Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y34: Properties of Biological Membranes |
Hide Abstracts |
Sponsoring Units: DBP Chair: Dobrin Bossev, Indiana University Room: Colorado Convention Center 404 |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y34.00001: Bending Elasticity of Bio-Membranes Studied by Neutron Spin-Echo Zheng Yi, Dobrin Bossev We have used neutron spin echo (NSE) spectroscopy to study the effects of the unsaturated double bond and the hydrocarbon chain length on the bending elasticity of lipid membranes. The bending elasticity \textit{$\kappa $} of bilayer vesicles made of 1,2-Dioleoyl-\textit{sn}-Glycero-3-phosphocholine(18:1 PC), has been measured in the fluid (L$_{\alpha })$ phase in different temperatures. When lipid bilayers made of DOPC are in fluid phase, the temperature effect on bending elasticity is minimal. The bending elasticities of 14:1 PC and 16:1 PC were measured in fluid phase in 30\r{ }C. We found that the lipid bilayers with longer chains have higher bending elasticities. Our data confirms that the stiffening of lipid bilayers increases with increasing chain length of the lipid molecules. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y34.00002: Using Neutron Spectroscopy to Study Collective Dynamics of Biological and Model Membrane Systems Maikel Rheinstadter Only recently, it has become possible to study collective dynamics of planar lipid bilayers using neutron spectroscopy techniques. By combining different neutron scattering techniques, namely three-axis, backscattering and spin-echo spectroscopy, we present measurements of short and long wavelength collective fluctuations in biomimetic and biological membranes in a large range in momentum and energy transfer, covering time scales from about 0.1ps to almost 1$\mu $s and length scales from 3{\AA} to about 0.1$\mu $m [1-4]. The measurements offer a large window of length and time scales to test and refine theoretical models of dynamics of biomimetic and biological membranes. The objective of this project is to establish dynamics-function relationships in artificial and biological membranes to relate in particular the collective dynamics, i.e., phonons, to key functions of the membranes, as, e.g., transport processes within and across the bilayers. M.C. Rheinst\"{a}dter, C. Ollinger, G. Fragneto, F. Demmel, T. Salditt, \textit{Phys. Rev. Lett.} \textbf{93}, 108107 (2004).$^{2}$ Maikel C. Rheinst\"{a}dter, Tilo Seydel, Franz Demmel, Tim Salditt, \textit{Phys. Rev. E} \textbf{71}, 061908 (2005).$^{3}$ Maikel C. Rheinst\"{a}dter, Wolfgang H\"{a}u{\ss}ler, Tim Salditt, \textit{Phys. Rev. Lett.} \textbf{97}, 048103 (2006).$^{4}$ Maikel C. Rheinst\"{a}dter, Tilo Seydel, Tim Salditt, submitted to PRE, cond-mat/0607514. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y34.00003: Damping of the thermal undulations of bio-membranes Dobrin Bossev, Zheng Yi In this work we discuss the damping mechanisms of the thermal undulation of lipid membranes. In the past, we have attempted to determine the bending elasticity of bio membranes by neutron spin-echo spectroscopy (NSE) as a function of the temperature, molecular structure of the phospholipids, ionic strength of the surrounding aqueous environment, and presence of cholesterol. NSE is ideal for studies of the thermal undulations of the biomembranes because it probes the short correlation times (0.01--100 ns) and length scales (10--100 {\AA}) that are characteristic for the biomembrane undulations. The bending modulus of elasticity is obtained through analysis of the intermediate scattering function I(Q,t) using Zilman-Granek theory, which considers the solution viscosity as the only damping mechanism for the thermal undulations. As a result the absolute k values are about an order of magnitude greater than those measured by other methods and predicted by simulations. Here we report measurements in water/glycerol mixtures in attempt to modify the bulk viscosity and to clarify the contribution of the different energy dissipation mechanisms. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y34.00004: Interaction Forces and Mechanics of Cellular Membranes using Novel Atomic Force Microscopy Probes Benjamin Almquist, Nicholas Melosh In order to probe the nature of nanostructure-membrane interfaces, we have developed an AFM probe platform that can quantitatively measure the interaction forces between specifically functionalized layers and the cell membrane. This platform consists of a cantilever with a post-style tip that ends in a hetero-metallic layer. This metallic layer can be selectively functionalized with various molecules of interest. Once functionalized, the layer is inserted into the hydrophobic region of the cell membrane. By varying the molecular species and examining the associated penetration and extraction forces, we will be able to correlate the molecule-membrane interaction forces to the molecular structure. This, in turn, will allow us to determine the role of molecular size, hydrophobicity, and disorder. In addition, the effects of functional layer thickness and post geometry will be examined. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y34.00005: Measuring Surface Potential of Zwitterionic Lipid Bilayers with Atomic Force Microscope Yi Yang, Kathryn Mayer, Jason Hafner Electrostatic potential was measured near supported zwitterionic lipid bilayer membrane surfaces with atomic force microscope. In our recent work, two methods were developed to measure the surface charge density of the membrane surface, Fluid electric force microscopy (FEFM) which creates a two- dimensional map of a surface charge density with a corresponding topographic map simultaneously and quantitative measurement method which based on tip-sample force curve analysis.Both FEFM and tip-sample force curve analysis showed that the surface of a DOPC (dioleoylphosphatidylcholine) lipid bilayer carries a negative electrostatic potential.This is an interesting and surprising result, for the head group of DOPC is carrying zero net charge over a broad range of pH where both the choline and phosphate groups are ionized.Two sources are proposed to explain the origin of this negative charge.The bilayers could carry a net charge density due to the counterions from the electrolyte binding to the lipid head groups.Alternatively, the dipole density in the DOPC lipid head group layer could cause an effective surface potential outside the membrane region.To study the source of this negative potential, Charge densities of supported DOPC bilayers under different ion concentrations were measured and compared with both of these two charge mechanisms. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y34.00006: Molecular Organization and Dynamics of Cholesterol Nanodomains in Fluid Lipid Bilayers Kwan Cheng, Brian Cannon, Qing Zhu, Mark Vaughn, Juyang Huang The molecular organization and dynamics of cholesterol nanodomains in lipid bilayers containing phospholipid (PL) and cholesterol (CHOL) were examined using FTIR, time-resolved fluorescence and surface-acting cholesterol oxidase enzyme (COD). In binary PL/CHOL system, abrupt changes in the PL C=O frequency, fluorescence lifetime and rotation rate of chain labeled PL, and the rate of cholesterol oxidation by COD were observed at $\sim $ 40 mole{\%} of CHO. For ternary PL$_{1}$/PL$_{2}$/CHOL system composed of two dissimilar PL's of different chain lengths or headgroup sizes, abrupt changes at PL$_{1}$/PL$_{2} \quad \sim $ 2 were found. The above critical lipid compositions agree favorably with the theoretical compositions predicted by the lipid superlattice model, suggesting that PL of different structures and CHOL can form regularly distributed, or superlattice-like, nanodomains at the polar headgroup and the acyl chain levels, respectively. The feasibility of the coexistence of headgroup and acyl chain nanodomains was demonstrated by a spacing filling model and MD simulations. We speculate that lipid superlattice domains may play an important role in the regulation of protein/lipid interaction in cells. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y34.00007: Temperature and Composition Dependent Phase Behavior in Two ``Raft-Like'' Ternary Membrane Mixtures: DPPC/DLPC/Cholesterol and DPPC/DOPC/Cholesterol Jeffrey Buboltz, Geoffrey Siegel, Matthew Schutzer, Krystle Williams, Charles Bwalya, Santiago Reyes For the last several years, so-called ``lipid-raft'' membrane domains have been the subject of intense research activity. As part of this effort, we have been carrying out experiments based on Probe-Partitioning FRET, a technique specifically designed to map out both phase boundaries and tie lines in artificial membrane mixtures. Specifically, we have studied two cholesterol-rich ternary mixtures, DPPC/DLPC/Cholesterol and DPPC/DOPC/Cholesterol, that mimic lipid-raft phase behavior. By studying more than 3000 independently prepared samples, we have gained insight into the general features (i.e., both temperature and composition dependence) that characterize the phase behavior in these two ternary systems. As we work toward extending our studies to other raft-like ternary mixtures, we are also adapting a different, purely thermodynamic technique (Equilibrium Surface Pressure Analysis) for the purpose of corroborating tie line patterns inferred from PP-FRET. [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y34.00008: On the interactions between neutral lipid bilayers Oscar Calvo, Marian Manciu, Eli Ruckenstein The stability of many colloids is thought as a balance between attractive van der Waals interactions and double layer repulsive forces. However, the latter does not exist for neutral lipid bilayers, for which the repulsive forces are supposed to be provided by a combination between hydration forces and Helfrich forces, due to the suppression of the thermal undulation, when two bilayers approach each other. Hydration forces are related to the structuring of water near surfaces, which is likely to be decreased by the thermal undulations. Helfrich forces have a longer range than the attractive forces and cannot lead by themselves to a stable minimum. We will show that the polarization model for the hydration forces combined with a statistical treatment for undulating bilayers might explain the interactions between neutral lipid bilayers. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y34.00009: Effect of dipolar moments in domain sizes of lipid bilayers and monolayers Alex Travesset Lipid domains are found in systems such as multi-component bilayer membranes and single component monolayers at the air water interface. It was shown by McConnell and collaborators that in monolayers the size of the domains results from balancing the line tension, which favors the formation of a large singular single circular domain, against the electrostatic cost of assembling the dipolar moments of the lipids. In this talk, I will generalize this argument to include effects of ionic strenght, dielectric discontinuities (or image charges) and the polarizability of the dipoles and extend the results to bilayer membranes. I will finish with a discussion on the experimental implications of the calculations. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y34.00010: Non-equilibrium dynamics of heterogeneous lipid membranes Mikko Haataja, Jun Fan, Maria Sammalkorpi Plasma membranes surrounding mammalian cells play a key role in regulating the exchange of information and matter between the cells and their surroundings. The unique properties of these membranes arise from the interactions between amphiphilic lipid molecules, sterols (incl. cholesterol), and proteins. It has been proposed that the plasma membrane displays dynamic heterogeneities (lipid rafts) in the local lipid composition. While such rafts have not yet been observed directly in vivo, there is ample indirect evidence that supports their existence. From a fundamental biophysical perspective, processes which may control the aggregation and stability of these rafts are poorly understood at the moment. Here, we address this issue by introducing a continuum model for the local lipid composition which incorporates non-equilibrium aspects of lipid recycling to and from the membrane. We show that recycling leads to coherent structures with a characteristic size which depends on both the recycling rate and the tendency of the components to phase separate in the absence of recycling. We argue that incorporating non-equilibrium effects is crucial in understanding the biophysical properties of the plasma membrane. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y34.00011: Mechanisms of protein transduction domains: HIV TAT and ANTP penetratin as prototypical cases Abhijit Mishra, Nathan Schmidt, Vernita Gordon, Gerard Wong Biologically active molecules such as proteins and oligonucleotides can be transduced across cell membranes with high efficiency when covalently linked to a Protein Transduction Domain (PTD), such as the TAT domain in the HIV virus and ANTP from the fruitfly. All PTDs have a high content of basic amino acids resulting in a net positive charge. Electrostatic interactions between cationic PTDs and the negatively charged phospholipids that constitute the plasma membrane are likely to be responsible for peptide uptake, but no detailed structural studies exist. We examined membrane structures induced by the cationic TAT domain and those induced by other cationic polypeptides as a function of membrane composition using synchrotron x-ray scattering. We find that both the TAT PTD and ANTP generate negative Gaussian curvature, which is necessary for pore formation, and produce a bicontinuous Pn3m double diamond cubic phase. A general mechanism is proposed. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y34.00012: Molecular Insights into Phospholipid -- NSAID Interactions Mohan Babu Boggara, Ramanan Krishnamoorti Non steroidal anti inflammatory drugs (NSAIDs) e.g. Aspirin and Ibuprofen, with chronic usage cause gastro intestinal (GI) toxicity. It has been shown experimentally that NSAIDs pre-associated with phospholipids reduce the GI toxicity and also increase the therapeutic activity of these drugs compared to the unmodified ones. Using all atomistic simulations and two different methodologies, we studied the partitioning behavior of~ two model NSAIDs (Aspirin and Ibuprofen) as a function of pH and drug loading. The results from two methodologies are consistent in describing the equilibrium drug distribution in the bilayers. Additionally, the heterogeneity in density and polarity of the bilayer in the normal direction along with the fact that NSAIDs are amphiphilic (all of them have a carboxylic acid group and a non-polar part consisting of aromatic moieties), indicate that the diffusion mechanism in the bilayer is far different compared to the same in a bulk medium. This study summarizes the various effects of NSAIDs and their behavior inside the lipid bilayer both as a function of pH and drug concentration. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y34.00013: Non-equilibrium Lipid Distributions in a Simulated Three-Species Biomembrane Andrew P. Paradis, Susan R. McKay, Samuel T. Hess Cellular biomembranes are in continual states of flux, yet theoretical models of biomembranes have primarily focused on equilibrium behavior, where constituent species interact but are not driven. This study examines the complex phase behavior of a three-species biomembrane driven out of equilibrium through frequent, simulated endo- and exo-cytosis events. The three species, representing unsaturated lipids, saturated lipids, and cholesterol, move and interact on a two-dimensional triangular lattice, simulated using a Metropolis algorithm. Two types of phase behavior are specifically investigated and discussed: cholesterol super-lattice structures and phase separation of saturated and unsaturated lipids, both as functions of cholesterol mole fraction and temperature. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y34.00014: Simulations of the Pore Structures for a M2GlyR Derived Channel Forming Peptide in Different Membrane Environments A. Al-Rawi, A. Herrera, J. Tomich, T. Rahman As part of an effort to develop a peptide-based compound suitable for clinical use as a channel replacement therapeutic for treating channelopathies such as cystic fibrosis, we present a reductionist model that appears to grasp the characteristics of ion channeling peptides. In particular we present the observed changes in the functional characteristics of NK$_{4}$-M2GlyR p22 (KKKKPARVGLGITTVLTMTTQS), a M2 GlyR derived channel forming peptide. Starting with a structure determined by multidimensional NMR (800 MHz) in SDS, a potential from CHARMM force-field was used to relax the structure of NK$_{4}$-M2GlyR p22. Following the relaxation, numerous pore structures were generated for the symmetric five-helix assembly with geometries varying from cylindrical to conical. As it is difficult \textit{a priori} to assign accurately the orientation of the hydrophilic portion of M2GlyR derived amphipath towards the inside of the pore, we tilted and rotated the helical structure by five different angles about the backbone axis before forming the pore. Energy minimization of the channel was performed in vacuum, in phosphotidylcholine (POPC) membrane, and 60{\%} POPC 30{\%} phosphotidylethanolamine (POPE) in order to determine the effect of the environment surrounding on the structure on its energy minimization. We will present the various pore assemblies, in the different membrane environments, used to predict the most probably membrane bound structure. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y34.00015: The Effects of Polyunsaturated Lipid Components on bilayer Structure Y. Pramudya, A. Kiss, Lam T. Nguyen, J. Yuan, Linda S. Hirst Polyunsaturated fatty acids (PUFAs), such as DHA (Docosahexanoic Acid) and AA (Alphalinoleic Acid) have been the focus of much research attention in recent years, due to their apparent health benefits and effects on cell physiology. They are found in a variety of biological membranes and have been implicated with lipid raft formation and possible function, particularly in the retinal rod cells and the central nervous system. In this work lipid bilayer structure has been investigated in lipid mixtures, incorporating polyunsaturated fatty acid moieties. The structural effects of increasing concentrations of both symmetric and asymmetric PUFA materials on the bilayer structure are investigated via synchrotron x-ray diffraction on solution samples. We observe bilayer spacings to increase with the percentage of unsaturated fatty acid lipid in the membrane, whilst the degree of ordering significantly decreases. In fact above 20{\%} of fatty acid, well defined bilayers are no longer observed to form. Evidence of phase separation can be clearly seen from these x-ray results and in combination with AFM measurements. [Preview Abstract] |
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