Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A45: Focus Session: Structure and Dynamics of Biomembranes I |
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Sponsoring Units: DBIO DPOLY Chair: Fredrick Heberle, Oak Ridge National Laboratory Room: Hilton Baltimore Holiday Ballroom 4 |
Monday, March 18, 2013 8:00AM - 8:36AM |
A45.00001: Molecular simulation studies of edges in bilayers and bicelles Invited Speaker: James Kindt The instability of the free edge of a lipid bilayer can be quantified by a line tension, or excess free energy per unit length of the edge. Atomistic simulations of bilayer ribbons composed of a series of lipids with varying tail lengths and degrees of saturation have been performed to determine line tensions, with the goal of relating edge stability to structural and elastic properties of the bilayer. Line tensions are relevant to the mechanical stability of bilayer membranes, and can be reduced or eliminated by the inclusion of edge-stabilizing molecules (edge-actants) to the bilayer system. Mixtures of long- and short-tailed phospholipids are known to form aggregates known as ``bicelles'' that contain bilayers with stable edges. Simulations of ``bicelle'' mixtures using coarse-grained and atomistic lipid models have been performed to study the partitioning of short-tailed lipids towards the edge and the flexibility of the stabilized edge. Input from these simulations has been used in the development of simple thermodynamic models to rationalize some aspects of bicelle aggregate morphology. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A45.00002: Shear-induced alignment of ``bicellar'' phospholipid membranes Mu-Ping Nieh, Ming Li, Norbert Kucerka ``Bicellar'' phospholipid mixtures, composed of two types of lipids (i.e., long-chain and short-chain lipids), self-assemble into a magnetically alignable bilayered structure. As a result, the model membranes have been used as membrane substrates for the structural study on membrane-associated proteins in many nuclear magnetic resonance experiments. In this presentation, I will demonstrate the shear-alignability of the bicellar model membranes through an in-situ neutron diffraction study under shear flows, the important controlling parameters and their applications and biological implications. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A45.00003: Estimation of Structural Properties Of The Thermally Fluctuated Membrane Based on The Small-Angle Neutron Scattering Data Takumi Hawa, Victor Lee SANS (Small-angle neutron scattering) and SAXS (Small-angle X-ray scattering) experiments are one of the most important laboratory techniques to determine nanoscale structure of biological and nanotechnology-related systems. These experimental techniques provide extensive information due to the sensitivity of about 1-1000 nm and 2-25 nm length scales for SANS and SAXS, respectively. Recently, the author and his collaborator, Dr. Nagao, studied swollen lamellar structure systems consisting of nonionic surfactant, water, and oil using SANS/NSE (Neutron Spin Echo) and MD (molecular dynamics) simulation. They proposed a new experimental technique to measure the thickness fluctuations of surfactant layers and verified their approach using MD simulations. In this talk we derive a simple mathematical model to estimate the thickness of the membrane as well as the amplitude and the wavelength of the surfactant layers in the membrane. The model is developed based on the harmonic motion of the surfactant layers. We consider both bending and thickness fluctuation motions of the membrane. The membrane thickness estimated from the proposed approach showed an excellent agreement with the SANS experimental results available in the literatures. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A45.00004: Lipid bilayer dynamics: Effects of segregation between DMPC and DSPC Michihiro Nagao, Paul Butler, Andrea Woodka, Rana Ashkar Dynamics in lipid bilayers are believed to play a key role in membrane stabilization. During the past decade, neutron spin echo (NSE) has been used to study the bending elastic behavior of large unilamellar vesicles (radius of around 50 nm). These results reveal that above Tm, where the lipid tails display liquid ordering, the bending modulus is on the order of 10 kT. Below Tm, the value increases by more than an order of magnitude. Recently NSE revealed thickness fluctuations of lipid bilayers above Tm, while none are discernable below Tm. The estimated amplitude of the observed membrane thickness fluctuations is approximately 4 angstroms and the time scale of the motion is on the order of 100 ns. In the present research, structure and dynamics of mixed lipid between dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) were investigated using small-angle neutron scattering (SANS) and NSE. DSPC has a higher Tm than DMPC. The mixed lipid systems show segregation between domains in the temperature range between Tm of DMPC and DSPC. The SANS and NSE measurements were performed with changing temperature from above to below the Tm of DSPC. The result indicates a slow down of thickness fluctuations once the segregation takes place. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A45.00005: X-ray reflectivity study of a DPPC floating bilayer: Effect of Ca$^{2+}$ ions and temperature Sambhunath Bera, Sajal Ghosh, Yicong Ma, Curt DeCaro, Zhang Jiang, Laurence Lurio, Sunil Sinha We have used a floating bilayer of DPPC (1,2- dipalmitoyl-sn-glycero-3-phosphocholine) to examine the effects on Ca$^{++}$ ion concentration on membrane fluctuations. The density profile of the double bilayer system normal to the support was measured via x-ray specular reflectivity. We find an increase in membrane spacing with ion concentration which we attribute to Ca$^{++}$ ions preferentially binding to the head group of the bilayer. We also find an increase in interfacial roughness which we attribute to the Ca$^{++}$ ions causing a reduction in the layer's bending modulus. These effects are studied as a function of temperature up to the temperature at which the bilayer if found to unbind from the support. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A45.00006: Deposition of Homogeneous Single-supported DMPG Lipid Membranes onto a Silica Substrate for Quasielastic Neutron Scattering Experiments Andrew Miskowiec, Mia Brown, Jason Cooley, Renee Jiji, Haskell Taub, Justin Grayer, Gavin King, Helmut Kaiser, Flemming Hansen, Madhusudan Tyagi We report deposition of single bilayers of dimyristoyl-phosphoglycerol (DMPG) lipid onto a SiO$_{2}$-coated Si(100) substrate. These anionic membranes have large-scale homogeneity comparable to that achieved with single-supported uncharged DMPC membranes used for neutron scattering studies.\footnote{M. Bai, \textit{et al.} Europhys. Lett. \textbf{98}, 48006 (2012).} Optimum deposition conditions were found by systematically varying the lipid concentration and both the monovalent and divalent buffer salt concentrations. Plausible mechanisms for the bilayer stability will be discussed. In addition, we report Atomic Force Microscopy measurements of the membrane thickness as a function of temperature near the gel-to-fluid phase transition. We also report initial results of elastic neutron intensity scans vs. temperature taken on the backscattering spectrometer at NIST that probe the freezing of the membrane-associated water for comparison with results obtained with the neutral membrane DMPC.\footnote{Ibid.} [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A45.00007: Mobility of water and selected atoms in DMPG lipid bilayer membranes F.Y. Hansen, A. Roennest, G.H. Peters, H. Taub, A. Miskowiec Molecular dynamics simulations have been used to study the structure and mobility of water and selected atoms in dimyristoyl-phosphoglycerol (DMPG) lipids forming a fully hydrated free standing bilayer membrane at 310 K. The effect of the anionic headgroup in DMPG on structure and dynamics has been studied by comparison with simulation\footnote{F.Y. Hansen {\em et al.}, J. Chem. Phys., in press.} and experimental\footnote{M. Bai {\em et al.}, Europhys. Lett. {\bf 98}, 48006 (2012).} results for bilayer membranes of dimyristoyl-phosphorylcholine (DMPC) lipids, which have a neutral head group and the same aliphatic tails. The membrane is found to be in the fluid phase with monovalent sodium counter ions and in the gel phase with divalent calcium counter ions as evidenced by an area/lipid change and the NMR order parameter. The simulation results are compared with preliminary neutron scattering results. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A45.00008: High-speed Membrane Imaging with Digital Holography Thomas Dimiduk, Amy Chen, Laura Arriaga, Vinothan Manoharan Lipid membranes can change on timescales faster than traditional three dimensional imaging tools can follow. Digital holography offers a the potential to observe observe membranes in 3D at 1000 Hz or greater to resolve dynamics down to thermal fluctuations. This works because holography encodes 3D information into a single 2D image, allowing imaging limited only by camera speed. However, precise quantitative interpretation of holograms has proved challenging for samples of any complexity. To address this limitation, I am developing methods based on the discrete dipole approximation and a new mathematical approach to solving inverse problems. I will present these methods and preliminary measurements of membrane dynamics using holography. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A45.00009: Compositional interface dynamics within symmetric and asymmetric planar lipid bilayer membranes Tao Han, Mikko Haataja Compositional domains within multicomponent lipid bilayer membranes are believed to facilitate many important cellular processes. In this work, we will first develop a general model of planar lipid bilayer membrane within a phase field framework, which includes not only advective and diffusive lipid transport mechanism, but also incorporates an asymmetry between the lipid compositions and thermodynamic behavior between the two leaflets, as well as an intermonolayer thermodynamic coupling and friction effects. Then, we will derive the general equations that describe the dynamics of compositional domains within planar membranes with asymmetry in leaflet properties and in the presence of a thermodynamic coupling between the leaflets. These equations are then employed to develop analytical solutions to the dynamics of the recurrence of registration for circular domains in the case of weak coupling. The validity of the analytical solutions is established by a direct comparison between the predicted dynamics and those obtained from numerical simulations of the the phase-field model. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A45.00010: The effects of cholesterol concentration in lipid packing and domain registration in ternary mixture lipid multilayer Yicong Ma, Sajal Ghosh, Laura Connelly, Ratneshwar Lal, Sunil Sinha The effects of cholesterol in membrane rafts formation remain a mystery even until today. In our study of model membrane multilayer systems consisting of DPPC/DOPC/Cholesterol, we have characterized the morphology changes using AFM and optical microscopy, and the bilayer electron density profile using X-ray reflectivity, as a function of cholesterol concentration. In this presentation, we shall discuss how the cholesterol concentration affects the lipid packing within the bilayer, as well as the interlayer coupling of phase separated domains. X-ray scattering, AFM and optical microscopy which look at different length scales would constitute a complete picture. Our results may shed new light on the understanding of the role of cholesterol in raft formation in biological membranes. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A45.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:36AM - 10:48AM |
A45.00012: The Effects of Ca$^{2+}$ on the Dynamics of PIP$_2$ containing Lipid Bilayers Ian McCabe, Martin Forstner Changes of intercellular Ca$^{2+}$ concentrations are one of the most ubiquitous signaling events that accompany or precede large scale cellular responses. We are in particular interested in the direct modulation of phosphatidylinositol 4,5-bisphosphate (PIP$_2$) organization in the membrane due to Ca $^{+2}$. At physiological conditions, PIP$_2$'s headgroup is multiply negatively charged (\textgreater\ 3 effective charges) and interacts with the cationic Ca$^{2+}$. By coordinating several PIP$_2$ head-groups, calcium ions can induce condensation and aggregation of PIP$_2$. A series of experiments were conducted on supported lipid bilayers containing physiological quantities of PIP$_2$. Fluorescence correlation spectroscopy (FCS) was used to study the response of the PIP$_2$ to changes in the concentration of Ca$^{2+}$ ions. As Ca$^{2+}$ concentration increases, the FCS indicates that PIP$_2$ goes from a freely diffusing single species to a multiple species system. The diffusion rates of the additional species decrease with increasing [Ca$^{2+}$], thus indicating increasing aggregate sizes with increasing, but physiological relevant Ca$^{2+}$ concentrations. An intriguing effect was observed at very low Ca$^{2+}$ levels. The diffusion rate was consistently measured to increase upon addition of small concentrations of Ca$^{2+}$ before decreasing as the concentrations increased. A series of polymer cushioned bilayers were used to attempt to gain greater insight into the nature of the membrane/support interaction and the nature of this effect. [Preview Abstract] |
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