenylalanine in switch-2 of Myo1E transformed the myosin into a low duty ratio motor. These data together with structural considerations propose a role of switch-2 in fine-tuning ADP release through a mechanism, where the class-specific tyrosine together with surrounding residues contributes to the coordination of Mg2+ and ADP. Our results highlight the importance of conserved switch-2 residues in class-1 myosins for efficient chemo-mechanical coupling, revealing that switch-2 is important to adjust the duty ratio of the amoeboid class-1 myosins for performing movement, transport or gating functions.

Kinetic Signatures, Dawn‐Dusk Asymmetries, and Flux Transfer Events Associated With Mercury's Dayside Magnetopause Reconnection From 3D MHD‐AEPIC Simulations

AbstractMercury possesses a miniature yet dynamic magnetosphere driven primarily by magnetic reconnection occurring regularly at the magnetopause and in the magnetotail. Using the newly developed Magnetohydrodynamics with Adaptively Embedded Particle‐in‐Cell (MHD‐AEPIC) model coupled with planetary interior, we have performed a series of global simulations with a range of upstream conditions to study in detail the kinetic signatures, asymmetries, and flux transfer events (FTEs) associated with Mercury's dayside magnetopause reconnection. By treating both ions and electrons kinetically, the embedded PIC model reveals crescent‐shaped phase‐space distributions near reconnection sites, counter‐streaming ion populations in the cusp region, and temperature anisotropies within FTEs. A novel metric and algorithm are developed to automatically identify reconnection X‐lines in our 3D simulations. The spatial distribution of reconnection sites as modeled by the PIC code exhibits notable dawn‐dusk asymmetries, likely due to such kinetic effects as X‐line spreading and Hall effects. Across all simulations, simulated FTEs occur quasi‐periodically every 4–9 s. The properties of simulated FTEs show clear dependencies on the upstream solar wind Alfvénic Mach number (MA) and the interplanetary magnetic field orientation, consistent with MESSENGER observations and previous Hall‐MHD simulations. FTEs formed in our MHD‐AEPIC model tend to carry a large amount of open flux, contributing ∼3%–36% of the total open flux generated at the dayside. Taken together, our MHD‐AEPIC simulations provide new insights into the kinetic processes associated with Mercury's magnetopause reconnection that should prove useful for interpreting spacecraft observations, such as those from MESSENGER and BepiColombo.

Exploring self-consistent 2.5D flare simulations with MPI-AMRVAC

Context. Multidimensional solar flare simulations have not yet included a detailed analysis of the lower atmospheric responses, such as downflowing chromospheric compressions and chromospheric evaporation processes. Aims. We present an analysis of multidimensional flare simulations, including an analysis of chromospheric upflows and downflows that provides important groundwork for comparing 1D and multidimensional models. Methods. We followed the evolution of a magnetohydrodynamic standard solar flare model that includes electron beams and in which localized anomalous resistivity initiates magnetic reconnection. We varied the background magnetic field strength to produce simulations that cover a large span of observationally reported solar flare strengths. Chromospheric energy fluxes and energy density maps were used to analyze the transport of energy from the corona to the lower atmosphere, and the resultant evolution of the flare. Quantities traced along 1D field lines allowed for detailed comparisons with 1D evaporation models. Results. The flares produced by varying the background coronal field strength between 20 G and 65 G have GOES classifications between B1.5 and M2.3. All produce a lobster claw reconnection outflow and a fast shock in the tail of this flow with a similar maximum Alfvén Mach number of ∼10. The impact of the reconnection outflow on the lower atmosphere and the heat conduction are the key agents driving the chromospheric evaporation and “downflowing chromospheric compressions”. The peak electron beam heating flux in the lower atmospheres varies between 1.4 × 109 and 4.7 × 1010 erg cm−2 s−1 across the simulations. The downflowing chromospheric compressions have kinetic energy signatures that reach the photosphere, but at subsonic speeds they would not generate sunquakes. The weakest flare generates a relatively dense flare loop system, despite having a negative net mass flux, through the top of the chromosphere, that is to say, more mass is supplied downward than is evaporated upward. The stronger flares all produce positive mass fluxes. Plasmoids form in the current sheets of the stronger flares due to tearing, and in all experiments the loop tops contain turbulent eddies that ring via a magnetic tuning fork process. Conclusions. The presented flares have chromospheric evaporation driven by thermal conduction and the impact and rebound of the reconnection outflow, in contrast to most 1D models where this process is driven by the beam electrons. Several multidimensional phenomena are critical in determining plasma behavior but are not generally considered in 1D flare simulations. They include loop-top turbulence, reconnection outflow jets, heat diffusion, compressive heating from the multidimensional expansion of the flux tubes due to changing pressures, and the interactions of upward and downward flows from the evaporation meeting the material squeezed downward from the loop tops.

Fluid-mediated exchange reaction induces clumped isotope resetting: Insights from aragonite dolomitization experiments

Carbon and oxygen isotopic exchange between external and internal fluids of carbonate minerals can reset the clumped isotope composition (Δ47) of paleotemperature archives. Understanding the nature of this exchange during carbonate diagenetic alterations is essential for correctly interpreting the paleoenvironmental significance of Δ47-derived temperatures. Through batch hydrothermal experiments on aragonitic coral carbonates, we examine the kinetic signatures of clumped isotope fractionation from the initial disequilibrium to final equilibrium states during the dolomitization of the aragonite. By analyzing oxygen isotope fractionation and Sr partitioning between carbonate and fluid, we simulate the effect of exchange reactions on the Δ47 values. As the resetting feature of Δ47 values in our aragonite samples disagrees with the fractionation pattern observed in previous aragonite solid-state reordering experiment, we rule out the effect of the solid-state reordering. Our findings demonstrate that isotope exchanges with internal or external fluids coexist, particularly during the early stage of aragonite self-recrystallization. The results of this study indicate that the Δ47 value of coral aragonite is susceptible to internal exchange reactions in Mg-bearing solutions at 160–220 °C. Additionally, by modeling the bulk recrystallization and internal fluid-mineral exchange and evaluating their relative importance, we determine that the internal exchange reaction results in a nonlinear correlation between δ18O and Δ47 values. Importantly, we highlight the previously unrecognized diagenetic scenario in which the internal fluid-mineral exchange effect on the Δ47 value can outweigh the bulk recrystallization effect with external fluid coexisting. Consequently, our findings emphasize the need for a comprehensive assessment on the reaction kinetics for different types of recrystallization effects, to support the application of clumped isotope proxies for altered carbonate samples in diagenetic environments.

Local Proton Heating at Magnetic Discontinuities in Alfvénic and Non-Alfvénic Solar Wind

We investigate the local proton energization at magnetic discontinuities/intermittent structures and the corresponding kinetic signatures in velocity phase space in Alfvénic (high cross helicity) and non-Alfvénic (low cross helicity) wind streams observed by Parker Solar Probe. By means of the partial variance of increments method, we find that the hottest proton populations are localized around compressible, coherent magnetic structures in both types of wind. Analysis of parallel and perpendicular temperature distributions suggest that the Alfvénic wind undergoes preferential enhancements of T ∥ at such structures, whereas the non-Alfvénic wind experiences preferential T ⊥ enhancements. Although proton beams are present in both types of wind, the proton velocity distribution function displays distinct features. Hot beams, i.e., beams with beam-to-core perpendicular temperature T ⊥,b /T ⊥,c up to three times larger than the total distribution anisotropy, are found in the non-Alfvénic wind, whereas colder beams are in the Alfvénic wind. Our data analysis is complemented by 2.5D hybrid simulations in different geometrical setups, which support the idea that proton beams in Alfvénic and non-Alfvénic wind have different kinetic properties and different origins. The development of a perpendicular nonlinear cascade, favored in balanced turbulence, allows a preferential relative enhancement of the perpendicular plasma temperature and the formation of hot beams. Cold field-aligned beams are instead favored by Alfvén wave steepening. Non-Maxwellian distribution functions are found near discontinuities and intermittent structures, pointing to the fact that the nonlinear formation of small-scale structures is intrinsically related to the development of highly nonthermal features in collisionless plasmas. Our results contribute to understanding the role of different coherent structures in proton energization and their implication in collisionless energy dissipation processes in space plasmas.

Hydrogen tunneling with an atypically small KIE measured in the mediated decomposition of the Co(CH3COOH)+ complex.

Quantum mechanical tunneling (QMT) is a well-documented phenomenon in the C-H bond activation mechanism and is commonly identified by large KIE values. Herein we present surprising findings in the kinetic study of hydrogen tunneling in the Co+ mediated decomposition of acetic acid and its perdeuterated isotopologue, conducted with the energy resolved single photon initiated dissociative rearrangement reaction (SPIDRR) technique. Following laser activation, the reaction proceeds along parallel product channels Co(CH4O)+ + CO and Co(C2H2O)+ + H2O. An energetic threshold is observed in the energy dependence of the unimolecular microcanonical rate constants, k(E). This is interpreted as the reacting population surmounting a rate-limiting Eyring barrier in the reaction's potential energy surface. Measurements of the heavier isotopologue's reaction kinetics supports this interpretation. Kinetic signatures measured at energies below the Eyring barrier are attributed to H/D QMT. The below-the-barrier tunneling kinetics presents an unusually linear energy dependence and a staggeringly small tunneling KIE of ∼1.4 over a wide energy range. We explain this surprising observation in terms of a narrow tunneling barrier, wherein the electronic structure of the Co+ metal plays a pivotal role in enhanced reactivity by promoting efficient tunneling. These results suggest that hydrogen tunneling could play important functions in transition metal chemistry, such as that found in enzymatic mechanisms, even if small KIE values are measured.

A focus on capacitive cation sensing under flow: play it again SAM.

We highlight an article by Patrick et al. (S. C. Patrick, R. Hein, P. D. Beer and J. J. Davis, Chem. Sci., 2024, https://doi.org/10.1039/D4SC05271D) that integrates capacitance spectroscopy with a continuous flow assay to achieve reagentless electrochemical sensing. Injection of analytes leads to ion recruitment by the self-assembled monolayer (SAM) of benzo-15-crown-5 receptors generating spikes in the sensorgram. Injecting the supporting electrolyte regenerates the vacant receptor, returning the value to baseline. Binding and debinding kinetic signatures also emerge. The continuous and sensitive reporting of interfacial host-guest binding in real-time follows the precision of a well-composed melody that can be played again and again.

Analysis and interpretation of first passage time distributions featuring rare events.

In this contribution we consider theory and associated computational tools to treat the kinetics associated with competing pathways on multifunnel energy landscapes. Multifunnel landscapes are associated with molecular switches and multifunctional materials, and are expected to exhibit multiple relaxation time scales and associated thermodynamic signatures in the heat capacity. Our focus here is on the first passage time distribution, which is encoded in a kinetic transition network containing all the locally stable states and the pathways between them. This network can be renormalised to reduce the dimensionality, while exactly conserving the mean first passage time and approximately conserving the full distribution. The structure of the reduced network can be visualised using disconnectivity graphs. We show how features in the first passage time distribution can be associated with specific kinetic traps, and how the appearance of competing relaxation time scales depends on the starting conditions. The theory is tested for two model landscapes and applied to an atomic cluster and a disordered peptide. Our most important contribution is probably the reconstruction of the full distribution for long time scales, where numerical problems prevent direct calculations. Here we combine accurate treatment of the mean first passage time with the reliable part of the distribution corresponding to faster time scales. Hence we now have a fundamental understanding of both thermodynamic and kinetic signatures of multifunnel landscapes.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores.

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Reduction of metmyoglobin by inorganic disulfide species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by inorganic disulfide species has been studied by combined spectroscopic and kinetic analyses, under argon atmosphere. The process is kinetically characterized by biexponential time traces, for variable ratios of excess disulfide to protein, in the pH interval 6.6–8.0. Using UV–vis and resonance Raman spectroscopies, we observed that MbFeIII is converted into a low spin hexacoordinated ferric complex, tentatively assigned as MbFeIII(HSS−)/MbFeIII(SS2−), in an initial fast step. The complex is slowly converted into a pentacoordinated ferrous form, assigned as MbFeII according to the resonance Raman records. The reduction is a pH-dependent process, but independent of the initial disulfide concentration, suggesting the unimolecular decomposition of the intermediate complex following a reductive homolysis. We estimated the rate of the fast formation of the complex at pH 7.4 (kon = 3.7 × 103 M−1 s−1), and a pKa2 = 7.5 for the equilibrium MbFeIII(HSS−)/MbFeIII(SS2−). Also, we estimated the rate for the slow reduction at the same pH (kred = 10−2 s−1). A reaction mechanism compliant with the experimental results is proposed. This mechanistic study provides a differential kinetic signature for the reactions of disulfide compared to sulfide species on metmyoglobin, which may be considered in other hemeprotein systems.

Nanoparticle-Coupled Single-Molecule Kinetic Fingerprinting for Enzymatic Activity Detection.

The sensitive and accurate detection of biomarkers plays an important role in clinical diagnosis and drug discovery. Currently, amplification-based methods for biomarker detection are widely explored. However, the key challenges of these methods are limited reproducibility and high background noise. To overcome these limitations, we develop a robust plasmonic nanoparticle-coupled single-molecule kinetic fingerprinting (PNP-SMKF) method to achieve ultrasensitive detection of protein kinase A (PKA). Transient binding of a short fluorescent probe with the genuine target produces a distinct kinetic signature that is completely different from that of the background signal, allowing us to recognize PKA sensitively. Importantly, integrating a plasmonic nanoparticle efficiently breaks the concentration limit of the imager strand for single-molecule imaging, thus achieving a much faster imaging speed. A limit of detection (LOD) of as low as 0.0005 U/mL is readily realized. This method holds great potential as a versatile platform for enzyme detection and inhibitor screening in the future.

Abstract 5301: Detection of arginine posttranslational modifications by single-molecule protein sequencing on the Quantum-Si platform

Abstract Aberrant post-translational modifications (PTMs) on arginine residues, such as methylation and citrullination, are closely linked to oncogenic processes. Detection of these PTMs is challenging with current technology and can be hampered by low abundance and mass differences that are difficult to resolve by mass spectrometry. To overcome these challenges, we developed methods for PTM detection using Quantum-Si's Platinum single-molecule protein sequencing platform. Proteins are sequenced on Quantum-Si's Platinum instrument using our library prep protocol. Briefly, proteins are digested into peptide fragments, conjugated to linkers, then immobilized at the bottom of nanoscale reaction chambers on a semiconductor chip, resulting in exposed N-termini for sequencing. During sequencing, the immobilized peptides are exposed to a solution of dye-labeled N-terminal amino acid (NAA) recognizers that reversibly bind to their cognate NAAs with distinct kinetic properties. Aminopeptidases sequentially remove NAAs to expose subsequent amino acids for recognition. Fluorescence lifetime, intensity, and kinetic data are collected in real time and analyzed to determine primary sequence and PTM content. The data output consists of distinct pulsing regions called recognition segments (RSs). Each RS corresponds to a period of time between aminopeptidase cleavage events, during which an NAA recognizer binds on/off to its exposed target NAA. Chemical modification to a NAA or to a nearby downstream amino acid can modulate recognizer affinity, resulting in a characteristic change in the average pulse duration (PD) during an RS. To demonstrate the detection and differentiation of arginine PTMs, we applied our platform to distinguish between asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) and between citrulline and native arginine residues. Sequencing traces revealed that arginine and ADMA bind the arginine recognizer (PS621) with similar PD, whereas SDMA exhibited no binding. These results indicate that SDMA, in contrast to ADMA, has reduced binding affinity with PS621, providing a clear kinetic difference between these isomeric arginine PTMs. Moreover, citrulline and arginine side chains also exhibited distinguishable kinetic signatures. Citrullination eliminated the N-terminal arginine recognized by PS621 and resulted in a large increase in the median PD of preceding amino acid residues. Taken together, single-molecule protein sequencing offers an alternative approach to detection of arginine PTMs that is not based on m/z, but rather on the kinetic signature of binding between recognizers and N-terminal amino acids. The ability to directly detect arginine PTMs offers potential for biomedical research. We envisage applications using Quantum-Si's platform to key areas of cancer research including biomarker development and drug discovery. Citation Format: Kenneth Skinner, David Kamber, Brian Reed. Detection of arginine posttranslational modifications by single-molecule protein sequencing on the Quantum-Si platform. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5301.

Kinetic Nonequilibrium Signatures in the Distribution Function of Earth-Escaping Hydrogen Atoms

The lightest atmospheric gas constituents, like H- and He- atoms, are known to escape from planetary gravitational fields to open space. Hereby it counts that not only the uppermost atmospheric layer, the so-called exobase, contributes to this planetary gas escape, but the layers below do contribute as well, especially since from below a nonthermal character of the final particle outflow is induced.We consider the outflow of H - atoms from lower levels of a planetary oxygen-dominated upper atmosphere, stratified by the planet‘s gravitational field - as given in case of the Earth. The terrestrial H-atom outflow is locally modified by elastic collisions of upwards flying H-atoms with the heavy major atmospheric background constituent. This causes a collision–induced velocity-modulation of the upwards directed H-atom flow and induces nonthermal kinetic signatures of the local H ˗ atom distribution function. An important, hitherto unrespected point hereby is that angle-integrated elastic O ˗ H- collision cross sections are velocity-dependent, falling off with increasing velocity v like (1/v). Consequently this modulation influences low velocity H-atoms stronger than high-velocity ones, which changes the kinetic profile of the escaping H-atoms and causes deviations from the classic Jeans escape. Deeply down in the lower thermosphere the local H-atoms, like as well the O-atoms, indeed are in a thermodynamical equilibrium characterized by Maxwell distributions with a common temperature TH = TO. Nevertheless, at the upper exobase border of the atmosphere the resulting H-atom escape flow turns out to be a, non-equilibrium flow with non-thermal escape-relevant properties. Here we describe this collisional modification of the H-escape flow and quantify the upcome of kinetic non-equilibrium features like power laws in the wings of the H-distribution function. This collisional modulation via velocity-dependent collision cross-sections acts as a typical process to convert equilibrium distributions into non-equilibrium kappa-like distribution functions. On the basis of this theoretical approach and stabilizing the upper atmosphere by the type of so-called "iso-baric Kappa functions" which all represent the same H ˗ atom- pressure we can calculate the effective escape flux of H- atoms to open space and can quantify its difference with respect to the classical Jeans escape value. While finding again that the classical Jeans formula slightly overestimates the actual escape flux, we now for the first time taking into account the nonthermal influence of collisional modulations can show that the actual escape with respect to the actual Jeans value is even enhanced by factors 2 to 3.

Multivariate curve Resolution-Alternating least squares coupled with Raman microspectroscopy: new insights into the kinetic response of primary oral squamous carcinoma cells to cisplatin.

Raman MicroSpectroscopy (RMS) is a powerful label-free tool to probe the effects of drugs at a cellular/subcellular level. It is important, however, to be able to extract relevant biochemical and kinetic spectroscopic signatures of the specific cellular responses. In the present study, a combination of Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) and Principal Component Analysis (PCA) is used to analyse the RMS data for the example of exposure of primary Oral Squamous Carcinoma Cells (OSCC) to the chemotherapeutic agent cisplatin. Dosing regimens were established by cytotoxicity assays, and the effects of the drug on cellular spectral profiles were monitored from 16 to 72 hours post-exposure using an apoptosis assay, to establish the relative populations of viable (V), early (EA) and late apoptotic/dead (LA/D) cells after the drug treatment. Based on a kinetic model of the progression from V > EA > D, MCR-ALS regression analysis of the RMS responses was able to extract spectral profiles associated with each stage of the cellular responses, enabling a quantitative comparison of the response rates for the respective drug treatments. Moreover, PCA was used to compare the spectral profiles of the viable cells exposed to the drug. Spectral differences were highlighted in the early stages (16 hours exposure), indicative of the initial cellular response to the drug treatment, and also in the late stages (48-72 hours exposure), representing the cell death pathway. The study demonstrates that RMS coupled with multivariate analysis can be used to quantitatively monitor the progression of cellular responses to different drugs, towards future applications for label-free, in vitro, pre-clinical screening.

Linear viscoelastic shear and bulk relaxation moduli in poly(tetramethylene oxide) (PTMO) using united-atom molecular dynamics

A quantitative understanding and prediction of the viscoelastic relaxation behavior of polymer melts is required to build up predictive multiscale models, which can be utilized for the practical design of viscoelastic materials. We report the application of the Green–Kubo method to compute the common shear relaxation modulus and the rarely reported bulk relaxation modulus for a realistic united atom model of poly(tetramethylene oxide) (PTMO), which is one key components of the segmented polyurethane (PU) elastomers. The temperature and molecular weight dependent viscoelastic behavior is investigated in detail by computing the bulk relaxation modulus K(t) along with shear relaxation modulus G(t). Our results provide new rheological data for PTMO melt based on the united atom model, which incorporates the chemical details of the polymer backbone. The predicted stress relaxation are mapped onto master curves using the time–temperature superposition principle with horizontal and vertical shift factors. The computed shift factors agree with the Williams–Landel–Ferry equation and appear to be similar for the shear and bulk relaxation modulus. The underlying dynamics for the shear and bulk relaxation modulus appear to stem from the same mechanistic origin as both processes seem to sample similar kinetic signature of the reference time scale of the underlying thermally activated processes in the liquid. Our results demonstrate furthermore, that MD simulations with the chemical details beyond coarse-grained models can sample the Rouse dynamics and the transitions towards entanglement, which is evident by the emergence of a plateau in shear relaxation modulus and scaling relations in the diffusion behavior of the monomer at larger molecular weights.

Direct Exciton Harvesting from a Bound Triplet Pair.

Singlet fission is commonly defined as the generation of two triplet excitons from a single absorbed photon. However, ambiguities within this definition arise due to the complexity of the various double triplet states that exist in SF chromophores and the corresponding interconversion processes. To clarify this process, singlet fission is frequently depicted as sequential two-step conversion in which a singlet exciton decays into a bound triplet-pair biexciton state that dissociates into two "free" triplet excitons. However, this model discounts the potential for direct harvesting from the coupled biexciton state. Here, it is demonstrated that individual triplet excitons can be extracted directly from a bound triplet pair. It is demonstrated that due to the requirement for geminate triplet-triplet annihilation in intramolecular singlet fission compounds, unique spectral and kinetic signatures can be used to quantify triplet-pair harvesting yields. An internal quantum efficiency for triplet exciton transfer from the triplet pair of >50%, limited only by the solubility of the compounds is achieved. The harvesting process is not dependent on the net multiplicity of the triplet-pair state, suggesting that an explicit, independent dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

Electron Signatures of Reconnection in a Global eVlasiator Simulation.

Geospace plasma simulations have progressed toward more realistic descriptions of the solar wind–magnetosphere interaction from magnetohydrodynamic to hybrid ion‐kinetic, such as the state‐of‐the‐art Vlasiator model. Despite computational advances, electron scales have been out of reach in a global setting. eVlasiator, a novel Vlasiator submodule, shows for the first time how electromagnetic fields driven by global hybrid‐ion kinetics influence electrons, resulting in kinetic signatures. We analyze simulated electron distributions associated with reconnection sites and compare them with Magnetospheric Multiscale (MMS) spacecraft observations. Comparison with MMS shows that key electron features, such as reconnection inflows, heated outflows, flat‐top distributions, and bidirectional streaming, are in remarkable agreement. Thus, we show that many reconnection‐related features can be reproduced despite strongly truncated electron physics and an ion‐scale spatial resolution. Ion‐scale dynamics and ion‐driven magnetic fields are shown to be significantly responsible for the environment that produces electron dynamics observed by spacecraft in near‐Earth plasmas.