Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W31: Focus Session: Interaction of Polymers with Biological Systems |
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Sponsoring Units: FIAP DPOLY Chair: Roland Faller, University of California-Davis Room: LACC 503 |
Thursday, March 24, 2005 2:30PM - 3:06PM |
W31.00001: Modeling the Dynamic Interactions between Polymeric Membranes and Target Species Invited Speaker: One of the most important properties of biological membranes is their ability to regulate the passage of various chemical components in and out of a cell. Inspired by the elegance and robustness of this process, we examine a polymeric membrane that encircles an interior polymeric domain and examine the extent to which this synthetic membrane can be driven to engulf various targets and incorporate these species into the interior domain. To carry out this study, we developed a computational, dynamic mean-field model. We examine how the physical characteristics of both the target species and the interior polymeric domain affect the ability of the target to enter the system and the membrane to close after the target is localized in the core. The findings can provide guidelines for designing synthetic vesicles that can be used in applications that involve the active uptake and removal of species from a fluid environment. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:42PM |
W31.00002: Lipid Corralling and Polymer Squeeze-out in Membranes Invited Speaker: Ka Yee C. Lee Victims of electrical trauma suffer extensive loss of structural integrity of cell membranes. Stable structural defects -- ``pores'' in the range of 0.1 mm -- have been demonstrated in electroporated cell membranes. Poloxamer 188, a triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) is known to help seal electroporated cell membranes, arresting the leakage of intracellular materials of the damaged cell. Using a monolayer to mimic the outer leaflet of the cell membrane, we have examined the interaction between the poloxamer and zwitterionic and anionic phospholipid monolayers. With synchrotron x-ray reflectivity and grazing-incidence x-ray diffraction, both the out-of-plane and in-plane structures of mixed phospholipid-poloxamer 188 monolayers were investigated at the air-water interface. P188 selectively inserts into low lipid-density regions of the membrane and ``corrals'' lipid molecules to pack tightly, leading to unexpected Bragg peaks at low nominal lipid density and inducing the film to separate into P188-rich and -poor phases. At tighter lipid packing, the once inserted P188 is squeezed out, providing a route for the poloxamer to gracefully exit when the membrane integrity is restored. Cryo-electron microscopy shows that the poloxamer can associate with the lipid in a reversible two-state fashion, depending on the physical state of the lipid. At temperatures above the main transition temperature (T$_{M})$ where the lipid is fluid, the poloxamer can incorporate itself into lipid vesicles, resulting in a more monodispersity vesicle population. At temperatures below T$_{M, }$the poloxamer cannot penetrate into the gel-like lipid layer, but instead corrals the lipid molecules to self-assemble into bilayer disks with the edges stabilized by the poloxamer. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W31.00003: Computer simulation of C60 permeation crossing dimyristoylphosphatidylethanolamine Liwei Li, Dmitry Bedrov, Grant Smith All-atom MD simulations have been carried out to investigate the permeation process of C60 crossing dimyristoylphosphatidylethanolamine(DMPC) lipid bilayer. The C60 has been constrained along the normal of bilayer from the surrounding water outside DMPC to the center of DMPC. The potential of mean forces(POMF) of C60 has been obtained by integrating the averaged forces acting on the C60. Local diffusion coefficient was calculated from the time autocorrelation function of instantaneous velocities. The overall resistance to C60 has been calculated in terms of POMF and the local diffusion coefficient. The C60 assembly behavior in the hydrophobic interior of DMPC was investigated by examining the POMF between two C60 molecules along lateral direction in the DMPC bilayer and in the bulk melt of alkyl chains. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W31.00004: Towards Improving the Targeting Efficiency of End-Functionalized Polymer Brushes Chun-Chung Chen, Elena E. Dormidontova Functionalized protective polymer layers are actively used in drug/gene delivery. By means of computer simulations, we study the interactions between a polymer layer end-functionalized by ligands and a surface containing receptors. We analyze the influence of ligand valence and the polymer layer architecture on binding efficiency of the ligands. Multivalent ligands are shown to substantially improve the efficiency of targeting through cooperative binding but the distance between the functional groups relative to the spacing of receptor sites is crucial to the success of the targeting. Thus, multivalent ligands with a branch length shorter than the receptor spacing turned out to be less efficient than monovalent ligands. We also studied bidisperse polymer brushes consisting of short non-functional and long functionalized polymers. We found that bidisperse structure of the polymer layer considerably improves accessibility of the functional groups leading to the stronger attraction between the polymer layer and receptor surface. The bidisperse structure of a polymer brush can be combined with multivalent ligands in order to greatly improve the binding of a functionalized polymer brush with receptor surfaces. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W31.00005: Connect the Dots: Tracking the Motion of Single Particles Stephen Anthony, Liangfang Zhang, Steve Granick The motion of individual molecules, both individually and as ensemble averages, contains information about many fundamental properties. Improvement in tracking algorithms has allowed the motion of single molecules to be observed and followed in movies. The information contained in these trajectories allows the determination of surface diffusion rates, as in fluorescent correlation spectroscopy, but unlike FCS, this technique also allows the detection of defects and domains within the liquid surface. Specific studies will be presented on the one hand of adsorbed polymers, and on the other hand of diffusion in supported lipid bilayers. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W31.00006: Polymer Diffusion in Lipid Membranes Ashok Prasad, Jane' Kondev Motivated by experiments on fluorescently labeled DNA molecules on a supported lipid bilayer\footnote{B. Maier and J. O. Radler, {\it Phys. Rev. Lett.} {\bf 82},1911, (1999)}, we have examined theoretically diffusion of polymers in two dimensions. The key experimental finding we focus on is the scaling of the diffusion constant of the center of mass, $D\sim 1/N$. This implies that no effective hydrodynamic coupling exists between the diffusing DNA segments in the membrane. We construct our theoretical model using the phenomenological hydrodynamic model of supported membranes proposed by Evans and Sackmann\footnote{ E. Evans and E. Sackmann, {\it J. Fluid Mech.} {\bf 194}, 553 (1988)}. Our model is based on the pre-averaged Oseen tensor, and is similar to the model of Komura and Seki\footnote{S. Komura. and K. Seki, {\it J. Phys. II France} {\bf 5},5 (1995)}, but elaborated and extended to take explicit account of self-avoidance. We find that the $1/N$ scaling of $D$ can be understood as a consequence of membrane hydrodynamics in the presence of a supporting surface. Further experimental consequences of the model, in particular the diffusion constant for DNA in free standing membranes, will also be discussed. This work was supported by the NSF through grants DMR-9984471 and DMR-0403997. JK is a Cottrell Scholar of Research Corporation. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W31.00007: Self-assembly between DNA and anionic membranes Hongjun Liang, Daniel Harries, Gerard Wong The self-assembly between anionic membranes and anionic polyelectrolytes has been investigated using synchrotron small angle x-ray scattering and confocal microscopy. Like-charged DNA rods and lipid membranes can condense into complexes with a rich polymorphism of structures in the presence of divalent cations, since cations can in principle generate attractions between rods and membranes, membranes and membranes, and rods and rods. The structures of these complexes have been examined as a function of the charge membrane charge density and the global counterion concentration. Lamellar and inverted hexagonal phases can be formed, as can phases with no direct analog in DNA-cationic membrane systems. The competition between electrostatic and membrane elastic properties in the determination of phase stability will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W31.00008: Mobility of DNA on Supported Lipid Bilayers Chakradhar Padala, Sanat Kumar, Ravi Kane The study of the dynamics of biopolymers such as DNA at interfaces is not only of fundamental interest but also useful in the development of biosensors and novel DNA separation strategies. In this work, we investigate the mobility of DNA adsorbed onto supported lipid bilayers. DNA was adsorbed onto a supported lipid bilayer whose mobility could be tuned by varying the temperature. Fluorescence Recovery after photo bleaching (FRAP) was used to determine the diffusivities of the lipid and that of DNA adsorbed on the bilayer. FRAP analysis reveals an interesting interplay between the mobility of the substrate and the mobility of the DNA. We will discuss these results, as well as results for DNA diffusion on solid surfaces. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W31.00009: Polymers Slaved Diffusion in Phospholipid Bilayers---A Study Using Single-Molecule Fluorescence Liangfang Zhang, Steve Granick The translational diffusion of phospholipids DLPC (1,2-dillauroyl-sn-Glycero-3-phosphocholine) in supported fluid bilayers splits into two populations when polyelectrolytes, quaternized poly-4-vinylpyridine (QPVP), adsorb at incomplete surface coverage. Spatially-resolved measurements using fluorescence correlation spectroscopy (FCS) with two-photon excitation show that a slow mode, whose magnitude scales inversely with the degree of polymerization of the adsorbate, coexists with a fast mode characteristic of naked lipid diffusion. Inner and outer leaflets of the bilayer are affected nearly equally. This offers a new mechanism to explain how nano-sized domains with reduced mobility arise in lipid membranes. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W31.00010: A Molecular Imprinting Strategy Employing Polyelectrolyte Multilayers Solar C. Olugebefola, Anne M. Mayes, Michael F. Rubner Polyelectrolyte multilayers were assembled from poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA) and poly(acrylic acid-r-vinyl benzyl acetate) (xPAA), derivatized from PAA. The pHs of polymer assembly solutions were controlled to yield high surface area film morphologies for adsorption. Assembled films were photo crosslinked in the presence of adsorbed template molecules and the template removed to yield selective binding sites. Quartz crystal microbalance measurements of adsorption onto films templated with bovine serum albumin show higher affinity for BSA compared to films crosslinked with no templating. [Preview Abstract] |
Thursday, March 24, 2005 5:18PM - 5:30PM |
W31.00011: DNA Molecules Adsorbed on Rippled Supported Cationic Lipid Membranes -- A new way to stretch DNAs Leonardo Golubovic We discuss a novel approach to control to shapes of DNA molecules. We elucidate the recent experimental work of M. Hochrein, L. Golubovic and J. Raedler, on the conformational behavior of DNA molecules adsorbed on lipid membranes that are supported on grooved micro-structured surfaces. We explain the striking ability of the edges formed on these supported membranes to adsorb and completely orient (stretch) very long DNA molecules. Here we explain the experimentally observed DNA stretching effect in terms of the surface curvature dependent electrostatic potential seen by the adsorbed DNA molecules. On the curved, rippled membrane, we show that the DNA molecules undergo localization transitions causing them to stretch by binding to the ripple edges of the supported membrane. In the future, this stretching will allow to directly image, by the common fluorescence microscopy, fundamental biological processes of the interactions between DNA and single protein molecules. [Preview Abstract] |
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W31.00012: Stretching Helical Macromolecules Gustavo A. Carri, Vikas Varshney We study the elasticity of a homopolypeptide under extension using Monte Carlo simulations based on the Wang-Landau algorithm. The effect of external mechanical forces is described with an extension of a model for helical polymers (V. Varshney \textit{et. al}., \textit{Macromolecules} \textbf{2004}, $37$, 8794). We find that the application of a mechanical force first increases the helix-to-coil transition temperature and then decreases it. This non-monotonic behavior is a consequence of a change in the nature of the helix-coil transition which becomes a helix-extended-coil transition for strong forces. We also find that the force-elongation curve at constant temperature displays three different behaviors depending on the temperature of the system. At temperatures below or slightly above the helix-coil transition temperature the force-elongation curve shows one or two coexistence regions, respectively. In these regions helical sequences and random coil domains coexist. At high temperatures our model recovers the elastic behavior of a random coil. We present a quantitative comparison with the theoretical results of Buhot and Halperin, and very good agreement is observed. [Preview Abstract] |
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