Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B17: Lipid Bilayers: Structure and Function |
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Sponsoring Units: DBP Chair: Linda Hirst, Florida State University Room: Morial Convention Center 209 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B17.00001: Zoology of giant unilamellar vesicles Yan Yu, Stephen M. Anthony, 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. For example, tube-like membranes are formed when flow is introduced during hydration and when certain types of lipids or polymers are inserted into the membrane. This talk will describe the evolution of GUVs from spherical to pearl-like and to tube-like shapes, and back again reversibly. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B17.00002: Adhesion induces and localizes phase separation in lipid membranes Markus Deserno, Vernita Gordon, Caroline Andrew, Stefan Egelhaaf, Wilson Poon We study, using confocal microscopy, model membranes in the form of giant unilamellar vesicles (GUVs) consisting of a mixture of two lipids. We demonstrate that, near a demixing transition, adhesion can favor phase separation, and thus induce the formation of well-defined, localized heterogeneities in a variety of lipid systems. We outline a theoretical framework in which this may be understood as the result of suppressing thermal fluctuations in the adhering areas and consequently favoring demixing. Our findings have important implications for the mechanisms by which biomembranes may create and stabilize functional heterogeneities, such as rafts and focal adhesion sites. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B17.00003: Cholesterol Effect on Phase Behavior in Ternary Lipid Membrane---X-ray Diffraction and AFM. Jing Yuan, Alex Kiss, Yohanes Pramudya, Lam Nguyen, Linda Hirst There is growing evidence that lipid membranes are not uniform, but contain lipid microdomains or ``rafts'', which are enriched in cholesterol, saturated long-chained lipids, and particular proteins. The effects of cholesterol on lipid ordering and phase separation in lipid-rafts-contained model membrane systems have been investigated by Synchrotron X-ray Diffraction and Atomic Force Microscope (AFM). We have measured bilayer d-spacings in two ternary lipid mixtures: DOPC/eSM/Cholesterol and DOPC/DPPC/Cholesterol, as cholesterol content is varied. Mixtures containing intermediate amounts of cholesterol exhibited two phases, and for DOPC/eSM/Cholesterol with 10{\%} and 12{\%} cholesterol, three d-spacings were observed, indicating the possible coexistence of three different phases: liquid disordered (ld) phase, liquid ordered (Lo) phase, and gel phase. AFM images of supported lipid bilayers on mica substrates contained clearly visible raft-like micro-domains in the similar cholesterol amount range. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B17.00004: Thermodynamic properties of planar membranes: applications to stripped phases Francisco J. Solis, Chloe Funkhouser, Katsuyo Thornton Multicomponent membranes can have shapes that are planar at large length scales while retaining complex morphologies at smaller scales. We explore the properties of these membranes that arise from the planarity condition. We show that planarity requires that the average stress tensor of the membrane be parallel to the planar directions. We apply this description of planar membranes to the case of striped (lamellar) morphologies. In this case we determine the possible shapes of membranes, their stability and the thermodynamic equations of state satisfied by their intensive variables. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B17.00005: Phospholipid Membranes Restructure Locally Where Nanoparticles Bind Bo Wang, Yan Yu, Stephen Anthony, Sung Chul Bae, Steve Granick In the field of surface science, it is known that metal and semiconductor surfaces may respond to their environment by restructuring. Similar issues are more significant in nanoscience, since large populations of the atoms/molecules reside on the surfaces. It is natural to inquire whether analogous restructuring might also be characteristic of phospholipid membranes, bearing mind that no bulk exists at all in this case. We show here that the two categories of reconstructions, phase state and local curvature, of unicomponent lipid bilayers can occur through non-specific interactions when charged nanoparticles adsorb. This coupling not only modulates the short-range molecular orientation and packing, but also is believed to be responsible for long-range interaction and transportation on fluctuating membranes. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B17.00006: Multiscale Modeling of supported lipid bilayers Roland Faller, Chenyue Xing, Matthew Hoopes The study of lipid structure and phase behavior at the nano scale length is of importance due to implications in understanding the role of the lipids in biochemical membrane processes. We performed a variety of simulations in homogeneous and heterogeneous membrane systems to elucidate such behaviors. Our simulations demonstrate that various coarse grained simulation models can predict different aspects of lipid phase separation and describe the change of the system under the influence of a support. The simulations are performed using models at different length scales ranging from the all atom scale to a scale where lipids are modeled by only three interaction sites. We are able to follow transformations, such as lipids phase transitions. These phase transitions are determined by analyzing parameters like area per lipid head group, the deuterium order parameter and dynamic properties. Additionally, we characterize individual lipid molecules using rotational correlation functions to classify different dynamic populations and we study the stability of artificially designed patterns. We discuss the changes of the system phase behavior as well as differences between the two leaflets as induced by the support. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B17.00007: Synergy of Membrane Curvature-Stabilization and Electrostatic Interaction leads to Formation of Block Liposomes by Colossal Charged Lipids Alexandra Zidovska, Kai K. Ewert, Cyrus R. Safinya, Joel Quispe, Bridget Carragher, Clinton S. Potter Recently, we have reported block liposomes (BLs), a new vesicle phase formed in mixtures of MVLBG2, DOPC and water (A. Zidovska et al., \textit{Submitted}, 2007), where MVLBG2 is a newly synthesized highly charged (16+) lipid (K. Ewert et al., \textit{JACS, }2006) with giant dendrimer-like headgroup. BLs are liposomes consisting of distinctly shaped nanoscale spheres, pears, tubes, or rods connected into blocks. In this work we investigate the contribution of spontaneous curvature and membrane charge density to the formation of BLs. By comparing with a system of matching membrane charge density but zero spontaneous curvature and by screening the charge of MVLBG2 but keeping the curvature constant, we were able to identify both, spontaneous curvature and membrane charge, as critical parameters for BLs-formation. The effect of salt and pH on the shape evolution of the BLs was also carefully studied. Funding provided by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B17.00008: Photo-induced Phase Separation Phenomena in Lipid Tubules Linda Hirst, Jing Yuan The self-assembly of biological amphiphiles has proved a fascinating topic in recent years, the hollow cylindrical lipid tubule morphology being of particular interest due to its potential relevance to intercellular transporting channels and applicability to controlled-release systems, chemical micro-reactors and nano-conduits. Co-existence of the liquid-ordered and liquid-disordered phases in the lipid bilayer has recently been observed in biologically-relevant three-component giant unilamellar vesicles. We have generated stable, photo-induced micron-scale phase separation in lipid tubules formed from ternary lipid mixtures, inducing a new bilayer disc structure. This investigation not only aids in our understanding of lipid sorting phenomena in cell membranes (suggesting a mechanism for bilayer disc formation in retinal rod-cells), but is also a fascinating route to the generation of new, functional structures. This work is supported by the MARTECH and the Institute of Molecular Biophysics, both at Florida State University. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B17.00009: Dynamics of Multi-Component Model Membranes Studied via Light and X-Ray Scattering Kevin Johnson, Maikel Rheinst\"adter We study the dynamics of multicomponent biological model membranes (phospholipid, ethanol, cholesterol systems) via X-ray and light scattering to probe the dynamics of such membranes in solid supported and freestanding configurations. Collective molecular motions may play a significant role in different biological functions such as transmembrane transport and pore opening processes. Our main research objective is to quantify collective molecular motions in membranes and establish relationships to key physiological and biological functions of the bilayers. The phase diagram of this system with varying cholesterol and ethanol concentrations at set temperatures is determined using X-ray diffraction techniques and the mesoscopic membrane dynamics is then measured using time correlation light scattering techniques. The results can be compared to molecular dynamics simulations in a coarse grained membrane model. The dynamics shows propagating and relaxating processes, which allow to determine, e.g. the elasticity parameters of the bilayers. By understanding the mesoscopic properties of membranes with selected composition, membranes with specific properties can be designed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B17.00010: The Impact of Collective Molecular Dynamics on Physiological and Biological Functionalities of Artificial and Biological Membranes Maikel Rheinstadter We use neutron, X-ray and light scattering techniques to determine dynamical and structural properties of artificial and biological membranes. The combination of various techniques enlarges the window to length scales from the nearest-neighbor distances of lipid molecules to more than 10$^{-6}$m, covering time scales from about 0.1 ps to 1 s. The main research objective is to quantify collective molecular fluctuations in these systems and to establish relationships to physiological and biological functions of the bilayers, such as transmembrane transport. The motivation for this project is twofold: 1) By understanding fundamental properties of bilayers at the microscopic and mesoscopic level, we aim to tailor membranes with specific properties such as permeability and elasticity. 2) By relating dynamical fluctuations to physiological and biological functions, we can gain a deeper understanding of the bilayers on a molecular scale that may help optimizing the transmembrane transport of certain drugs. We show how bilayer permeability, elasticity and inter protein excitations can be determined from the experiments. M.C. Rheinst\"{a}dter et al., Phys. Rev. Lett. 93, 108107 (2004); Phys. Rev. Lett. 97, 048103 (2006); Phys. Rev. E 75, 011907 (2007);J. Vac. Soc. Technol. A 24, 1191 (2006). [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B17.00011: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 1:27PM - 1:39PM |
B17.00012: Effect of anesthetics on bending elasticity of lipid membranes Zheng Yi, Nagao Michihiro, Dobrin Bossev Change in physical and chemical properties of bio-membranes is of great interest for understanding the mechanism of anesthetic action on membranes. Hypothetically the anesthetic alters the lipid membrane structure (promoting pore formation across membranes or at least switching transmembrane channels) and therefore the biophysical properties of the membrane. We have used neutron spin echo (NSE) spectroscopy to study the effect of anesthetic molecule, lidocaine, on the bending elasticity (BE) of lipid membranes. BE of lipid bilayers made of (1,2-Dimyristoyl-\textit{sn}-Glycero-3-Phosphocholine) DMPC and 1,2-Dipalmitoyl-\textit{sn}-Glycero-3-Phosphocholine (DPPC) have been measured at different temperatures and different in the fluid (L$_{\alpha })$ phase. Using Zilman-Granek theory the BE were obtained from the decay of the NSE intermediate scattering function. We have found that in the presence of lidocaine the BE of DMPC and DPPC bilayers increases. The results were correlated with those from differential scanning calorimetry. Increase in the lidocaine concentration leads to decrease in the liquid/crystalline transition temperature. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B17.00013: Interactions between non-steroidal anti-inflammatory drugs and lipid membranes Mohan Boggara, Ramanan Krishnamoorti Chronic usage of Non-steroidal anti-inflammatory drugs(NSAIDs) leads to gastrointestinal toxicity and clinical evidences point the cause to direct interactions between NSAIDs and phospholipid membranes. Also, NSAIDs pre-associated with phospholipid vesicles are shown to be safer and therapeutically more effective than unmodified ones. Our initial experiments and simulations on the partitioning of Aspirin and Ibuprofen clearly indicate role played by the drug structure in drug-membrane interactions. Those results motivated systematic molecular dynamics simulations of membranes with NSAIDs of different size, structure and pKa values. Our results suggest high partition coefficients for these NSAIDs in the membrane compared to water and thinning effect on the bilayer. Our small angle neutron scattering and reflectivity studies on DMPC-Ibuprofen systems indicate that the drug affects both $\sim $5 nm thick bilayer and overall $\sim $100 nm diameter vesicle, indicating that NSAIDs affect vesicles on various length scales. We will discuss the structural perturbations to membranes due to NSAIDs at clinically relevant molar ratios and their implications on the use of vesicles as delivery vehicles for NSAIDs. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B17.00014: Calorimetric and Optical Studies of Cholesterol-Rich Filamentous, Helical Ribbon, and Crystal Microstructures Klaida Kashuri, Germano S. Iannacchione, Yekaterina A. Miroshnikova, Yevgeniya V. Zastavker Calorimetry (differential-scanning and modulation) and optical phase contrast microscopy studies have been performed on the filamentous, helical ribbon, and crystal microstructures formed in Chemically Defined Lipid Concentrate (CDLC). CDLC is a quaternary sterol system consisting of a cholesterol, bilayer-forming amphiphiles, micelle-forming amphiphiles, and water. Phase contrast microscopy confirms the presence of the three microstructure types in all samples studied. Sample size and temperature scan rate were varied on samples ranging from 1 to 20 mg and rates from 0$:$017 to 1 degree C/min, respectively. Thermal profiles are strongly dependent on sample size, scan rate, and thermal history. These scans also reveal numerous ``transition'' features, likely due to melting of various microstructures in CDLC, that generally shift to higher temperatures with increasing sample size. These results indicate that the filamentous, helical ribbon, and crystal microstructures formed in CDLC may be coexisting in a meta-stable chemical equilibrium with each other and the solvent environment from which they grow. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B17.00015: Structural and dynamical properties of water at the bilayer DPPC membrane interface Jianping Gao, Charles Cleveland, Uzi Landman The properties of interfacial water near the DPPC lipid bilayer surface are probed by a nano-size quartz tip through large scale atomistic molecular dynamics simulations. The water films confined between the bilayer membrane surface and a crystalline wetting quartz surface are kept in contact with a water reservoir at 293K. The distance between the solid tip and the gel phase membrane is varied between 0.5 to 2 nanometers. Layering of the confined water film is found near the solid tip while the water at the membrane surface remains unlayered. Some water is trapped in the cavities between the head groups of the lipid molecules. The recorded solvation force does not show oscillations due to the rough nature of the membrane surface. [Preview Abstract] |
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