Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G15: Membranes: Biological and Synthetic |
Hide Abstracts |
Sponsoring Units: DBIO Chair: Mark Bowick, Syracuse University Room: 304 |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G15.00001: Dynamical Clustering and the Origin of Raft-like Structures in a Model Lipid Membrane Francis Starr We investigate the dynamical heterogeneity of a model single-component lipid membrane using simulations of a coarse-grained representation of lipid molecules. In the liquid-ordered (LO) phase, lipid diffusion is hindered by the transient trapping of molecules by their neighbors, giving rise to two distinct mobility groups: low-mobility lipids which are temporarily ``caged'', and lipids with displacements on the scale of the intermolecular spacing. The lipid molecules within these distinct mobility states cluster, giving rise to transient ``islands'' of enhanced mobility having the size and time scale expected for lipid ``rafts''. These clusters are strikingly similar to the dynamical clusters found in glass-forming fluids, and distinct from phase-separation clusters. Such dynamic heterogeneity is ubiquitous in disordered condensed-phase systems. Thus, we hypothesize that rafts may originate from this universal mechanism, explaining why raft-like regions should arise, regardless of lipid structural or compositional details. This perspective provides a new approach to understand membrane transport. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G15.00002: Undulatory motion of bilayer membrane structures and fluctuation amplitudes on SANS/SAXS profile for large membrane wavelength Takumi Hawa, Victor Lee In this study, we have extended our previous investigation of the effect of bilayer membrane structures and fluctuation amplitude on small angles scatter (SAS) profiles to the cases with undulatory bending motions using 2D harmonic motion model. We consider the case that the aspect ratio (AR) = membrane wavelength (Y) / membrane thickness is larger than 5. Thicknesses of the bilayer membrane are accurately estimated based on the formula we have derived in our previous study. We have identified the shifting direction of the peak location, q$_{\mathrm{peak}}$, which is distinctively different from the cases of AR $<$ 1. We also identified a form factor q$_Y$, from which the value of Y (and consequently AR also) can be estimated accurately. We also found the relationship among AR, q$_Y$, and the amplitude and developed a chart which can be used to estimate the value of the membrane amplitude. Even though the applicability of the new chart is limited and some assumptions may be required, the values obtained through the new chart showed good agreements to the experimental results obtained in the past experiments. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G15.00003: The Cooperative Behaviour of $\alpha $-Helical Antimicrobial Peptides in Different Environments Marco Pinna, Janping Wang, Manuela Mura, Yuhua Zhou, Andrei Zvelindovsky, Sarah Dennison, David Phoenix A systematic analysis of the antimicrobial peptides (AMPs) cooperative action is performed by means of a full atomistic molecular dynamics simulation. The following peptide analogues: Aurein 2.5-COOH, Aurein 2.6-COOH and Aurein 3.1--COOH are investigated in different environments including aqueous solution, trifluoroethanol (TFE), palmitoyloleoylphosphatidylethanolamine (POPE), and palmitoyloleoylphosphatidylglycerol (POPG) lipid bilayers. Simulations conducted for monomer and trimer peptide highlight the importance of the cooperative behaviour and reveal the different mechanisms of antimicrobial peptides action in different lipid bilayers. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G15.00004: Line Tension of Multi-Component Bilayer Membranes Ashkan Dehghan, Kyle Pastor, An-Chang Shi The line tension of self-assembled multi-component bilayers is investigated using self-consistent field theory. The bilayer membranes are self-assembled from amphiphilic AB/ED diblock copolymers in a solvent modelled as C-homopolymers. We examine the effects of copolymer composition, geometrical shape and interactions on the line tension of bilayer membranes. Specifically, we calculate the line tension for membranes composed of symmetric, cone and inverse-cone shape amphiphilic molecules with neutral and/or repulsive E/D interactions. We show that an increase in the concentration of the cone shaped species results in a decrease in the pore line tension. In contrast, we found that adding inverse-cone shaped copolymers results in an increase in the line tension of the bilayer membrane. By examining the density profile of the membrane we determined that the different amphiphilic species phase separate within the membrane according to their local curvature. Our theoretical predictions are shown to be consistent with available experiments and theories. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G15.00005: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G15.00006: Molecular Transport Studies Through Unsupported Lipid Membranes William Rock, Sapun Parekh, Mischa Bonn Dendrimers, spherical polymeric nanoparticles made from branched monomers around a central core, show great promise as drug delivery vehicles. Dendrimer size, core contents, and surface functionality can be synthetically tuned, providing unprecedented versatility. Polyamidoamine (PAMAM) dendrimers have been shown to enter cells; however, questions remain about their biophysical interactions with the cell membrane, specifically about the presence and size of transient pores. We monitor dendrimer-lipid bilayer interactions using unsupported black lipid membranes (BLMs) as model cell membranes. Custom bilayer slides contain two vertically stacked aqueous chambers separated by a 25 $\mu$m Teflon sheet with a 120 $\mu$m aperture where the bilayer is formed. We vary the composition of model membranes (cholesterol content and lipid phase) to create biomimetic systems and study the interaction of PAMAM G6 and G3 dendrimers with these bilayers. Dendrimers, dextran cargo, and bilayers are monitored and quantified using time-lapse fluorescence imaging. Electrical capacitance measurements are simultaneously recorded to determine if the membrane is porous, and the pore size is deduced by monitoring transport of fluorescent dextrans of increasing molecular weight. These experiments shed light on the importance of cholesterol content and lipid phase on the interaction of dendrimer nanoparticles with membranes. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G15.00007: Diffusion of water on supported bilayer lipid membranes Andrew Miskowiec, Zachary Buck, Helmut Kaiser, Haskell Taub, Flemming Hansen, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig We compare the temperature dependence of quasielastic incoherent neutron scattering from water associated with fully hydrated single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid supported on a SiO$_{\mathrm{2}}$-coated silicon substrate to that of water in proximity to a similarly supported \textit{anionic} DMPG (dimyristoylphosphoglycerol) bilayer. The diffusion constant of water near the DMPC membrane decreases on cooling in two step-like transitions: 1) at the freezing point of bulk-like water (267 K); and 2) at a second transition of unknown origin at 261 K [2]. In contrast, we observe on cooling only a continuous decrease in the diffusion constant of water in proximity to the DMPG membrane. Water remains mobile to lower temperature on the anionic membrane; however, its diffusion is systematically slower than on DMPC in the temperature range above 255 K where water is mobile in both systems. [2] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G15.00008: Wetting and freezing of water on supported bilayer lipid membranes Zachary Buck, Andrew Miskowiec, Mia Brown, Helmut Kaiser, Gavin King, Renee Jiji, Jason Cooley, Haskell Taub, Flemming Hansen, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig Temperature-dependent elastic incoherent neutron scattering shows qualitatively different behavior for water associated with single bilayers of the charge-neutral DMPC (dimyristoylphosphocholine) lipid than for the anionic DMPG (dimyristoylphosphoglycerol) bilayer supported on an SiO$_{\mathrm{2}}$-coated silicon substrate. For the neutral DMPC membrane, the membrane-associated water shows a step-like freezing transition somewhat below the bulk freezing point followed by a continuous freezing behavior and, on heating, a step-like melting transition at the bulk melting point of 273 K [2]. In contrast, water near the anionic DMPG membrane shows only continuous freezing that extends to much lower temperatures than for DMPC and continuous melting that is complete well below the bulk melting point. We suggest that these results may be explained by a film-like water structure in the DMPG case owing to the hydrophilic nature of the membrane surface, while most of the water in the DMPC system is bulk-like and dewets from this more hydrophobic membrane surface. [2] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G15.00009: Euler buckling, membrane corrugation and pore formation induced by antimicrobial peptide Leonardo Golubovic, Lianghui Gao, Licui Chen, Nana Jia, Weihai Fang Antimicrobial peptides serve as defense weapons against bacteria. They are secreted by organisms of plants and animals and have a wide variety in composition and structure. In this study, we theoretically explore the effects of the antimicrobial peptides on the lipid bilayer membrane by using analytic arguments and the coarse grained dissipative particle dynamics simulations. We study peptide/lipid membrane complexes by considering peptides with various structure, hydrophobicity and peptide/lipid interaction strength. The role of lipid/water interaction is also discussed. We discuss a rich variety of membrane morphological changes induced by peptides, such as pore formation, membrane corrugation and Euler buckling. Such buckled membrane states have been indeed seen in a number of experiments with bacteria affected by peptide, yet this is the first theoretical study addressing these phenomena more deeply. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G15.00010: Logarithmic Domain Growth in Ternary Mixture Lipid Multilayer Systems with Cholesterol Yicong Ma, Sajal Ghosh, David DiLena, Laura Connelly, Nirav Patel, Fernando Teran Arce, Ratnesh Lal, Sunil Sinha Cholesterol, one essential constituent of the cell membrane, is shown to perform an important function in maintaining the membrane integrity and fluidity. However, the role of cholesterol in mixed membrane phase behavior is still not completely understood. In our study of model membrane multilayer systems consisting of 1:1 DPPC/DOPC with 0 to 30{\%} Cholesterol, we have studied the kinetics of domain growth as a function of time. X-ray diffraction measurements reveal two different time scales of domain growth: fast growth at small time scale of minutes, and slow growth at large time scale which exhibits a logarithmic growth law. This logarithmic growth law may indicate that the barriers to domain growth increase linearly with the domain size [1,2]. Our detailed analysis of domain lipid structure from electron density profiles with different cholesterol concentrations, and comparison of the growth law time constants should shed new light on the effects of cholesterol on membrane domains. [1] J.D Shore, M.Holzer, and J.P.Sethna, Phys. Rev. B 46, 11376 (1992). [2] Z. W. Lai, G. F. Mazenko, and O. T. Valls, Phys. Rev. B 37,9481 (1988). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G15.00011: Measuring Lipid Membrane Viscosity Using Rotational and Translational Tracer Diffusion Tristan Hormel, Sarah Kurihara, Matthew Reyer, Raghuveer Parthasarathy The two-dimensional fluidity of lipid membranes enables the motion of membrane-bound macromolecules and is therefore crucial to biological function. However, current methods of measuring membrane viscosity rely on particular membrane lipid compositions or geometries, making the comparison of different measurements difficult. We address this with a new technique for measuring lipid membrane viscosity, in which determination of both the rotational and translational diffusion coefficients of tracer particles enables quantification of viscosity as well as the effective size of the tracers. This technique is general, and can be applied to different model membrane systems to determine the effects of membrane composition and protein modulation. We present measurements of lipid membrane viscosity for two different lipids with phosphatidylcholine headgroups, finding a surprisingly wide distribution of effective tracer sizes, due presumably to a large variety of couplings to the membrane. We also compare the effective viscosity of two different structures -- black lipid membranes and membrane multilayers -- as well as changes in viscosity induced by peripheral protein binding. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G15.00012: Aspirin Increases the Solubility of Cholesterol in Lipid Membranes Richard Alsop, Matthew Barrett, Sonbo Zheng, Hannah Dies, Maikel Rheinstadter Aspirin (ASA) is often prescribed for patients with high levels of cholesterol for the secondary prevention of myocardial events, a regimen known as the Low-Dose Aspirin Therapy. We have recently shown that Aspirin partitions in lipid bilayers [1]. However, a direct interplay between ASA and cholesterol has not been investigated. Cholesterol is known to insert itself into the membrane in a dispersed state at moderate concentrations (under $\sim$37.5\%) [2] and decrease fluidity of membranes. We prepared model lipid membranes containing varying amounts of both ASA and cholesterol molecules. The structure of the bilayers as a function of ASA and cholesterol concentration was determined using high-resolution X-ray diffraction. At cholesterol levels of more than 40mol\%, immiscible cholesterol plaques formed. Adding ASA to the membranes was found to dissolve the cholesterol plaques, leading to a fluid lipid bilayer structure [3]. We present first direct evidence for an interaction between ASA and cholesterol on the level of the cell membrane. [1] MA Barrett, S Zheng, G Roshankar, {\bf RJ Alsop} {\em et al.} PLoS ONE 7, e34357, 2012 [2] MA Barrett, S Zheng, LA Toppozini, {\bf RJ Alsop} {\em et al.}Soft Matter 9, 9342-9351, 2013 [3] {\bf RJ Alsop} {\em et al.}, submitted [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G15.00013: Formation of Vesicles in Lipid -- Liquid Crystal Colloidal Mixtures Jeffrey Peters, Germano Iannacchione The formation and phase ordering / evolution has been studied in lipid and liquid crystal (LC) colloidal mixtures as a function of LC concentration and thermal history. The lipid used was 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) while the liquid crystal was pentylcyanobiphenyl (5CB). POPC is a naturally occurring lipid in eukaryotic cell membranes and mimics many of the properties of human cell walls. 5CB is a polar liquid crystal that exhibits a thermodynamically stable orientationally ordered (nematic) state at room temperature. Colloidal dispersions were made at various 5CB and POPC concentrations in water and studied via optical microscopy (phase contrast, confocal, florescence, and cross-polarizing) to probe phase order and evolution as well as by calorimetry to study phase transformations. Very large vesicles were observed to form that appear to use 5CB droplets as scaffolds as well as a unique promotion of lipid crystallization within defect regions of nematic domains. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G15.00014: Topological defects of tetratic liquid-crystal order on a soft spherical surface Yao Li, Han Miao, Hongru Ma, Jeff Chen We model the orientational and positional order of tetratically shaped molecules, each having four-fold structural symmetry, confined on a spherical surface. Our Monte Carlo simulation shows that at a high molecular density, a tetratic orientational order develops in the system, accompanied by eight disclinations arranged in an anticube form on the hard spherical surface. We also consider an elastic-energy model, which consists of both Helfrich and Frank energies for a soft surface; the solution confirms the Monte Carlo study and further predicts the tetratic morphology that can be realized on the surface of a soft vesicle. Assuming that the induced interaction between these disclinations are repulsive, we demonstrate that the anticube structure has a lower free energy than, for example, the cubic structure. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G15.00015: Tetratic order on elastic membranes: defects and geometry Oksana Manyuhina, Mark Bowick One of the fascinating problems in soft condensed matter is finding equilibrium shapes of elastic membranes subjected to additional constraints. It is known that confining nematic liquid crystal (with point symmetry group $D_{\infty h}$) on a surface of a sphere results in topological defects, which influence the overall shape of the closed surface and favour the formation of faceted tetrahedron (topologically equivalent to sphere). Nevertheless, because of the interplay between the local nematic orientational order and the global geometry of the closed membrane, identifying the ground state is not straightforward and requires numerical computation. We propose a phenomenological model to describe the tetratic order formed by particles with point symmetry group $D_{4h}$ on surfaces with various Gaussian curvature. Self-organization of defect patterns as well as buckling instability of membranes are discussed and further connections with other soft matter systems are established. [Preview Abstract] |
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