Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H39: Lipid Bilayers: Structure and Function I |
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Sponsoring Units: DBP Chair: Alex Levine, University of California, Los Angeles Room: 411 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H39.00001: A New Mechanism for Domain Size Selection in Curved Lipid Membranes Fangfu Ye, Jonathan Selinger Lipid membranes, composed of saturated lipids, unsaturated phospholipids and cholesterols, play important roles in maintaining cellular activities. It is now well established that lipid membranes under proper conditions separate into saturated-lipid-enriched liquid-ordered (L$_{o})$ phase regions and unsaturated-lipid-enriched liquid-disordered (L$_{d})$ phase regions, with the L$_{o}$ phase having a larger bending modulus than the L$_{d}$ phase. In this project, we study how the bending modulus difference between L$_{o}$ and L$_{d}$ phases may affect the phase separation behavior of uniformly curved lipid membranes. We predict that, for membranes of a spherical lipid vesicle, when the line tension between the L$_{o}$ phase and L$_{d}$ phase is small the phase separation process is truncated and the underlying curvature leads to formation of stable L$_{o}$-phase domains of finite size. We also compare these predictions with experiments on lipid rafts. [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H39.00002: Making Sense of the Polymorphous Shapes of Giant Liposomes Yan Yu, Stephen Anthony, Julie Vroman, Sung Chul Bae, Steve Granick Lipid vesicles, especially giant unilamellar vesicles (GUVs) are often used as simplified models for biological membranes, but their polymorphous panoply of shapes and shape changes is notorious to those who work with them. This affords opportunities to study why phospholipid membranes so often fail to minimize their surface area to adopt spherical shapes. Instabilities can be triggered by the tension caused by optical tweezers, osmotic perturbations, or polymer anchorage. This talk will describe the evolution of GUVs from spherical to pearl-like and to tube-like shapes, and back again reversibly. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H39.00003: Near-Field structural studies of lipid bilayers Merrell Johnson, Ricardo Decca We use a Near-field Scanning Optical Microscope (NSOM) in conjunction with a Photo Elastic Modulator (PEM) to conduct birefringence $(\mathop n\nolimits_e -\mathop n\nolimits_o )$ measurements with a spatial resolution of $\sim $80nm. With our current setup we are able to distinguish changes in retardance $\alpha =\frac{2\pi (\mathop n\nolimits_e -\mathop n\nolimits_o )}{\lambda }$ on the order of $5\times 10^{-3}$ radians. Simultaneously while gathering information about $\alpha $ we extract information about the samples optical orientation$\theta $, reference to the system's axis, with an accuracy of $3.64\times 10^{-3}$radians. We use our system on 1,2-dipalmitoylphosphatidylchorline (DPPC) bilayers, which at room temperature are in the gel state,( i.e.: their acyl chains have a $\sim $32 degree azimulthal tilt with respect to the membranes normal). Modeling the membrane as a uniaxial crystal we are able determine the position of the acyl chains by measuring the birefringence and optical orientation. By controlling the temperature of our sample we hope to better study the structural changes that occur during phase transitions from gel to liquid states. The investigation of other lipid mixtures and the transformations they undergo during different phases will also be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H39.00004: X-ray insight into cholesterol-phospholipid interactions David Gidalevitz The mechanism of nonideal cholesterol-lipids mixing yet remains controversial. We report on a systematic study of cholesterol-phospholipid interactions in lipid monolayers using Langmuir isotherms, synchrotron X-ray reflectivity (XR), and grazing-incidence X-ray diffraction (GIXD) techniques. Lipid monolayers consisted of cholesterol-DPPC mixtures with cholesterol mole fractions $\chi _{CHOL}$ varying from 0 to 1. GIXD reveals that at both $\chi _{CHOL}$ and $\chi _{DPPC}$ above .85 mixed films exhibit packing order of a prevalent lipid. In between, cholesterol seizes places in DPPC crystalline lattice at the stoichiometry similar as that of the mixture inducing short-range regular-hexagonal packing order with increasing spacing between molecules as a function of cholesterol content. XR shows that cholesterol tends to stay in DPPC acyl chains at low $\chi _{CHOL}$ while gradually descending to a subphase at higher $\chi _{CHOL}$ accompanied by rearrangement of DPPC headgroups. Thus, a desire of highly nonpolar cholesterol to avoid contacts with polar water molecules and/or DPPC headgroups defines a mode of cholesterol-lipid interactions. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H39.00005: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H39.00006: Monte Carlo Simulation of Coexisting Phases in DOPC/DSPC/Cholesterol Ternary Mixtures Rejwan Ali, Jian Dai, Juyang Huang Lipid raft domain has been a topic of current interest in both computational and experimental membrane biophysics. Understanding raft domain will open up path for modeling many cellular phenomena. Extensive studies on model raft consists of DOPC/DSPC/cholesterol ternary system have been reported by many experimental groups. We report Monte Carlo simulation to reconstruct experimental phase diagram. Both pair-wise and multi-body interactions have been used to simulate the phase boundary of liquid-ordered phase and liquid-disordered phase coexistence region. A new algorithm, named the ``Composition Evaluation Method,'' was implemented to determine the compositions of the coexisting phases in simulations. The new method is about 20$\sim $50 times faster in determining phase boundaries, comparing to the traditional free energy calculation. In addition, pair correlation functions were used to map the phase boundaries in the critical region. We found that pair-wise interactions can reproduce the experimental critical point as well as the slope of tie lines, but not the compositions of the coexisting phases. Simulations with multi-body interactions produced a much better fit to the experimental phase diagram. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H39.00007: Structural studies of mixed lipid bilayers on solid substrates using x-ray reflectivity Gang Chen, Mrinmay Mukhopadhyay, Yicong Ma, Sunil Sinha, Zhang Jiang, Curt DeCaro, Justin Berry, Laurence Lurio, Adrian Brozell, Atul Parikh The lipid bilayers of natural membranes generally exist in a fluid state which occurs above the gel to liquid crystalline phase transition temperature. Knowledge of the structure of such bilayers is important for understanding fundamental biological processes mediated by or occurring within membranes. We have performed systematic measurements on bilayers of 1,2-Dipalmitoyl-\textit{sn}-Glycero-3-Phosphoethanolamine (DPPE) and its mixture with 1,2-Dioleoyl-\textit{sn}-Glycero-3-Phosphocholine (DOPC) and cholesterol (CH) on silicon substrates with x-ray reflectivity both below and above their phase transition temperatures. Structural variations as a function of temperature are demonstrated by fitting the reflectivity data with both a model dependent and a model independent routine. Studies of Au nanoparticle labeled DOPC and DOPC + DPPE + CH mixture are also performed and the location of Au nanoparticles in these bilayers is established by analyzing the x-ray reflectivity data. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H39.00008: Cellular adhesion and dynamic membrane tether extraction Sarah Nowak, Tom Chou We consider the energetics and dynamics of pulling a ligand bound to an integral membrane receptor. Deformation of the cell membrane and cytoskeleton is considered as the ligand is pulled. We assume that deformation of the cytoskeleton obeys Hook's law up to a critical force at which the cell membrane locally detaches from the cytoskeleton and a membrane tether forms. Depending on the pulling velocity and force, a membrane tether of varying length may form before the receptor-ligand bond breaks. We study the probability of tether formation and the mean tether length at the moment of ligand detachment as a function of system parameters. This problem is applicable to AFM studies of cellular adhesion molecules, and to the biological problem of leukocyte rolling. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H39.00009: Lipid Gymnastics: Tethers and Fingers in membrane Lobat Tayebi, Gregory Miller, Atul Parikh A significant body of evidence now links local mesoscopic structure (e.g., shape and composition) of the cell membrane with its function; the mechanisms by which cellular membranes adopt the specific shapes remain poorly understood. Among all the different structures adopted by cellular membranes, the tubular shape is one of the most surprising one. While their formation is typically attributed to the reorganization of membrane cytoskeleton, many exceptions exist. We report the instantaneous formation of tubular membrane mesophases following the hydration under specific thermal conditions. The shapes emerge in a bimodal way where we have two distinct diameter ranges for tubes, $\sim $20$\mu $m and $\sim $1$\mu $m, namely fat fingers and narrow tethers. We study the roughening of hydrated drops of 3 lipids in 3 different spontaneous curvatures at various temp. and ionic strength to figure out the dominant effect in selection of tethers and fingers. Dynamics of the tubes are of particular interest where we observe four distinct steps of birth, coiling, uncoiling and retraction with different lifetime on different thermal condition. These dynamics appear to reflect interplay between membrane elasticity, surface adhesion, and thermal or hydrodynamic gradient. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H39.00010: Pseudo-phase Diagram of Cholesterol-Rich Filamentous, Helical Ribbon, and Crystal Microstructures Y.A. Miroshnikova, M. Elsenbeck, Guanqing Ou, Y.V. Zastavker, K. Kashuri, G.S. Iannacchione Optical and calorimetric techniques are employed to study temperature and concentration dependence of three self-assembled microstructure types formed in Chemically Defined Lipid Concentrate (CDLC): filaments, helical ribbons, and crystals. CDLC consists of cholesterol, bilayer-forming amphiphiles, and micelle-forming amphiphiles in water, and is considered to be a model system for cholesterol crystallization in gallbladder bile. Phase contrast and DIC microscopy indicate the presence of all three microstructure types in all samples studied. Optically observed structural evolution indicates that filaments first bend to form helical ribbons followed by clustering and ``straightening'' of these structures into short and increasingly thickening filaments that dissolve with increasing temperature. Complementary calorimetric studies (differential-scanning and modulation) reveal thermal signatures that correspond to this observed structural evolution, which occurs throughout a large region of metastable chemical coexistence. These results suggest that a pseudo-phase diagram for the microstructures formed in CDLC may be developed to explain the observed behavior of the system. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H39.00011: PTEN interaction with tethered bilayer lipid membranes containing PI(4,5)P$_{2}$ R. Moldovan, S. Shenoy, P. Shekhar, A. Kalinowski, A. Gericke, F. Heinrich, M. Loesche Synthetic lipid membrane models are frequently used for the study of biophysical processes at cell membranes. We use a robust membrane model, the tethered bilayer lipid membrane (tBLM), based on a (C14)$_{2}$-(PEO)$_{6}$-thiol anchor, WC14 [1]. Such membranes can be prepared to contain single phospholipids or complex lipid mixtures [2], including functional lipids involved in cell signaling, such as the highly charged phosphatidylinositol phosphates (PIPs). To study the interaction between the tumor suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10) and model membranes we have incorporated phosphatidylinositol-4,5-bisphosphate (PI(4,5)P$_{2}$) in tBLMs and use fluorescence correlation spectroscopy (FCS), neutron reflectometry (NR) and surface plasmon resonance (SPR) for their characterization. NR shows that tBLMs formed with PI(4,5)P$_{2}$ are complete. FCS of labeled PI(4,5)P$_{2}$ shows that diffusion occurs at the time scale characteristic of membrane-incorporated lipid. Finally, SPR shows specific binding of PTEN to the model membrane thus confirming the incorporation of PI(4,5)P$_{2}$ into the tBLM. [1] McGillivray et al, Biointerphases 2, 21-33 (2007) [2] Heinrich et al, Langmuir, submitted [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H39.00012: Subdiffusion and diffusion of lipid atoms and molecules in phospholipid bilayers Elijah Flenner, Jhuma Das, Maikel Rheinstadter, Ioan Kosztin We examine the dynamics of lipid atoms and molecules using a 0.1 $\mu$s all-atom molecular dynamics simulation of a hydrated diyristoyl-phosphatidycholine (DMPC) lipid bilayer. We identify three well separated time regimes in the mean square displacement, $\langle \delta r^2(t) \rangle$, of the lipid atoms and molecules: (1) a ballistic regime for $t<10$ femptoseconds; (2) a subdiffusive regime where $\langle \delta r^2(t) \rangle \sim t^\beta$ and $\beta < 1$ for times between 10 picoseconds and 10 nanoseconds; and (3) a Fickian diffusion regime where $\langle \delta r^2(t) \rangle \sim t$ for $t > 30$ nanoseconds. We propose a memory function approach for describing the mean square displacement over the whole time range, and find that the lateral self diffusion coefficient found from the memory function approach agrees well with the one determined from the mean square displacement. We use the cumulant expansion of the self-intermediate scattering function to connect the three time scales in the mean square displacement to the interpretation of neutron scattering signals. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H39.00013: Suppression of fluid membrane fluctuations by a periodic pinning potential: Applications to red blood cells. Mark L. Henle, Alex J. Levine The membrane of the red blood cell (RBC) is tethered to a two- dimensional triangular network of semi-flexible elastic spectrin filaments. This network allows the cell to maintain its structural integrity during the large shape deformations that occur as it circulates through the microvasculature. The lipid membrane is anchored to the spectrin filaments at the nodes of the network. Consequently, these attachments impose a two-dimensional periodic pinning potential upon the membrane. In this talk, we investigate the effect of this pinning potential on the thermal bending fluctuations of the membrane. We show that there is an exact mapping of this system onto the classic problem of non-interacting electrons subject to a periodic potential; we exploit this mapping to obtain an exact analytic solution for a defect-free triangular array of harmonic pinning sites. The pinning potential affects both the local and global structure of the bending fluctuations. To investigate the local structure we consider the bending correlations between two nearby points in the membrane, while for the global structure we consider the total area stored in the fluctuations. We also investigate the effective area modulus of the membrane/spectrin composite structure. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H39.00014: Rough-Smooth-Rough Interface transition in a supported lipid bilayer system Piyush Verma, Nick Melosh Dynamic evolution of interfaces with quenched disorder is common in nature including fluid flow in porous media, granular particle flow and bacterial colony growth. These interfaces, which are either modeled using the Quenched-KPZ equation or the Quenched-Edward-Wilkinson (QEW) equation, expand due to a driving force while the edge profile roughens monotonically over time due to a distribution of disordered trapping defects. We studied interface expansion of a supported phospholipid bilayer, which is an ideal two dimensional viscoelastic material. Surprisingly, we observed a unique rough-smooth-rough bilayer interface transition on chromium oxide which has never been reported before. This transition was found to be a result of the viscoelasticity of the lipid bilayer and could be modeled using a modified QEW equation, which includes a spring-like term to account for the bilayer elasticity. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H39.00015: The effect of curvature on the undulation spectrum of Red Blood Cell membranes Tatiana Kuriabova, Mark L. Henle, Alex J. Levine The human red blood cell (RBC) membrane has a composite structure of a fluid lipid bilayer tethered to an elastic 2D spectrin network. The study of the mechanical properties of RBCs is crucial to our understanding of their ability withstand large amplitude deformations during their passage through the microvasculature. The linear mechanical response of this composite membrane can be measured by observing its undulatory dynamics in thermal equilibrium, i.e. microrheology. Previous models of these dynamics postulated an effective surface tension. In this talk, we show that surface tension is not necessary. Rather, the coupling of membrane bending to spectrin network compression by curvature can account for the observed dynamics. We use a simplified theoretical model to describe the undulatory dynamics of RBCs, measured experimentally by the Popescu group.\footnote{G. Popescu et al. ``Imaging red blood cell dynamics by quantitative phase microscopy, Blood Cells, Molecules, and Diseases, (2008), in print''} Analyzing their data using our model, we observe dramatic changes in RBC membrane elasticity associated with cells' morphological transition from discocytes to echinocyte to spherocyte. [Preview Abstract] |
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