Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Z11: Lipid Bilayers II |
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Sponsoring Units: DBP Room: A107-A109 |
Friday, March 19, 2010 11:15AM - 11:27AM |
Z11.00001: Stress Induced Domain Formation in Multilamellar Lipid Bilayers Lobat Tayebi, Sean Gillmore, Atul Parikh Domain formation in lipid mixtures due to phase separation of the components is a well-known phenomenon that has been studied in mono- and bi-molecular lipid configurations. We report same phenomenon, however, in multilamellar configurations consisting of thousands of lamellae where the domain pattern in each layer is interestingly aligned with the other lamellae. In this process, both dehydration and hydration of lipid cake can act as the driving force to separate two phases of liquid ordered and liquid disordered. In a controlled experiment with a stack lipid saturated with water, mechanical perturbation can induce domain formation too. Series of experiments of this kind reaches us to the conclusion that any sort of stress in special condition may cause domain formation. We use a combination of microscopy tools including AFM, fluorescence confocal and bright-field microscopy to determine the influence of interaction between the line tension and key elastic properties of the lipid bilayers. As a particular interest we studied the dynamics of the domain pattern formation and the interactions between the domains such as long-term fusion. [Preview Abstract] |
Friday, March 19, 2010 11:27AM - 11:39AM |
Z11.00002: Antibiotics and the mechanics of cellular bulging in gram-negative bacteria K. Daly, Ned S. Wingreen, Ranjan Mukhopahyay For most bacteria, the cell wall, consisting of a cross-linked polymer network, is the primary stress-bearing structure. Due to the high osmotic pressure difference across the cell membrane, the presence of the cell wall is essential for cell stability. Recent experiments have addressed the effect of cell-wall defects induced by antibiotics such as vancomycin, and find that in Gram-negative bacteria, antibiotics can lead to pronounced bulging of the cell membrane and eventually to lysis. Here we address the mechanics of bulging and its relationship to cell-wall defects. We estimate the critical defect size for bulging and discuss the biological implications of our results. We also discuss the relevance of our physical model to blebbing and vesiculation in eukaryotic cells. [Preview Abstract] |
Friday, March 19, 2010 11:39AM - 11:51AM |
Z11.00003: Compositional Heterogeneity in Ternary Models for the Cell Membrane Robin Smith, Frederick Heberle, Jing Wu, Gerald Feigenson Ternary models for the cell membrane comprised of cholesterol (Chol) plus high and low melting temperature lipids exhibit rich phase behavior as a function of temperature and composition. Of particular interest is a region of coexisting disordered and ordered fluid phases that is thought to indicate how lipids organize to promote protein function in the cell membrane. We have used fluorescence resonance energy transfer to investigate the ternary mixtures DOPC(1,2-dioleoyl-sn-glycero-3-phosphocholine)/bSM (porcine brainsphingomyelin)/Chol and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine)/bSM/Chol at high compositional resolution. We confirmed liquid coexistence for DOPC/bSM/Chol at 15 and 25C that melts by 35C, but in contrast to previous studies we detected no fluid-phase compositional heterogeneity for POPC/bSM/Chol from 5-35C. If domains exist, they must be smaller than the approximately 5 nm sensitivity provided by the fluorescent lipid analogs employed. We propose electron spin resonance and x-ray scattering for measuring whether liquid-phase compositional heterogeneity occurs for POPC/bSM/Chol. Understanding POPC/bSM/Chol phase behavior will provide a framework for investigating peptide/lipid interactions in a biologically relevant lipid mixture. [Preview Abstract] |
Friday, March 19, 2010 11:51AM - 12:03PM |
Z11.00004: Field-theoretic model of membrane/protein assemblies Kang Chen, Glenn Fredrickson, Kim Rasmussen, Paul Welch We developed a coarse-grained field-theoretic model of an assembling membrane/protein system that includes immovable cylindrical transmembrane protein(s) and assembling membrane species including neutral and charged lipids, counterions, and water. This field-theoretic model is able to capture the molecular architecture of lipids that constitute the membrane. Our study focused on the following aspects: i) membrane thickness fluctuation due to hydrophobic mismatch, ii) the corresponding lipid tail stretching near the protein, iii) ion and cationic lipid distributions at various levels of protein head charge, iv) protein tilting, v) membrane-mediated protein-protein interactions, and vi) in-plane packing structures of proteins. Our simulations were used to evaluate free energies and structure that enabled the quantitative analysis of features such as lipid tail stretching and membrane/protein structural stability. [Preview Abstract] |
Friday, March 19, 2010 12:03PM - 12:15PM |
Z11.00005: Effect of biocompatible polymers on the structural integrity of lipid bilayers under external stimuli Jia-Yu Wang, Ravinath Kausik, Chi-Yuan Chen, Song-I. Han, Jeremy Marks, Ka Yee Lee Cell membrane dysfunction due to loss of structural integrity is the pathology of tissue death in trauma and common diseases. It is now established that certain biocompatible polymers, such as Poloxamer 188, Poloxamine 1107 and polyethylene glycol (PEG), are effective in sealing of injured cell membranes, and able to prevent acute necrosis. Despite these broad applications of these polymers for human health, the fundamental mechanisms by which these polymers interact with cell membranes are still under debate. Here, the effects of a group of biocompatible polymers on phospholipid membrane integrity under osmotic and oxidative stress were explored using giant unilamellar vesicles as model cell membranes. Our results suggest that the adsorption of the polymers on the membrane surface is responsible for the cell membrane resealing process due to its capability of slowing down the surface hydration dynamics. [Preview Abstract] |
Friday, March 19, 2010 12:15PM - 12:27PM |
Z11.00006: Towards general design rules for membrane active antimicrobials Lori Sanders, Nathan Schmidt, Abhijit Mishra, Vernita Gordon, Gerard Wong Membrane active antimicrobials are short amphipathic peptides that selectively disrupt and lyse bacterial cell membranes. While it is believed that the combination of peptide hydrophobicity and cationic charge is essential for function, the detailed molecular mechanism of selective membrane permeation remains unclear. We use synchrotron small angle x-ray scattering (SAXS) to investigate the interaction of model bacterial and eukaryotic cell membranes with archetypes from each of the three defensin subfamilies found in mammals. The relationship between membrane composition and peptide induced changes in membrane curvature and topology is examined. By comparing the membrane rearrangement and corresponding phase behavior induced by these different peptides we will discuss the importance of amino acid composition and placement on antimicrobial peptide design. [Preview Abstract] |
Friday, March 19, 2010 12:27PM - 12:39PM |
Z11.00007: pH-dependent Differential Scanning Calorimetry and Dynamic Light Scattering Studies of 21:0 PC and 18:0 PS Lipid Binary System Rejwan Ali Large unilamallar vesicle has been a model system to study many membrane functions. High Tg lipid systems offer many potential biomedical applications in lipid-based delivery applications. While the optimized vesicle functionalities are achieved by Polyethylene Glycol (PEG) polymer, modified PEG and other functional molecule incorporation, however, the host binary lipid system plays the pivotal role in pH-dependent phase transition based lipid vehicular methods. We have investigated a lipid binary system composed of 21:0 PC (1,2-dihenarachidoyl-sn-glycero-3-phosphocholine) and 18:0 PS(1,2-distearoyl-sn-glycero-3-phospho-L-serine). Preliminary studies implementing differential scanning calorimetry shows pH plays key role in temperature shift and thermotropic phase behavior of the binary system. While dynamic light scattering study shows lipid vesicle size is almost independent of pH changes. We will also present pH-dependent thermodynamic parameters to correlate underlying molecular mechanism in relevant pH-range. [Preview Abstract] |
Friday, March 19, 2010 12:39PM - 12:51PM |
Z11.00008: Quantitative Imaging of Membrane Shape Transformation and Pearling Stephen Anthony, Yan Yu, Steve Granick Experiments show, in areas from vesicle budding, to pearling and even stochastic fluctuation of shape, the ubiquity of non-spherical shape in phospholipid assemblies. Here we focus on pearling and the massive stochastic fluctuations which precede it when nanoparticles induce this transformation by adsorption to the inner leaflet of a giant unilamellar vesicle (GUV). Novel methods to quantify non-spherical contours in movies with massive numbers of frames allow us to imagine membrane fluctuations frame-by-frame, even in the case of low signal-to-noise. [Preview Abstract] |
Friday, March 19, 2010 12:51PM - 1:03PM |
Z11.00009: Criticality in Plasma Membranes Ben Machta, James Sethna, Sarah Veatch, Stefanos Papanikolaou Recent work in giant plasma membrane vesicles (GPMVs) isolated from living cells demonstrated that they can be tuned with a single parameter (temperature) to criticality, not far from in vivo temperatures [1,2]. Criticality requires the fine-tuning of two parameters suggesting important biological function, and its presence resolves many of the paradoxes associated with putative lipid rafts. Here we present a minimal model of membrane inhomogeneities. We incorporate criticality using a conserved order parameter Ising model coupled to a simple actin cytoskeleton interacting through fields which act as point-like pinning sites. Using this model we make a host of experimentally testable predictions that are in line with recent published findings. At physiological temperatures we find inhomogeneities in the form of critical fluctuations with a length scale of roughly 20nm. Individual constituents making up these liquid domains are mobile, though they diffuse anomalously, but the correlated regions themselves can last as long as the cytoskeleton persists. We explain this by considering the effective long ranged interaction mediated by the Ising order parameter. In general we find Ising criticality organizes and spatially segregates membrane components by providing a channel for interaction over large distances. [1] Veatch et al., ACS Chem Biol. 2008 3(5):287-93 [2] Honerkamp-Smith, Veatch, and Keller, Biochim Biophys Acta. 2008 (in press) [Preview Abstract] |
Friday, March 19, 2010 1:03PM - 1:15PM |
Z11.00010: Topological Transitions in Mitochondrial Membranes controlled by Apoptotic Proteins Ghee Hwee Lai, Lori K. Sanders, Abhijit Mishra, Nathan W. Schmidt, Gerard C.L. Wong, Olena Ivashyna, Paul H. Schlesinger The Bcl-2 family comprises pro-apoptotic proteins, capable of permeabilizing the mitochondrial membrane, and anti-apoptotic members interacting in an antagonistic fashion to regulate programmed cell death (apoptosis). They offer potential therapeutic targets to re-engage cellular suicide in tumor cells but the extensive network of implicated protein-protein interactions has impeded full understanding of the decision pathway. We show, using synchrotron x-ray diffraction, that pro-apoptotic proteins interact with mitochondrial-like model membranes to generate saddle-splay (negative Gaussian) curvature topologically required for pore formation, while anti-apoptotic proteins can deactivate curvature generation by molecules drastically different from Bcl-2 family members and offer evidence for membrane-curvature mediated interactions general enough to affect very disparate systems. [Preview Abstract] |
Friday, March 19, 2010 1:15PM - 1:27PM |
Z11.00011: The buckling transition of ionic shells and electrostatics Monica Olvera de la Cruz, Rastko Sknepnek, Graziano Vernizzi Can one design the morphology of a shell with diverse symmetries by coassembling oppositely charged molecules? We present the results of numerical simulations of a model for an ionic shell at different stoichiometric ratios. The tendency of electrostatic interactions to organize a system of charges (globally electroneutral) along flat planes, competes with the curved geometry of the shell. An ``electrostatic buckling'' instability ensues, and at low-temperatures a variety of shapes arise, beyond the icosahedral one typical of large viruses, large fullerenes, and catanionic-anionic vesicles. We study also the effects of temperature, different dielectric environments, and screening salt. [Preview Abstract] |
Friday, March 19, 2010 1:27PM - 1:39PM |
Z11.00012: Facilitation of Electron Transfer in the Presence of Mitochondria-Targeting Molecule SS31 Tetiana Nosach, Mark Ebrahim, Yuhang Ren, Shaun Darrah, Hazel Szeto Electron transfer (ET) processes in mitochondria are very important for the production of adenosine triphosphate (ATP), the common source of the chemical energy. The inability to transfer electrons efficiently in mitochondrial ET chain plays a major role in age associated diseases, including diabetes and cancer. In this work, we used the time dependent absorption and photoluminescence spectroscopy to study the electron transfer kinetics along the ET chain of mitochondria. Our spectroscopic results suggest that SS31, a small peptide molecule targeting to the mitochondrial inner membrane, can facilitate electron transfer and increase ATP production. We show that SS31 targets cytochrome c to both increase the availability of state and also potentially reduce the energy barrier required to reduce cytochrome c. [Preview Abstract] |
Friday, March 19, 2010 1:39PM - 1:51PM |
Z11.00013: Mean-field overcharging of macromolecules via charge or surface modulation Jonathan Landy It is well known that multivalent counterions can at times overcharge macromolecules in electrolyte solutions. In this talk, I will discuss a simple mean-field mechanism that can allow for this effect: modulation of source charge and/or surface geometry can induce additional charge condensation sometimes resulting in overcharging. The qualitative features of this mechanism will be related to experimental observations. In addition, an experimental method by which one may be able test for modulation effects will be discussed. [Preview Abstract] |
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