The attainable resolution of fluorescence microscopy has reached the subnanometer range, but this technique still fails to image the morphology of single proteins or small molecular complexes. Here, we expand the specimens at least tenfold, label them with conventional fluorophores and image them with conventional light microscopes, acquiring videos in which we analyze fluorescence fluctuations. One-step nanoscale expansion (ONE) microscopy enables the visualization of the shapes of individual membrane and soluble proteins, achieving around 1-nm resolution. We show that conformational changes are readily observable, such as those undergone by the ~17-kDa protein calmodulin upon Ca2+ binding. ONE is also applied to clinical samples, analyzing the morphology of protein aggregates in cerebrospinal fluid from persons with Parkinson disease, potentially aiding disease diagnosis. This technology bridges the gap between high-resolution structural biology techniques and light microscopy, providing new avenues for discoveries in biology and medicine.

Exchange of material across two membranes, as in the case of synaptic neurotransmitter release from a vesicle, involves the formation and poration of a hemifusion diaphragm (HD). The nontrivial geometry of the HD leads to environment-dependent control, regarding the stability and dynamics of the pores required for this kind of exocytosis. This work combines particle simulations, field-based calculations, and phenomenological modeling to explore the factors influencing the stability, dynamics and possible control mechanisms of pores in HDs. We find that pores preferentially form at the HD rim, and that their stability is sensitive to a number of factors, including the three line tensions, membrane tension, HD size, and the ability of lipids to ‘flip-flop’ across leaflets. Along with a detailed analysis of these factors, we discuss ways that vesicles or cells may use them to open and close pores and thereby quickly and efficiently transport material.

The ability of diblock copolymers to self-assemble into periodic microstructures with length scales in the nanometer range offers many opportunities for fundamental research and applications. For practical applications, it is often desirable that the microstructures have a high degree of order on macroscopic length scales and are oriented in a desired direction. This can be achieved in a large volume by shearing the copolymer melt. In experiments, different orientations are observed depending on the copolymer characteristics and the applied shear conditions. However, details of the orientation mechanism under shear are not completely understood. Studying structurally and thermodynamically symmetric, lamellae-forming diblock copolymers by molecular simulation using a highly coarse-grained model, we analyze the effect of dynamical asymmetry on the stable orientation in steady-shear flow. We control the dynamical asymmetry via (i) the segmental friction in our dissipative particle dynamics DPD simulation or via (ii) slip springs, which mimic physical entanglements of the polymers. We study the kinetics of structure formation after a quench from the disordered state in the presence of shear and the ordering of a system, initially comprised of two orthogonally oriented lamellar grains, under shear. In both simulation settings and for both mechanisms of dynamical asymmetry, the perpendicular orientation, where the lamellae normals are perpendicular to the shear gradient, is preferred for approximately equal dynamics of the two blocks, whereas the parallel orientation becomes stable when the ratio of the relaxation times of the blocks exceeds an order of magnitude. We rationalize this finding by the minimum of the Rayleighian, i.e., the energy dissipation rate of the nonequilibrium steady state. We compare these simulation results to experimental diblock copolymer model systems, polystyrene-b-poly-2-vinylpyridine, with slightly different glass transition temperatures of the two polymer blocks. Adjustment of the polymer block mobility by different temperatures for alignment experiments confirms the trend toward a parallel orientation with increasing dynamical asymmetry of the polymer blocks, when the rigid lamellae slide past the opposing brushes of the more mobile polymer block.

Division of intracellular organelles often correlates with additional membrane wrapping, e.g., by the endoplasmic reticulum or the outer mitochondrial membrane. Such wrapping plays a vital role in proteome and lipidome organization. However, how an extra membrane impacts the mechanics of the division has not been investigated. Here we combine fluorescence and cryo-electron microscopy experiments with self-consistent field theory to explore the stress-induced instabilities imposed by membrane wrapping in a simple double-membrane tubular system. We find that, at physiologically relevant conditions, the outer membrane facilitates an alternative pathway for the inner-tube fission through the formation of a transient contact (hemi-fusion) between both membranes. A detailed molecular theory of the fission pathways in the double membrane system reveals the topological complexity of the process, resulting both in leaky and leakless intermediates, with energies and topologies predicting physiological events. While fission of single-membrane systems is well understood, the mechanism helping split double membranes (as in organelle division) is unclear. Here, the authors use experiment and theory to discover double membrane fission pathways, and find that a second membrane actually enables splitting.

The synaptic vesicle cluster (SVC) is an essential component of chemical synapses, which provides neurotransmitter-loaded vesicles during synaptic activity, at the same time as also controlling the local concentrations of numerous exo- and endocytosis cofactors. In addition, the SVC hosts molecules that participate in other aspects of synaptic function, from cytoskeletal components to adhesion proteins, and affects the location and function of organelles such as mitochondria and the endoplasmic reticulum. We argue here that these features extend the functional involvement of the SVC in synapse formation, signalling and plasticity, as well as synapse stabilization and metabolism. We also propose that changes in the size of the SVC coalesce with changes in the postsynaptic compartment, supporting the interplay between pre- and postsynaptic dynamics. Thereby, the SVC could be seen as an 'all-in-one' regulator of synaptic structure and function, which should be investigated in more detail, to reveal molecular mechanisms that control synaptic function and heterogeneity.

The division of a cellular compartment culminates with the scission of a highly constricted membrane neck. Scission requires lipid rearrangements, topology changes, and transient formation of nonbilayer intermediate structures driven by curvature stress. Often, a side effect of this stress is pore-formation, which may lead to content leakage and thus breaching of the membrane barrier function. In single-membrane systems, leakage is avoided through the formation of a hemifusion (HF) intermediate, whose structure is still a subject of debate. The consequences of curvature stress have not been explored in double-membrane systems, such as the mitochondrion. Here, we combine experimental and theoretical approaches to study neck constriction and scission driven by tension in biomimetic lipid systems, namely single- and double-membrane nanotubes (sNTs and dNTs), respectively. In sNTs, constriction by high tension gives rise to a metastable HF intermediate (seen as stalk or worm-like micelle), whereas poration is universally slower in a simple neck. In dNTs, high membrane tension causes sequential rupture of each membrane. In contrast, low tension leads to the HF of both membranes, which may lead to a leaky fusion pathway, or may progress to further fusion of the two membranes along a number of transformation pathways. These findings provide a new mechanistic basis for fundamental cellular processes.

The cellular environment, characterized by its intricate composition and spatial organization, hosts a variety of organelles, ranging from membrane-bound ones to membraneless structures that are formed through liquid-liquid phase separation. Cells show precise control over the position of such condensates. We demonstrate that organelle movement in external concentration gradients, diffusiophoresis, is distinct from the one of colloids because fluxes can remain finite inside the liquid-phase droplets and movement of the latter arises from incompressibility. Within cellular domains diffusiophoresis naturally arises from biochemical reactions that are driven by a chemical fuel and produce waste. Simulations and analytical arguments within a minimal model of reaction-driven phase separation reveal that the directed movement stems from two contributions: Fuel and waste are refilled or extracted at the boundary, resulting in concentration gradients, which (i) induce product fluxes via incompressibility and (ii) result in an asymmetric forward reaction in the droplet's surroundings (as well as asymmetric backward reaction inside the droplet), thereby shifting the droplet's position. We show that the former contribution dominates and sets the direction of the movement, toward or away from fuel source and waste sink, depending on the product molecules' affinity toward fuel and waste, respectively. The mechanism thus provides a simple means to organize condensates with different composition. Particle-based simulations and systems with more complex reaction cycles corroborate the robustness and universality of this mechanism.

Preoperative evaluation prior to listing for orthotopic liver transplantation (LT) requires a careful multidisciplinary approach with specialized teams including surgeons, hepatologists and anesthesiologists in order to improve short- and long-term clinical outcomes. Due to inadequate supply of donor organs and changing demographics, patients listed for LT have become older, sicker and share more comorbidities. As cardiovascular events are the leading cause for early mortality precise evaluation of risk factors is mandatory. This review focuses on the detection and management of coronary artery disease, cirrhotic cardiomyopathy, portopulmonary hypertension and hepatopulmonary syndrome in patients awaiting LT. Further insights are being given into scoring systems, patients with Acute-on-chronic-liver-failure (ACLF), frailty, NASH cirrhosis and into psychologic evaluation of patients with substance abuse.

The self-assembly of cylinder-forming diblock copolymer solutions in the course of solvent evaporation in the presence of an electric field is studied by particle-based simulations. The electric field provides additional control of the evaporation-induced self-assembly (EISA) and enlarges the processing window, which results in the desired formation of cylindrical domains that are perpendicularly oriented to the film surface. Two effects of the electric field are highlighted: (i) If the components of the AB block copolymer exhibit different permittivities, dielectrophoretic forces align the internal AB interfaces along the electric field, rendering parallel cylinders unstable. (ii) If shallow density gradients in the course of EISA give rise to the unfavorable morphology of perpendicular cylinders and subjacent layers of spherical micelles, the application of an electric field results in an elongation of the cylindrical domains and suppresses sphere formation. The beneficial effect of an electric field can be rationalized by the layer evolution model (LEM), previously developed for EISA in the absence of an electric field.

Block copolymer membranes offer a bottom-up approach to form isoporous membranes that are useful for ultrafiltration of functional macromolecules, colloids, and water purification. The fabrication of isoporous block copolymer membranes from a mixed film of an asymmetric block copolymer and two solvents involves two stages: First, the volatile solvent evaporates, creating a polymer skin, in which the block copolymer self-assembles into a top layer, comprised of perpendicularly oriented cylinders, via evaporation-induced self-assembly (EISA). This top layer imparts selectivity onto the membrane. Subsequently, the film is brought into contact with a nonsolvent, and the exchange between the remaining nonvolatile solvent and nonsolvent through the self-assembled top layer results in nonsolvent-induced phase separation (NIPS). Thereby, a macroporous support for the functional top layer that imparts mechanical stability onto the system without significantly affecting permeability is fabricated. We use a single, particle-based simulation technique to investigate the sequence of both processes, EISA and NIPS. The simulations identify a process window, which allows for the successful in silico fabrication of integral-asymmetric, isoporous diblock copolymer membranes, and provide direct insights into the spatiotemporal structure formation and arrest. The role of the different thermodynamic (e.g., solvent selectivity for the block copolymer components) and kinetic (e.g., plasticizing effect of the solvent) characteristics is discussed.

Conventional theories of weak polyelectrolytes are either computationally prohibitive to account for the multidimensional inhomogeneity of polymer ionization in a liquid environment or oversimplistic in describing the coupling effects of ion-explicit electrostatic interactions and long-range intrachain correlations. To bridge this gap, we implement the Ising density functional theory (iDFT) for ionizable polymer systems using the single-chain-in-mean-field algorithm. The single-chain-in-iDFT (sc-iDFT) shows significant improvements over conventional mean-field methods in describing segment-level dissociation equilibrium, specific ion effects, and long-range intrachain correlations. With an explicit consideration of the fluctuations of polymer configurations and the position-dependent ionization of individual polymer segments, sc-iDFT provides a faithful description of the structure and thermodynamic properties of inhomogeneous weak polyelectrolyte systems across multiple length scales.

Chemical reaction cycles are prototypical examples how to drive systems out of equilibrium and introduce novel, life-like properties into soft-matter systems. We report simulations of amphiphilic molecules in aqueous solution. The molecule's head group is permanently hydrophilic, whereas the reaction cycle switches the molecule's tail from hydrophilic (precursor) to hydrophobic (amphiphile) and vice versa. The reaction cycle leads to an arrest in coalescence and results in uniform vesicle sizes that can be controlled by the reaction rate. Using a continuum description and particle-based simulation, we study the scaling of the vesicle size with the reaction rate. The chemically active vesicles are inflated by precursor, imparting tension onto the membrane and, for specific parameters, stabilize pores.

The theological properties of polymer composites depend on the interfacial interactions between solid fillers and a polymer fluid. In highly coarse-grained (hCG) models, where one coarse-grained segment represents multiple monomeric repeat units, the solid surface of a filler appears smooth on the hCG scale. Thus, special simulation techniques are required to control the single-chain dynamics and friction at the solid-fluid contact. We devise a simulation strategy-the wall-spring (WASP) thermostat-where transient bonds are formed between the solid surface and the polymer segments, based on a grand canonical Monte Carlo (MC) algorithm. These transient bonds mimic strong, specific interactions of the polymer segments with the solid. The attraction, induced by the transient bonds, can be compensated with a permanent, analytically known potential such that static properties do not differ from the system without WASPs. The single-chain and collective dynamics of the polymer fluid at the surface can be tailored by the areal density of transient bonds and their lifetime. The WASP thermostat allows us to capture dynamic heterogeneities at surfaces, such as those quantified by the non-Gaussian behavior of the van Hove self-correlation of polybutadiene at silica surfaces, obtained by atomistic simulations. The parametrized hCG model enables us to explore the dynamics of polymers at solid surfaces for a wide range of molecular weights. We study the Navier-slip boundary condition and demonstrate that both the slip length and the position of the hydrodynamic boundary increase like the polymer's end-to-end distance, R-e. Since both lengths are approximately equal, the velocity profile vanishes close to the narrow interface between polymer melt and solid.

Using the combination of a soft, coarse-grained, particle-based model, a free-energy functional that depends on the local composition, and a lattice model of local, metastable states, we study the structure and motion of a grain boundary between two orthogonal grains of cylindrical domains in asymmetric block copolymers. The particle-based model provides direct insights into the elementary class of transitions of the self-assembled morphology in the course of grain-boundary translation. These processes are correlated in space and time. We identify a minimal set of transitions, whose free-energy changes and barriers are obtained by describing the system by a free-energy functional of the local composition and calculating the minimum free-energy path (MFEP). The spatiotemporal correlation arises from the dependence of the free-energy characteristics on the local environment. We use this information to parametrize a lattice model of the correlated processes in the course of grain-boundary motion. This allows us to investigate the grain-boundary motion by kinetic Monte Carlo (kMC) simulation and determine its free energy landscape. Grain-boundary motion proceeds by nucleating a two-dimensional, anisotropic cluster inside the plane of the grain boundary.

A new mesophase in binary blends of A-b-(BA')(3) miktoarm star block copolymers and A homopolymers has recently been discovered experimentally and explored with field-theoretic simulations. This mesophase has been reported to consist of aperiodic discrete domains of A embedded in a continuous matrix of B up to very high concentrations of A. Because of the material's potential as a thermoplastic elastomer, a deeper understanding of its structural and dynamic-mechanical properties, including its domain connectivity, linear rheological behavior, response to shear, and response to uniaxial tension, is warranted. These properties are explored here using dissipative particle dynamics in three dimensions, for the first time. These simulations establish that the so-called "bricks-and-mortar" phase, while appearing discrete in two dimensions, is bicontinuous. The simulations focusing on dynamics establish that the role of molecular bridging dominates the mechanical behavior and outweighs the influence of microphase segregation (contributions from the interfacial tension alone) even at the highest homopolymer concentrations we study. Additionally, it appears that the bricks-and-mortar phase is sensitive to the application of sufficiently high shear, leading to nonisotropic mechanical responses, which has ramifications for the processability of such materials. We find that upon application of shear the phase becomes closer in structure to its speculated discrete nature. Molecular simulations on our longest accessible timescales show that the material is unable to relax back to its original structure, suggesting that the morphology observed depends heavily on the material process pathway.

Using analytical considerations and particle-based simulations of a coarse-grained model, we study the relaxation of a density modulation in a polymer system without nonbonded interactions. We demonstrate that shallow density modulations with identical amplitudes and wavevectors that have been prepared by different processes exhibit different nonexponential decay behaviors. Thus, in contrast to the popular assumption of dynamic self-consistent field theory, the density alone does not suffice to characterize the configuration of the polymer system. We provide an analytic description within Linear-Response Theory (LRT) and the Rouse model that quantitatively agree with the results of the particle-based simulations. LRT is equivalent to a generalized model-B dynamics with an Onsager coefficient that is nonlocal in space and time. Alternatively, the Rouse description can be cast into a dynamic density-functional theory that uses the full probability distribution of single-chain configurations as a dynamic variable and yields a memory-free description of the dynamics that quantitatively accounts for the dependence on the preparation process. An approximate scheme that only considers the joint distribution of the first two Rouse modes-the ellipsoid model-is also explored.

Using particle-based Monte Carlo simulations and continuum modeling, we study the self-assembly of asymmetric diblock copolymers in the course of solvent evaporation. We examine the effects of evaporation rate and solvent selectivity on the structure formation, especially the alignment of the cylindrical domains of the minority block. The comparison of the two simulation techniques facilitates identifying general trends upon parameter variation, while their inherent differences help us to understand the role of single-chain dynamics, fluctuations, and additional model details. In both cases, the simulation models feature a liquid and a gas phase with an explicit surface, across which solvent evaporates. We propose a "layer evolution model" that links processing parameters to the final morphology via the time dependence of layers, in which characteristic microphases, for example, spherical or cylindrical, can form. The evolution of these layers varies with the processing conditions and determines the morphology. This allows us to discuss the interplay of various experimentally accessible parameters, which we support by respective simulations. Our results single out two main factors to ensure the formation of minority-block cylinders, perpendicular to the film surface: (i) Fast evaporation rates induce a steep gradient in the polymer-density profile; that is, the polymer density immediately beneath the gas- liquid surface rapidly exceeds the critical value for cylinder formation. This confines cylinder formation into a layer that is thinner than the actual cylinder diameter, forcing a perpendicular alignment. (ii) A certain selectivity of the gas phase for the matrix-forming, majority block is necessary to disrupt an otherwise entropically favored surface layer of the minority block that would lead to parallel cylinder alignment.

A supercooled fluid close to the glass transition develops nonlocal shear-stress correlations that anticipate the emergence of elasticity. We performed molecular dynamics simulations of a binary Lennard-Jones mixture at different temperatures and investigated the spatiotemporal autocorrelation function of the shear stress for different wavevectors, q, from a locally measured and Fourier-transformed stress tensor. Anisotropic correlations are observed at non-zero wavevectors, exhibiting strongly damped oscillations with a characteristic frequency ω(q). A comparison with a recently developed hydrodynamic theory [Maier et al., Phys. Rev. Lett. 119, 265701 (2017)] shows a remarkably good quantitative agreement between particle-based simulations and theoretical predictions.

Cross-linking is a versatile strategy to restrain the single-chain and collective dynamics of polymers and thereby stabilize structures against transformations in response to variations of thermodynamic conditions, such as temperature. Using particle-based simulations and analytic calculations, we systematically study the dependence of the phase behavior of randomly cross-linked, symmetric diblock copolymer melts on the preparation state, in which the cross-linking has been performed. The irreversible nonselective cross-links that do not prefer any of the two segment species impart a memory of the preparation state onto the system, and therefore the phase diagram does not only depend on the incompatibility, x(m)N, at which the system is observed and the cross-link density but also on the incompatibility, x(p)N, in the preparation state.

Using computer simulations and phenomenological considerations, we study the interplay between elasticity and microphase separation in quasi-two-dimensional phantom networks, obtained by cross-linking AB diblock copolymers at their ends. In the limit of weak stretching, where the average distance, l(omega), of A cross-links (or the mesh-cell size of the regular network) in the disordered phase is much smaller than the lamellar spacing, L*, of the diblock copolymer melt, network elasticity plays only a minor role. Upon increasing the stretching, we find that the incompatibility chi N-db, at which the order-disorder transition occurs, decreases, and it becomes vanishingly small for l(u) >> L* and large networks. At intermediate stretching, we observe a multigrain state, where the lamellae tilt with respect to the network orientation.