Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session S16: Membranes, Micelles, and Vesicles |
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Sponsoring Units: DSOFT Chair: Chenxian Xu, University of Illinois Chicago Room: Room 208 |
Thursday, March 9, 2023 8:00AM - 8:12AM |
S16.00001: Modulating the Phase Properties of Biomimetic Ion Pair Amphiphile Membranes Chi-cheng Chiu Ion pair amphiphile (IPA), a molecular complex composed of a pair of cationic and anionic surfactants, can self-assemble into catanionic vesicles as potential liposome surrogates. Like lipid bilayers, an IPA membrane can be in either fluidic liquid-disorder (Lα) phase or in solid-like gel (Lβ) phase when above or below the main phase transition temperature (Tm), respectively. Modulating the phase behaviours of the IPA vesicles is thus critical for its delivery applications. Here, we utilized molecular simulations to systematically examine the phase properties of IPA bilayers with various compositions. Via varying the alkyl chain asymmetry, we first characterized the Tm response of the membranes to IPA mixtures and found deviations from the ideal mixture. Such deviation is partly originated from the transition between the tilted and non-tilted gel phases induced by IPA mixing. We further modulated the compositions of ionic charged groups of IPAs. Using molecular simulations, we constructed the database for the solvation free energies of various ions and the association free energies of possible ion pairs. Applying the law of matching water affinities (LMWA), we screened ion pair candidates to from pH responsive IPA membranes. Our simulations suggested that IPA membranes are most stable at pH = pKa of one of the components. The combined results provide useful insights into the designs of catanionic vesicles with the stimuli-responsive potentials, benefiting the future development of novel drug carriers. |
Thursday, March 9, 2023 8:12AM - 8:24AM |
S16.00002: Understanding Vesicle Fission and Fusion via Constrained Self-consistent Field Theory Luofu Liu, Chao Duan, Rui Wang Despite the wide existence of vesicles in living cells as well as their important applications like drug-delivery vehicles, the underlying mechanism of vesicle fission and fusion remains under debate. Classical models cannot fully explain the results observed in experiments and simulations. Here, we use vesicles formed by polyelectrolytes as a model system and apply the constrained self-consistent field theory to track the energy and morphology along the fission/fusion pathway. The theory can capture the coupling between the position-dependent interaction and the morphological evolution of the two vesicles. We show that there are two discontinuous transitions accompanied by topological changes between three different states: single-cavity vesicle, hemifission/hemifusion, and two separated vesicles. This is consistent with the predictions of the classical stalk model for fusion and the necking model for fission. Besides, we predict the possibility of the direct transition between the state of a single-cavity vesicle and the state of two separated vesicles without the hemifission/hemifusion intermediate if the repulsion between the two vesicles is strong enough, in agreement with simulation results of vesicles formed by amphiphilic molecules. Moreover, we study the micromechanics of different types of PE associates, including pearl-necklace (PN), sphere, and vesicle. In particular, the vesicle behaves like highly plastic materials, characterized by the long force plateau and constant necking thickness, while the PN and sphere are found similar to brittle and common ductile materials with a much smaller ultimate extension ratio. Although our results are based on a model system of PE vesicles, the theory and conclusions can be generalized to vesicles formed by amphiphilic molecules such as surfactants and block copolymers due to the essential topological feature. |
Thursday, March 9, 2023 8:24AM - 8:36AM |
S16.00003: Tail length dependence of acyl tail correlation dynamics and membrane viscosity in lipid bilayers Michihiro Nagao, Elizabeth G Kelley, Bela Farago, Piotr A Zolnierczuk, Paul D Butler Biological membranes are unique borders that define cell boundaries and separate the cell interior from its surrounding environment. These membranes are rigid enough to maintain the cell shape and protect the cell from collapsing and leaking its components, and at the same time, are soft enough to deform and exchange molecules into and out of the cell. The bending and compression elasticity describe the membrane rigidity and the extent of the deformation, but these properties do not explain how fast embedded components can move inside the membrane. Instead, the viscous properties of the membranes determine these response times. Among various techniques to measure membrane viscosity, acyl tail correlation dynamics measured by neutron scattering can help identify the molecular origins of membrane viscosity. In addition to the previous studies on dimyristoyl-phosphocholine (DMPC) bilayers, we performed further neutron spin echo studies on dipalmitoyl- and distearoyl-phosphocholine (DPPC and DSPC) bilayers. We will discuss how the tail length differences, with the number of carbons ranging from 14 to 18, changes the acyl tail correlation dynamics and compare the estimated values of the membrane viscosity with those estimated from the collective membrane thickness fluctuations. |
Thursday, March 9, 2023 8:36AM - 8:48AM |
S16.00004: Dynamic buckling of light-responsive lipid vesicles: rate-dependent mode excitation and relaxation Chris Oville, Arash Manafirad, Jithu Krishna, S Thai Thayumanavan, Anthony D Dinsmore Vesicles made from UV-responsive lipids undergo a dynamic buckling process when photoisomerization drives a rapid area expansion. The active lipids have an azobenzene-functionalized fatty acid tail which isomerizes from trans to cis under UV light and results in an increase in surface area of up to 12%. As the area expands toward a new equilibrium, the membrane is transiently under compression and the vesicles undergo deformations. By varying the rate of UV intensity increase, we can tune the effective compression rate and thereby alter the shape response as well as the post-buckling relaxation of the membrane. Slow compression rates lead to isometric area expansion while fast rates lead to high amplitude deformations in a narrow range of excited modes. In addition, some combinations of intensities and loading rates result in an onset of irreversible processes such as budding and tubulation. We map the excited mode amplitudes and onset of irreversibility to effective loading rates and lipid composition. The results show how dynamic loading may affect morphologies in cell membranes and in membrane-based materials. |
Thursday, March 9, 2023 8:48AM - 9:00AM |
S16.00005: A Neural Network-based Approach to Determining Vesicle Shapes in a Phase Field Model Yousef Rohanizadegan, Jeff Z Chen We introduce a novel framework to numerically solve the energetically optimized shape of a vesicle by a feed-forward neural network. This framework is akin to the variational formalism for minimizing the elastic bending energy of vesicle membranes subject to volume and area constraints, which was first formulated by Helfrich and was investigated further by Seifert et. al. We model the vesicles using a phase field approach proposed by Wang et. al. Following this approach, the inputs of the network are the coordinates of a point inside a three-dimensional domain of interest and the output is the value of the corresponding phase field. As such, the neural network corresponds to an ansatz for an equivalent energy-based variational problem, which converges to the solution by minimizing a loss function for the elastic bending energy with appropriate volume and surface area penalty terms. The three main axisymmetric shapes i.e., prolate or dumbbell, oblate or discocyte and stomatocyte, are obtained. Moreover, nonaxisymmetric shapes and shapes that correspond to a non-zero spontaneous curvature parameter in Helfrich’s energy equation are attainable by implementing this framework. |
Thursday, March 9, 2023 9:00AM - 9:12AM |
S16.00006: Dynamic and Static Light Scattering study of Polymeric Microgels of Varying Crosslinking Concentration. Kiril A Streletzky, Andrew L Scherer, Samantha C Tietjen, Patrick Herron, Samantha R Hudson, Krista G Freeman Structure and dynamics of polysaccharide polymeric microgels synthesized at various crosslinker concentrations were studied with Dynamic Light Scattering (DLS) and Static Light Scattering (SLS). DLS on these microgels revealed crosslinker-concentration-dependent dynamics below and above their volume phase transition. SLS with direct dn/dc measurements revealed the strong dependence of particle's structure and molecular weight on concentration of crosslinker. In particular, at higher crosslinker concentrations dn/dc showed a significant increase with increase of crosslinker concentration and under volume phase transition. Microgel scattering form factors were also found to depend on crosslinker concentration changing from shapes consistent with more spherical to less spherical shapes with the increase of the crosslinker amount. The overall light scattering analysis shows that the crosslinker concentration affects the apparent density and structure of microgels and could be related to non-uniform crosslinker distribution in microgels at higher crosslinker concentrations. Several potential explanations of the observed effects of crosslinker concentration are proposed. |
Thursday, March 9, 2023 9:12AM - 9:24AM |
S16.00007: Lipid Vesicles Induced Ordered Nanoassemblies of Janus Nanoparticles yu zhu, Abash Sharma, Eric J Spangler, Mohamed Laradji Using molecular dynamics of a coarse-grained implicit-solvent model, the adhesion modes of nanoparticles (NPs) on the outer side of lipid vesicles are explored. Specifically, Janus NPs are considered so that the amount by which they are wrapped by the vesicle is controlled. When two NPs adhere to the outer side of the vesicle, the curvature-mediated interaction leads to two main modes of adhesion. Namely, the NPs are either apart, or forming an in-plane dimer. More interesting, when the number of NPs on the vesicle is larger than two, they self-assembly into an array of highly ordered clusters, resulting from effective membrane-curvature mediated repulsion. These geometries include several deltahedra and three Platonic solids, corresponding to the tetrahedron, octahedron, and icosahedron. These results indicate that lipid vesicles can potentially be used as an alternative medium for bottom-up fabrication of tunable nanoassemblies with different geometries. |
Thursday, March 9, 2023 9:24AM - 9:36AM |
S16.00008: Investigating Nanoparticle-cancer cell membrane Interactions using a modified Jarzynski Free-Energy Estimator to Eliminate Non-conservative Forces Najla Hosseini, Mikael Lund, Mohammd Reza Ejtehadi Despite extensive research and progress in the field of drug design for cancer therapy, cancer is still a leading cause of death worldwide. Chemotherapy is standard practice for cancer treatment and has an enormous impact on patient quality of life, as it can be the only effective treatment for advanced metastatic tumors; however, it often leads to a chemoresistant state in which cancer cells no longer respond to administered drugs. To study the interactions of drugs and nanoparticles (NPs) with the lipidomic component of the membrane of chemoresistance cancer cells at the most basic level, we need to use computational methods. Computational methods to understand interactions in bio-systems are however limited to time scales typically much shorter than in Nature. For instance, on the molecular level, interactions between NPs and membranes are vital in complex biomolecular processes such as cellular uptake. This can be remedied by the application of e.g. Jarzynski’s equality where thermodynamic properties are extracted from non-equilibrium |
Thursday, March 9, 2023 9:36AM - 9:48AM |
S16.00009: Membrane tension and bending alter the morphology of solid domains on curved membranes Hao Wan, Geunwoong Jeon, Gregory M Grason, Maria M Santore Compared with crystal growth in a planar geometry, crystal growth on surfaces having non-zero Gaussian curvature is poorly understood, where calculations reveal the possibility that curvature-induced stress may lead to anisotropic crystal growth. However, there are very few experimental examples available so far. In this study, we demonstrate that the crystal formation resulting from the solidification of lipid membrane domains in giant unilamellar vesicles is altered by the membrane tension that pulls on the edge of a growing crystal, creating new morphologies. The scaling of this behavior with curvature acts in opposition to the curvature-induced stress effect seen for colloidal deposition on a curved template. During nucleation and growth, experimental parameters, such as cooling rate and water transport, affect the tension history of individual vesicles. The outcome is that flat compact domains are repeatedly found on small vesicles whereas flower shaped domains grow as the vesicle diameter increases. Quantitative micropipette experiments as well as theoretical modeling suggests that the emergence of flowers is due to higher membrane tension on larger vesicles and the interplay between bending energies and line tension. |
Thursday, March 9, 2023 9:48AM - 10:00AM |
S16.00010: Adhesion and Dimerization of Spherocylindrical Nanoparticles on Tensionless Lipid Bilayers Abash Sharma, Abash Sharma, yu zhu, Eric J Spangler, Mohamed Laradji We present a numerical investigation of the interaction of sphero-cylindrical nanoparticles (SCNPs) with tensionless planar lipid bilayers, using molecular dynamics simulations of an implicit-solvent model. The investigation is performed systematically with respect to the adhesion energy density, ξ, SCNP's diameter D, and aspect ratio α. A single SCNP adheres to the membrane through a parallel mode at weak ξ and through a normal mode at large ξ. The value of ξ, at the transition from the parallel to the normal mode, decreases with increasing D or α, in agreement with theoretical arguments based on the Helfrich Hamiltonian. As ξ is further increased, the SCNP undergoes endocytosis. The value of ξ, at the transition from the normal mode to endocytosis, is independent of α but decreases with increasing D. Similarly, we investigated the modes of adhesion of two SCNPs on tensionless lipid bilayers and found that, when the NPs are placed relatively close to each other initially, the SCNPs dimerize into a sequence of modes including in-plane dimers, V-shape dimers, and tubular dimers, as ξ is increased. Importantly, α is found to play a greater role in the endocytosis of two NPs than a single NP. The stabilities of the different modes are inferred from free energy calculations based on the weighted histogram analysis method and from free energy calculations based on the Helfrich Hamiltonian. |
Thursday, March 9, 2023 10:00AM - 10:12AM |
S16.00011: Surface Forces Sculpt Nanoscopic Mesas in Micellar Foam Films Chenxian Xu, Yiran Zhang, Subinur I Kemal, Vivek Sharma A complex interplay of curvature-dependent capillary pressure and thickness- |
Thursday, March 9, 2023 10:12AM - 10:24AM |
S16.00012: Peeling with (almost) no torque: three-phase contact lines of highly bendable films Benjamin Davidovitch, NARAYANAN MENON, Deepak Kumar, Nuoya Zhou Peeling a thin sheet off an adhesive substrate gives rise to 3-phase contact line in whose vicinity stresses are typically large. Here we elucidate the universal nature of this “extreme mechanics” by studying two distinct geometries – a one-dimensional (1D) geometry, in which one edge of a thin polymer film floating on water is lifted up in the air, and an axial geometry, in which an air bubble is locked between the liquid bath and the floating film. We show that in both geometries the curvature is substantially enhanced at the contact line, but varies continuously across it. Furthermore, the curvature profile is characterized by two distinct “microscopic” scales – the familiar bendo-capillary length (~10-100 micrometres in our experiment), and another, “bendo-tensile” scale, which is affected by the in-plane stress tensor, normal to the contact line. The former determines a discontinuity in the gradient of the curvature at the contact line, and the latter (which may be substantially smaller than the former) determines a ``penetration length” over which the excess curvature decays. These observations underlie a generalization of the classical peeling condition for solid layers from a rigid substrate (Obreimoff 1930) to peeling from a liquid foundation. |
Thursday, March 9, 2023 10:24AM - 10:36AM |
S16.00013: Trapped Either Way: Lose–Lose Diffusion in Ordered Porous Environments Tobias Dwyer, Timothy C Moore, Sharon C Glotzer Understanding diffusion through complex environments is crucial for a wide range of pharmaceutical and biomedical applications. Recently, highly ordered arrays of DNA nanocages have been synthesized with tunable site-specific interactions between nanocages (NCs) and diffusing nanoparticles (NPs), potentially allowing for diffusion control through porous DNA networks[1]. In this work, we simulate the diffusion of NPs through a network of NCs to elucidate the dependence of NP–NC interactions on NP diffusion. We find that both attractive and repulsive interactions decrease the diffusion coefficient of particles in these lattices—a “lose–lose” situation. Through particle tracking, we show that the diffusing particles spend more time inside the cages as we increase the NP–NC attraction. With increasing NP–NC repulsion, the diffusing particles spend increased time in cage-adjacent sites, which are geometrically identical to the cages but lack site-specific attraction or repulsion; this effect also decreases the overall diffusion coefficient. Finally, we show that the diffusion coefficient in these systems can be predicted via a coarse-grained master equation. |
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