Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J39: Lipid Bilayers: Structure and Function II |
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Sponsoring Units: DBP Chair: Wouter Ellenbroek, University of Pennsylvania Room: 411 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J39.00001: Membrane curvature sensing by the actin cytoskeleton Ghee Hwee Lai, Abhijit Mishra, Nathan Schmidt, Daniel Kamei, Timothy Deming, Gerard C. L. Wong Biological active molecules such as proteins and oligonucleotides can be transduced across cell membranes with high efficiency by cell penetrating peptides. It has been recently demonstrated using synchrotron x-ray diffraction that such peptides induce saddle-splay (negative Gaussian) membrane curvature, which is the topological requirement for pore formation. Here, we show how the actin cytoskeleton 'senses' and responds to negative Gaussian defects on a membrane, by examining the interaction between cell penetrating peptides and an active polymerizing cytoskeleton encapsulated within giant unilamellar vesicles, and compare the results to cell based studies. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J39.00002: Optically Induced Rotation of Laser-trapped Chiral Lipid Tubules by Linearly Polarized Light Nattaporn Chattham, Thanate Na Wichian, Apichart Pattanaporkratana, Jamras Limtrakul Chiral Phospholipids are found self-assembled into fascinating cylindrical tubules of 500 nm in diameter by helical winding of bilayer stripes under cooling in ethanol and water solution. Theoretical prediction and experimental evidence reported so far confirmed the modulated tilt direction in a helical striped pattern of the tubules. This molecular orientation morphology results in optically birefringent tubules. We investigate them under optical trap of 532 nm linearly polarized optical tweezers. We observed spontaneous rotation of lipid tubules induced by radiation torque. The tubule direction can be controlled by the alignment of polarization direction, and thus the rotation angle can be specified. Other related aspect on optical activity of the lipid tubules is also studied. This work is supported by Kasetsart University Research and Development and National Nanotechnology Center, Thailand. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J39.00003: Diffusion in Tethered Bilayer Lipid Membranes as observed by Z-Scan FCS S. Shenoy, R. Moldovan, S. Rauhala, M. Loesche Tethered bilayer lipid membranes (tBLMs, [1]) are resilient biomimetic systems stabilized by the proximity of an inorganic interface. Synthetic lipids with a hydrophilic oligomer covalently coupled to the substrate serve as membrane anchors while forming a nm-thick aqueous reservoir. This property can be exploited to investigate protein-membrane interactions at the molecular length scale. The anchor is chemisorbed into a self-assembled monolayer, either as a pure compound (densely tethered) or laterally diluted (sparsely tethered) by $\beta$ -mercaptoethanol ($\beta$ME), a small spacer. Phospholipids are then precipitated to complete the bilayer structure. Diffusion measurements were performed using both one-photon and two-photon fluorescence correlation spectroscopy using the Z-Scan approach[2]. While the aqueous reservoir decouples the bilayer from the substrate, we expect the presence of tethers in the inner leaflet to inhibit the free diffusion of lipids. Indeed, we see a drop in the apparent diffusion coefficient by a factor of 2 when comparing a densely tethered membrane to a sparsely-tethered membrane. Importantly, the diffusion coefficients in tBLMs compare favorably with those observed in giant unilamellar vesicles, indicating that tBLM dynamics are similar to those of free bilayers. [1] McGillivray et al., Biointerphases 2007(2): 21-33 [2] Benda et al., Langmuir 2003(19): 4120-4126 [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J39.00004: Cholesterol Perturbs Lipid Bilayers Non-Universally John Nagle, Jianjun Pan, Thalia Mills, Stephanie Tristram-Nagle Cholesterol is well known to modulate the physical properties of biomembranes. Using modern x-ray scattering methods, we have studied the effects of cholesterol on the bending modulus K$_{C}$, the thickness D$_{HH}$, and the orientational order parameter S$_{xray}$ of lipid bilayers. We find that the effects are different for at least three classes of phospholipids characterized by different numbers of saturated hydrocarbon chains. Most strikingly, cholesterol strongly increases K$_{C}$ when both chains of the phospholipid are fully saturated but not at all when there are two mono-unsaturated chains. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J39.00005: Barrier Function of Lipid Membrane in the Interaction with Nanostructures. Sergiy Minko, Yury Roiter Tiny details of the phospholipid (DMPC) membrane morphology in close vicinity to nanostructured silica surfaces have been discovered in the atomic force microscopy experiments. The structural features of the silica surface were varied in the experiments by the deposition of silica nanoparticles of different diameter on plane and smooth silica substrates. It was found that, due to the barrier function of the lipid membrane; only particles larger than 22 nm in diameter, with a smooth surface were completely enveloped by the lipid membrane. However, nanoparticles with bumpy surfaces (curvature diameter of bumps as that of particles $<$22 nm) were only partially enveloped by the lipid bilayer. For the range of nanostructure dimensions between 1.2 nm and 22 nm, the lipid membrane underwent structural rearrangements by forming pores (holes). The nanoparticles were accommodated into the pores but not enveloped by the lipid bilayer. The study also found that the lipid membrane conformed to the substrate with surface structures of dimensions less than 1.2 nm without losing the membrane integrity. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J39.00006: Direct nm-scale observation of lipid membrane fluctuations Sung Chul Bae, Yan Yu, Stephen M. Anthony, Steve Granick Thermal fluctuation of giant unilamellar phospholipids vesicles(GUVs) was observed by a combination of direct imaging using epifluorescence microscopy and forward laser beam scattering of a laser beam from the vesicle edge. The latter technique, owing to the refractive index mismatch, offers nanometer spatial and microsecond temporal resolution. When nanoparticles adsorb, this changes. We will discuss the changes of membrane rigidity caused by nanoparticles, based on membrane fluctuation data. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J39.00007: Effects of mobile membrane proteins on the structure and dynamics of lipid rafts Jun Fan, Maria Sammalkorpi, Mikko Haataja Compositional lipid domains (``lipid rafts''), which reside in the plasma membrane, are thought to facilitate many important cellular processes, including signal transduction and viral entry. Experimentally, raft dynamics have been probed mainly indirectly through observations of raft-associated membrane proteins, and the interpretation of the data relies heavily on assumptions about raft shape and viscosity. Previously, we have shown that strong interactions between rafts and immobile protein clusters may induce the formation of spatially extended raft aggregates [J. Fan et al., PRL 100, 178102 (2008)], thus complicating the interpretation of experimental data. In this work we correlate the dynamics of membrane proteins with the underlying time-dependent raft domain structure via a hybrid continuum-particle simulation scheme, and develop strategies for extracting quantitative information about raft dynamics from observations of the membrane proteins alone. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J39.00008: Multiscale Modeling of supported bilayers Roland Faller, Chenyue Xing, Matthew I. Hoopes Supported Lipid Bilayers are an abundant research platform for understanding the behavior of real cell membranes as they allow for additional mechanical stability. We studied systematically the changes that a support induces on a phospholipid bilayer using coarse-grained molecular modeling on different levels. We characterize the density and pressure profiles as well as the density imbalance inflicted on the membrane by the support. 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 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. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J39.00009: Thermotropic vibrational spectroscopy of newly developed self-forming PEGylated lipids Rajan Bista, Reinhard Bruch, Aaron Covington Vibrational spectroscopy can provide valuable structural information about lipids, which are important molecular components of biological membranes. In the present study, we have focused on the thermotropic vibrational spectroscopy of two newly developed synthetic PEGylated lipids trademarked as QuSomes{\texttrademark} to investigate the phase behaviors and associated changes in the conformational order. In contrast to conventional phospholipids, this new kind of lipid forms liposomes spontaneously upon hydration, without the supply of external activation energy. Variable-temperature thin-layered Fourier Transform Infrared (FTIR) spectroscopy and Raman spectro-microscopy have been developed and employed in order to plot the transition temperature profiles showing the phase behavior of these new lipids composed of 1,2-dimyristoyl-\textit{rac}-glycerol-3-dodecaethylene glycol (GDM-12) and 1,2-distearoyl-\textit{rac}-glycerol-3-triicosaethylene glycol (GDS-23). Furthermore, several spectral indicators were calculated and correlated which allowed for the deduction of various aspects of molecular structure as well as intramolecular motion and intermolecular interactions occurred during temperature change. To confirm the observations, differential scanning calorimetry (DSC) was applied and revealed a good agreement with the FTIR and Raman spectroscopic results. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J39.00010: Theory of Disk-to-Vesicle Transformation Jianfeng Li, An-Chang Shi Self-assembled membranes from amphiphilic molecules, such as lipids and block copolymers, can assume a variety of morphologies dictated by energy minimization of system. The membrane energy is characterized by a bending modulus (\textit{$\kappa $}), a Gaussian modulus (\textit{$\kappa $}$_{G})$, and the line tension (\textit{$\gamma $}) of the edge. Two basic morphologies of membranes are flat disks that minimize the bending energy at the cost of the edge energy, and enclosed vesicles that minimize the edge energy at the cost of bending energy. In our work, the transition from disk to vesicle is studied theoretically using the string method, which is designed to find the minimum energy path (MEP) or the most probable transition path between two local minima of an energy landscape. Previous studies of disk-to-vesicle transition usually approximate the transitional states by a series of spherical cups, and found that the spherical cups do not correspond to stable or meta-stable states of the system. Our calculation demonstrates that the intermediate shapes along the MEP are very different from spherical cups. Furthermore, some of these transitional states can be meta-stable. The disk-to-vesicle transition pathways are governed by two scaled parameters, \textit{$\kappa $}$_{G/}$\textit{$\kappa $} and \textit{$\gamma $R}$_{0}$\textit{/4$\kappa $}, where $R_{0}$ is the radius of the disk. In particular, a meta-stable intermediate state is predicted, which may correspond to the open morphologies observed in experiments and simulations. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J39.00011: Hybrid Lipid as Biological Surfactants Robert Brewster, Phil Pincus, Sam Safran Systems capable of forming finite-sized, equilibrium domains are of biological interest in the context of membrane rafts where it has been observed that certain cellular functions are mediated by small (nanometric to tens of nanometers) domains with specific lipid composition that differs from the average composition of the membrane. These small domains are composed mainly of lipids with completely saturated hydrocarbon tails that show good orientational order in the membrane. The surrounding phase consists mostly of lipids with at least one unsaturated bond in the hydrocarbon tails which forces a ``kink'' in the chain and inhibits ordering. In vitro, this phase separation can be replicated; however, the finite domains coarsen into macroscopic domains with time. We have extended a model for the interactions of lipids in the membrane, akin to that developed in the group of Schick (Elliott et al., PRL 2006 and Garbes Putzel and Schick, Biophys. J. 2008), which depends entirely on the local ordering of hydrocarbon tails. We generalize this model to an additional species and identify a biologically relevant component, a lipid with one fully saturated hydrocarbon chain and one chain with at least one unsaturated bond, that may serve as a line-active component, capable of reducing the line tension between domains to zero, thus stabilizing finite sized domains in equilibrium. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J39.00012: Distribution of Drug Molecules in Lipid Membranes: Neutron Diffraction and MD Simulations. Mohan Boggara, Ella Mihailescu, 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. In this study, using neutron diffraction, the DOPC lipid bilayer structure (with and without drug) as well as the distribution of a model NSAID (Ibuprofen) as a function of its position along the membrane normal was obtained at sub-nanometer resolution. It was found that the bilayer thickness reduces as the drug is added. Further, the results are successfully compared with atomistic Molecular Dynamics simulations. Based on this successful comparison and motivated by atomic details from MD, quasi-molecular modeling of the lipid membrane is being carried out and will be presented. The above study is expected to provide an effective methodology to design drug delivery nanoparticles based on a variety of soft condensed matter such as lipids or polymers. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J39.00013: Calcium-induced domain formation in mixed lipid monolayers Wouter G. Ellenbroek, David A. Christian, Ilya Levental, Andrea J. Liu, Paul A. Janmey Multivalent ions such as calcium play an important role in soft matter and biological systems. This role cannot be captured by a mean field treatment of the electrostatics such as the Poisson-Boltzmann equation, which neglects, for example, the fact that Ca$^{2+}$-ions can mediate attractions between negatively-charged objects. We show, both experimentally and theoretically, that Ca$^{2+}$-mediated attractions lead to phase separation of charged and neutral lipid molecules in mixed lipid monolayers, and discuss the dependence on pH, salt concentration and ion valency. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J39.00014: Membrane Disruption Mechanism of Antimicrobial Peptide Kin Lok H. Lam PG-1, a cationic antimicrobial peptide, kills bacteria by forming pores which increase membrane permeability to ions or larger molecules. It has been proposed that PG-1 selectively induces stable membrane pores in bacterial membranes over mammalian membranes. To study the mechanism of action of PG-1, we directly visualize the topological changes induced by PG-1 in model membranes via atomic force microscopy for the first time. PG-1 induces structural transformations in supported lipid bilayers, progressing from bilayer edge instability, to the formation of pores, and finally to a network of wormlike micelles in a zwitterionic dimyristoylphosphatidylcholine model membrane with increasing PG-1 concentrations. The structural transformation can be understood in the framework of the action of 1d detergent, with PG-1 acts as a line active agent. The results elucidate the mechanism by which PG-1 uses to induce leakage in bacterial cells. [Preview Abstract] |
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