Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B06: Membranes, Micelles and VesiclesLive
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Sponsoring Units: DSOFT Chair: Hendrik Spanke, ETH Zurich Room: 06 |
Monday, March 15, 2021 11:30AM - 11:42AM Live |
B06.00001: Dynamics of spontaneous wrapping of microparticles by floppy lipid membranes Hendrik Spanke, Robert Style, Eric R Dufresne In vivo, proteins are most probably the main actors of membrane deformation. The adsorption of inert particles could enable reproduction of the essential physics of membrane deformation by bound proteins. |
Monday, March 15, 2021 11:42AM - 11:54AM Live |
B06.00002: Solvent diffusivity and viscosity in graphene oxide membrane for water-ethanol separation. Gobin Acharya, Peter Hoffmann, Eugene Mamontov, Madhusudan Tyagi Graphene oxide (GO) membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Understanding microscopic diffusivity of water and ethanol in graphene oxide membranes is important for separations applications. It is also important to study the anisotropy of water and ethanol diffusivity between the direction perpendicular to the plane and in-plane direction. We used quasielastic neutron scattering(QENS) to measure the temperature dependence of the diffusivity of water and ethanol, and its anisotropy by the utilization of Q-dependence of QENS signals obtained from BASIS at Oak Ridge National Lab. We will also discuss how dynamic measurements from QENS can be correlated with atomic force microscopy measurements of the temperature dependence of viscosity in water/ethanol confined in GO. |
Monday, March 15, 2021 11:54AM - 12:06PM Live |
B06.00003: Monte Carlo simulation of fluid membranes with orientational order and free edges Lijie Ding, Robert Alan Pelcovits, Thomas R Powers Motivated by experiments on membranes composed of chiral rod-like viruses, we consider a triangular mesh of hard beads possessing directors joined by tethers. The beads, tethers and directors are free to move, reconnect or rotate at each Monte Carlo step. We model the system using the Canham-Helfrich energy in addition to a liquid crystalline energy, which include a Lebwohl-Lasher interaction, a tilt energy and a chiral energy that favors twist between neighboring directors. We find three membrane shapes: branched polymers, disks and vesicles. For the disk, we find the twist of directors is expelled to the edge of the membrane, and as the chiral coupling increases, the twist penetration depth increases until pi walls form inside the membrane. We also study the membrane shape when we pull apart two antipodal points on the membrane edge. As the distance between the antipodal points increases, the disk twists into a ribbon and the extension force increases eventually reaching a plateau. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B06.00004: Comparing microrheological methods for measuring lipid membrane viscosity Philip Jahl, Raghuveer Parthasarathy The fluidity of lipid membranes governs the motions of bound proteins and macromolecules. Despite this, measurements of the viscosity of a lipid bilayer remain challenging to perform and interpret. Two different microrheological methods for measuring bilayer viscosity have been developed in recent years, one involving tracking phase-separated domains in giant unilamellar vesicles, and the other involving tracking elliptical lipid-anchored tracer particles. The latter approach has so far been applied only to pore-spanning black lipid membranes. The membrane viscosity values obtained by these two methods differ by an order of magnitude, however, and it is unclear whether this indicates inaccuracy of one or both techniques, or whether it is due to the difference in membrane systems examined. To resolve this discrepancy, we applied both methods simultaneously to the same lipid vesicles, featuring both phase separated domains and bound elliptical beads. We show that when applied to identical systems these methods are in agreement. The elliptical tracer method is generally applicable to vesicles of arbitrary composition, and we use it to quantify the viscosity of bilayers composed of phosphatidylcholine lipids of different chain lengths. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B06.00005: Absolute/convective instabilities and front propagation in lipid membrane tubes Amaresh Sahu, Joel Tchoufag, Kranthi K Mandadapu Biological lipid membranes make up the cell boundary, and are often found in cylindrical shapes. For example, axonal flows bring lipids from the growth cone of a neuron to its cell body. We investigate the stability of membrane tubes with and without a base flow of lipids. We confirm tubes are stable at low tensions and unstable at high tensions, yet for unstable tubes we find increasing the base flow velocity changes the nature of the instability from absolute to convective. In the former case, an initially local perturbation will grow faster than it is convected downstream, and eventually will invade the entire domain. In the latter case, the perturbation is convected faster than it spreads, and at long times a stationary observer will see no disturbance---though the perturbation continues to grow. Nonlinear simulations reveal an initially localized disturbance results in propagating fronts, which leave a thin tube in their wake. Depending on the base tension, the thin tube is connected to the unperturbed regions via oscillatory or monotonic shapes---reminiscent of experimental observations in axons. Our study sheds light on the pattern selection mechanism in axonal shapes, and we determine the base tension at which the front dynamics undergo steady-to-oscillatory bifurcations. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B06.00006: Assessing Shear-induced Scission of Wormlike Micelles Using Flow-Small Angle Neutron Scattering Jiamin Zhang, Patrick Corona, L. Gary Leal, Matthew Helgeson Wormlike micelles (WLMs), long thread-like surfactant aggregates, remain an important class of surfactant-based materials in consumer products and oilfield applications. Although it is widely accepted that equilibrium micelle scission greatly influences the rheology of WLMs, due to the lack of direct structural measurements, there is still considerable theoretical debate regarding whether scission is affected by flow. We report new flow-small angle neutron scattering experiments on a carefully designed series of linear, weakly entangled WLMs. We find that the low-q intensity monotonically decreases with increasing shear rate, consistent with the net breakage of the WLMs. We use model calculations of scattering from deformable objects in flow to deconvolute the contributions to the scattering from changes in micelle orientation, stretching, and interactions from effects due to changes in micelle length in flow. We find that without including micelle scission, none of these other factors is able to explain the qualitative trends in experiments. This suggests that the length distribution of wormlike micelles is significantly affected by shear and opens up new possibilities for measuring and modeling the effects of flow on the reversible scission of WLMs. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B06.00007: Non-pulsatile lipid vesicles under osmotic stress Vinit Kumar, Jie Feng Understanding the physical mechanisms governing vesicle responses under osmotic imbalance is crucial for not only advancing knowledge of the osmoregulatory mechanisms of living cells, but also constraining design choices in biocompatible micro-solute transportation systems, and multicompartment microreactors. When placed under osmotic stress, vesicle response will diverge across one of three trajectories: simple engorgement, swelling with eventual pore formation, and irreversible explosion. In this talk, we present a comprehensive theoretical model that unifies these behavioral outcomes based on fundamental properties of the lipid membrane. We demonstrate how sudden osmotic shock, such as through light-triggered reactions, can lead to explosion, and outline key conditions that determine explosion or swell-burst-reseal dynamics. We expand this understanding by presenting recent advances for the phase diagram of vesicle fates. With this, the previously unappreciated contribution of membrane bending energy emerges as a significant determiner of mechanical response. This talk situates this energy contribution within the broader context of vesicle dynamics and helps in advancing a fundamental understanding of vesicle response in non-equilibrium environments. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B06.00008: Membrane remodeling in life processes: Unravelling membrane interactions, one vesicle at a time. Raya Sorkin, Raviv Dharan, Hila Shir-Shapira, Margherita Marchetti, wouter roos, Josep Rizo, Sander Groffen, Gijs Wuite Fundamental understanding of physiological processes that occur at biological membranes, such as membrane fusion, necessitates addressing not only of the biochemical aspects but also membrane mechanical properties and membrane deformation. In this talk, I will show how we combine a system of optical tweezers dedicated to accurate force measurements with confocal fluorescence microscopy to study membrane remodelling by calcium sensor proteins, which are crucial in neuronal communication. Using this setup, we discovered surprising differences between the action mechanisms of two structurally similar proteins, Doc2b and Synaptotagmin1 (Syt1), as determined by quantifying the strength and probabilities of protein-induced membrane-membrane interactions and lipid and content mixing assays. I will further describe how we use this approach to gain insight into the fusion mechanisms of enveloped viruses with host cell membranes. |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B06.00009: Structural Modifications Induced by Violacein in Lipid Membranes Ritika Gupta, Saheli Mitra, Subhadip Chowdhury, Gangadhar Das, Richa Priyadarshini, Mrinmay Mukhopadhyay, Sajal Ghosh Several gram negative bacteria produce a purple pigment called violacein which helps them to protect against the surrounding gram positive bacteria. Violacein possesses a high affinity towards the lipid membrane and understanding their interaction mechanism can help in facilitating the use of violacein in the pharmaceutical paradigm. The present study sheds light on the surface and structural modifications induced by violacein in zwitterionic, cationic, and anionic lipid membranes. The modified surface activity of lipid monolayers with the addition of violacein was manifested as change in transition temperature and in-plane elasticity. Increment in membrane thickness with the addition of violacein to zwitterionic and cationic lipid monolayers was confirmed from x-ray reflectivity measurements. Grazing incidence x-ray diffraction study on lipid multilayers suggests a decrease in lattice parameters and tilt angle confirming the increased compactness of the mixed system. |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B06.00010: Multiscale simulation of surfactant self assembly: An Integrated Particle And Field-Theoretic Simulation Approach Kevin Shen, My Nguyen, Nick Sherck, Brian Yoo, Stephan Kohler, Joshua Speros, Kris T Delaney, M. Scott Shell, Glenn H Fredrickson Surfactants self assemble into structures important for many soft matter applications. Resolving the self-assembled structures requires well-equilibrated, large scale simulations, but the sensitivity of surfactant self assembly to chemical details necessitates high-resolution, chemically accurate approaches. Here, we present a new strategy to tackle this multiscale modeling challenge by combining the strengths of both particle and field-theoretic simulations. Field theoretic simulations are uniquely suited to the rigorous study of large scale self assembly behaviors that are oftentimes inaccessible to all-atom simulations. However, the predictive power of field theoretic simulations are limited by the challenge of parameterizing its coarse grained, emergent (e.g. chi) parameters. Using sodium dodecyl sulfate as a model charged surfactant, we show how the chemical detail embodied by all atom simulations can be efficiently transferred to coarse-grained simulations with quantitative accuracy. Consequently, we are able to make de-novo, chemically faithful predictions of surfactant self assembly, such as the dependence of wormlike micelle shape and the critical micelle concentration on temperature and added salt. |
Monday, March 15, 2021 1:30PM - 1:42PM Live |
B06.00011: Mechanically mediated attractions and repulsions between solid domains in composite vesicles Hao Wu, Maria Santore, Gregory M Grason We explore the structure and interactions of solid/fluid composite vesicles using 2D and 3D continuum models compared with experimental studies of phase-separated vesicles. In contrast to well-known repulsive interactions in fluid-fluid phase separation, the shear rigidity of solid domains produces qualitatively new collective behaviors. Beyond simply an enhanced bending stiffness, shear rigidity of the solid domains tends to expel Gaussian curvature into fluid phase, an effect which generically competes with the global spherical topology of the vesicle. We show that fluid membrane bending elasticity mediates interactions between solid domains, controlled by their size and the vesicle area-to-volume ratio. For sufficiently tensed vesicles, fluid phase bending induces a depletion-like attraction between plate-like solid domains, driven by tendency to consolidate elastic high-bending “hinges” that flank the solid domains at high pressure, while in floppy vesicles domains repel at long range. This result suggests a new picture of plate-like solid domains as “2D colloids” dispersed in a fluid background, whose effective interactions are tunable through global vesicle properties and may be useful for engineering novel responsive and functional materials. |
Monday, March 15, 2021 1:42PM - 1:54PM Live |
B06.00012: Attractive and Repulsive Interactions between Plate-like Solid Membrane Domains in Phospholipid Membranes. Weiyue Xin, Hao Wu, Gregory M Grason, Maria Santore This project employed giant unilamellar phospholipid vesicles to study the interactions between solid membrane domains which act more generally as plate-like inclusions, and it made a comparison to a continuum model. The positions of multiple solid microscale membrane domains within single vesicles suggested the presence of an attractive minimum between domain pairs, short-range interdomain repulsions, and long-range inter-domain attractions. We quantified the range of the potential minimum in vesicles containing only two domains, reporting features of a true pair potential. As previous work has only reported repulsive interactions or attractive interactions individually, our discovery of a distinct minima translates to preferred domain positioning. These interactions result from the shear elasticity of the solid domains, which tend to expel Gaussian curvature into the fluid membrane phase, generically competes with the global spherical topology of the vesicle. We also discovered the ability to toggle interactions, and shift the targeted positioning, by osmotic adjustment or mechanical manipulation of the fluid membrane by employing micropipettes or osmotic pressure. |
Monday, March 15, 2021 1:54PM - 2:06PM Live |
B06.00013: Design of self-organizing peptide chassis materials for synthetic cells by machine learning, molecular modeling, and cell-free protein synthesis. Yutao Ma, Andrew Ferguson Synthetic cells are engineered cell-like compartments of interest for molecular encapsulation, reaction, and transport in non-biological environments. The fragility of cell-sized lipid vesicles outside of biological environments has motivated the exploration of alternative 'chassis' materials for synthetic cells. Elastin-like polypeptides (ELP) have been recently shown to form robust vesicular micro-compartments. This has opened up opportunities for the design of novel ELPs from which to self-assemble synthetic cells that can survive in harsher environments than lipid membranes and with greater biocompatibility and biofunctionalization than synthetic polymersome materials. In this study, we combine molecular simulation, machine learning, and cell-free protein synthesis to build predictive models for the discovery and design of ELP sequences and identify a small number of promising candidates for experimental testing. |
Monday, March 15, 2021 2:06PM - 2:18PM Live |
B06.00014: Pore Formation: How Cubic Lipid-siRNA Constructs Escape the Endosome Lining Zheng, Cecilia Leal Small interfering RNA (siRNA) silences gene expression and has shown great potential in medical applications. However, intracellular delivery of siRNA remains a great challenge. Lipid nanoparticles have been one of the most successful siRNA carriers to date, but their delivery efficiency is limited due to low endosomal release. We have shown that cubic structured lipid nanoparticles (cubosomes) loaded with siRNA - cuboplexes, show greater siRNA knockdown compared to traditional lamellar structured liposomes. In this work, we aim to elucidate the reason behind this difference in delivery efficiency. We will show confocal microscopy (CF), flow cytometry and live cell imaging data demonstrating higher cellular uptake of cubosomes compared to liposomes. CF, fluorescent based fusion assays and dynamic light scattering measurements indicate that compared to liposomes, cubosomes are more prone to fuse and form aggregates with endosomes, which implies a stronger pore forming capability that leads to siRNA escape. These results support our suggestion that cubosomes can lower the free energy required to promote endosome pore formation and establish a topologically active delivery mechanism. |
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