Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W10: Lipid Bilayers I |
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Sponsoring Units: DBP Chair: Christian Santangelo, University of Massachusetts Room: A106 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W10.00001: Lipid membrane rigidity from fluctuation measurement in nm scale Sung Chul Bae, Bo Wang, Chang-Ki Min, Steve Granick Thermal fluctuation of giant unilamellar phospholipids vesicles(GUVs) was observed by a forward laser beam scattering of a laser beam from the vesicle edge. This technique, owing to the refractive index mismatch, offers nanometer spatial and microsecond temporal resolution of membrane position. When nanoparticles adsorb on the membrane, this membrane fluctuation changes. We will discuss the changes of membrane rigidity caused by nanoparticles, based on membrane fluctuation data. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W10.00002: What is the difference between a supported and a free lipid bilayer? Roland Faller Supported Lipid Bilayers are an abundant research platform for understanding the behavior of real cell membranes as they allow for additional mechanical stability and enable characterization techniques not reachable otherwise. However, in computer simulations these systems have been studied only rarely up to now. Here we present a systematic study of the changes that a support inflicts on a phospholipid bilayer using coarse-grained molecular modeling. We characterize the density and pressure profiles as well as the density imbalance induced by the support. It turns out that the changes in pressure profile are strong enough that protein function should be impacted leading to a previously neglected mechanism of transmembrane protein malfunction in supported bilayers. We also determine the diffusion coefficients and characterize the influence of different corrugations of the support. We then determine the free energy of transfer of phospholipids between the proximal (close to the surface) and distal leaflet of a supported membrane using the coarse-grained Martini model. It turns out that there is at equilibrium about a 2-3\% higher density in the proximal leaflet. These results are in favorable agreement with recent data obtained by very large scale modeling using a water free model where flip-flop can be observed directly. We compare results of the free energy of transfer obtained by pulling the lipid across the membrane in different ways. There are small quantitative differences but the overall picture is consistent. We are additionally characterizing the intermediate states which determine the barrier height and therefore the rate of translocation. Simulations in atomistic detail are performed for selected systems in order to confirm the findings. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W10.00003: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W10.00004: Shapes of biological membranes due to curvature-inducing proteins Kiyotaka Akabori, Christian Santangelo In biological cells, lipid membranes are shaped by a variety of proteins. We develop a theoretical model for a class of anisotropic proteins that induce curvature along their backbone. Due to the interplay between rotational entropy and membrane bending energy, these proteins can locally induce either positive (spherical) or negative (saddle-splay) Gaussian curvature. When added to a lamellar phase, protein-induced saddle-splay favors the formation of bicontinuous structures by inducing screw dislocations in the lamellar order. We also study the effect of an additional transverse coupling to make contact with previous theoretical models of anisotropic, curvature-inducing proteins. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W10.00005: Microrheology of freestanding lipid bilayers Christopher Harland, Miranda Bradley, Raghuveer Parthasarathy The macroscopic material properties of cellular membranes, determined by the composition and interactions of their constituent lipids, are important factors in the structure and function of all living cells. Fluidity is a key material property of membranes, yet the underlying lipid bilayer viscosity and other rheological parameters remain poorly quantified. We adopt recently developed microrheological methods to study multiple composite freestanding ``black'' lipid membranes. Using high speed video particle tracking, we monitor dynamics of membrane-anchored nano- and micro-particles across a range of temperatures that span bilayer phase transitions. Two particle spatial correlation functions and the complex shear modulus are extracted from such measurements and provide information about fundamental membrane material properties. We find striking and previously unreported signatures of viscoelasticity in these lipid bilayers whose properties are sensitive to the bilayers' temperature dependent liquid ordered to liquid disordered phase transitions. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W10.00006: Correlated motion of membrane proteins and their effect on membrane viscosity Haim Diamant, Naomi Oppenheimer We extend the theory of membrane hydrodynamics to account for the correlated motion of membrane proteins, along with the effect of protein concentration on that correlation and on the response of the membrane to stresses. Expressions for the coupling diffusion coefficients of protein pairs and their concentration dependence are derived. The additional role of membrane viscosity as determining the characteristic length scale for membrane response leads to unusual concentration effects at large inter-protein separation. We treat a freely floating membrane, as well as a supported membrane lying at a finite distance from a solid substrate. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W10.00007: Effect of myristoylated N-terminus of Arf1 on the bending rigidity of phospholipid membranes Beatriz Burrola Gabilondo, Hernan Zhou, Paul A. Randazzo, Wolfgang Losert The protein Arf1 is part of the COPI vesicle transport process from the Golgi to the ER. It binds to membranes via a myristoylated N-terminus and it has been shown to tubulate Large Unilamellar Vesicles. The effect of the N-terminus of Arf1 on physical properties of membranes has not been studied, with the exception of curvature. We previously found that the myristoylated N-terminus increases the packing of the lipid molecules, but has no effect on the lateral mobility. We tested the hypothesis that myristoylated peptides affect the bending rigidity of phospholipid Giant Unilamellar Vesicles (GUV). We use optical tweezers to pull tethers from GUV and measure the force of pulling the tether, as well as the retraction speed of the tether once it is released. We also used flicker spectroscopy to estimate the values of the mechanical properties of GUV. We will present results of the force and tether retraction measurements, as well as mechanical properties estimates from flicker, for GUV in the presence of varying concentrations of myristoylated and non-myristoylated N-terminus of Arf1, and compare these with measurements for GUV in the absence of peptide. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W10.00008: Molecular dynamics simulation study of structural changes and pore formation in phospholipid bilayers in the presence of dimethylsulfoxide Dorel Moldovan, Raghava Alapati, Jieqiong Lin, Brian Novak, Ram Devireddy Understanding the structure and properties of cell membranes in the presence of various chemicals is of great importance for numerous applications in pharmaceuticals and biosciences. We present molecular dynamics simulations that delineate with atomistic details the effect of dimethylsulfoxide (DMSO) at various concentrations on dimyristoylphosphatidylcholine (DMPC) lipid bilayers. The simulations show that at low concentrations DMSO leads to bilayer thinning and increases permeability. When the DMSO concentration increases above a critical value ($\sim $6.0 wt{\%}), even in the absence of an external stress, our simulations show that hydrophilic pores are thermally nucleated and grow. We rationalize the nucleation process in terms of a simplified free energy model that includes the entropy of the pore shape. By estimating the line tensions within the lipid bilayers with and without DMSO, our simulations corroborate the pore growth model. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W10.00009: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W10.00010: Dynamics of inter-leaflet lipid exchange in membranes: Mechanism for stress relaxation Mark L. Henle, L. Mahadevan Fusion and fission events in the cell membrane play a crucial role in many important biological processes, including membrane budding and vesicle recycling in the synapse. The elastic properties of the cell membrane play a crucial role in these processes. Standard elastic models assume that the exchange of lipids between membrane leaflets is negligible. While this is valid for the phospholipids in the membrane, other lipids such as cholesterol undergo rapid flip-flop between leaflets [J. A. Hamilton, \emph{Curr. Opin. Lipidol.} {\bf 14}, 263 (2003)]. Such exchange allows a dynamic stress relaxation in the membrane via addition (removal) of lipids to expanded (compressed) regions of the leaflets. This can, for example, reduce the energetic barrier to neck formation in membrane budding, which simple elastic models have estimated to be as high as $500 k_B T$. In this talk, we present a non-equilibrium model for a membrane composed of two lipids, one of which is able to flip between leaflets. For a variety of simple geometries, we find that lipid exchange can dramatically reduce the energetic barriers for bending the membrane. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W10.00011: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W10.00012: Local measurements of phase transitions in Bacteriorhodopsin membrane R. Proksch, M.P. Nikiforov, S. Hohlbauch, W.P. King, S. Antoraz Contera, K. Vo\"Itchovsky, S.V. Kalinin Phase transitions play an important role in biology. Specifically the thermodynamic stability of internal membrane proteins is an important issue of biophysics. Purple membrane from Halobacterium halobium contain bacteriorhodopsin (bR), an integral protein 70-80{\%} of whole mass is intramembraneous. There are heated debates in the field about the parameters of thermal denaturation of bR, such as the denaturation temperature, enthalpy etc. Recently, bR was proposed as a component of biomolecular electronics. Thus, reliable information about the phase transitions of supported samples of bR membranes is necessary. Phase transitions in polymer/biopolymer materials are associated with the large changes in mechanical properties of the samples. We developed the technique for the measurements of the temperature dependence of the mechanical properties with high spatial resolution. This technique is based on the measurements of the contact stiffness of the atomic force microscopy tip -- sample system as a function of temperature. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W10.00013: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W10.00014: Particle/fluid interface replication as a means of producing topographically patterned surfaces: Substrates for supported lipid bilayers Anand Subramaniam, Sigolene Lecuyer, Kumaran Ramamurthi, Richard Losick, Howard Stone There is intense interest in the role of geometry in the thermodynamics and dynamics of such systems as lipid bilayers, membrane proteins and block copolymers. Topographically patterned surfaces that impose well-defined gradients of curvature on surface adsorbed layers are a potential model to study these geometrical effects. Here we report a method for producing topographically patterned surfaces by replicating a fluid-fluid interface studded with colloidal particles. With this method we have fabricated geometrically simple surfaces, such as arrays of spherical features on planar surfaces and also surfaces with complex geometries such as replicas of whole bacterial cells, tubular nanoclays, and even multi-walled carbon nanotubes. Furthermore, chemically heterogeneous surfaces composed of silica, polystyrene, epoxy or poly(dimethyl)siloxane (PDMS), and chemically homogeneous surfaces composed of PDMS or epoxy can be made. As an example of the potential applications of these surfaces, we show that lipid bilayers that are supported on all-PDMS topographically patterned substrates undergo curvature-modulated phase separation. [Preview Abstract] |
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