Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L18: Vesicles and Membranes |
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Sponsoring Units: DCMP GSNP Chair: E Lyman, University of Delaware Room: 403 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L18.00001: Mechanism of Cationic Nanoparticles and Cell-Penetrating Peptides Direct Translocate Across Cell Membranes Jiaqi Lin, Alfredo Alexander-Katz Cationic Nanoparticles (NPs) and cell-penetrating peptides (CPPs) are known effective intracellular delivery agents. These positively charged particles can bypass traditional endocytosis route to enter the cytosol, which is known as direct translocation. However, mechanism of direct translocation of both NPs and CPPs is not well understood. Using Coarse-grained (CG) molecular dynamics simulation, we found that gold nanoparticles (AuNPs) as well as HIV-1 Tat peptides can translocate across model biological membranes through nanoscale holes under a transmembrane (TM) potential. After the translocation, the TM is strongly weakened and the holes gradually reseal themselves, while the NPs/CPPs roam freely in the ``intracellular region.'' Both size and shape of the NPs/ CPPs are found to be a determine factor of their translocation behaviour, and the relationship between direct translocation and endocytosis is also discussed. The results provided here establish fundamental rules of direct translocation entry of NPs/CPPs, which may guide the rational design of cationic intracellular nanocarriers. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L18.00002: Hydrophobic matching between melittin and phosphocholine lipid bilayers having different thicknesses William Heller, Shuo Qian The lipid bilayer of the cellular membrane is more than a simple medium that houses proteins with specific function. Instead, it is an elastic medium that plays an active role in the function of the membrane and that both drives the function of membrane proteins and alters its properties in response to their presence. The conceptual simplicity of membrane active peptides makes them attractive model systems for studying membrane-protein interactions. Melittin, a 27 amino acid cationic peptide having a helix-hinge-helix motif, is one of the most extensively studied examples. Small-angle neutron scattering (SANS) measurements of melittin associated with lipid bilayer vesicles having different hydrocarbon thicknesses showed that the bilayer thickness stretches to match the thickness of the peptide in a manner consistent with a rigid, extended melittin having its helical axis oriented parallel to the bilayer normal. This behavior is surprising considering the helix-hinge-helix motif of the peptide and in contrast to studies indicating that transmembrane helices tilt with respect to the bilayer normal to accommodate differences in hydrophobic thicknesses. Possible sources of the discrepancy will be discussed and explored. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L18.00003: The Role of Osmotically-induced Tension in Binding of N-BAR to Lipid Vesicles Anthony D. Dinsmore, Jaime B. Hutchison, Derek A. Wood, Robert M. Weis We measured the binding affinity of a curvature-sensing protein domain (N-BAR) as a function of applied membrane tension while the membrane curvature was held nearly constant. We focus on the N-BAR domain of Drosophila amphiphysin, which participates in a range of key cell functions including synaptic vesicle endocytosis. We monitored N-BAR binding on unilamellar vesicles composed of 90 mol{\%} DOPC and 10 mol{\%} PIP. Controlled tension was applied by osmotic stress. We found that the bound fraction of N-BAR was enhanced by a factor 6.5 when the tension increased from zero to 2.6 mN/m. This tension-induced response can be explained by the hydrophobic insertion mechanism with a hydrophobic domain area that is consistent with known structure. These results suggest that membrane strain might explain the previously reported curvature affinity of N-BAR. This work was supported by the National Science Foundation through grant DMR-0907195. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L18.00004: The Molecular Structure of the Liquid Ordered Phase Edward Lyman Molecular dynamics simulations reveal substructures within the liquid-ordered phase of lipid bilayers. These substructures, identified in a 10 $\mu $sec all-atom trajectory of liquid-ordered/liquid-disordered coexistence (L$_{\mathrm{o}}$/L$_{\mathrm{d}})$, are composed of saturated hydrocarbon chains packed with local hexagonal order, and separated by interstitial regions enriched in cholesterol and unsaturated chains. Lipid hydrocarbon chain order parameters calculated from the L$_{\mathrm{o}}$ phase are in excellent agreement with $^{\mathrm{2}}$H NMR measurements; the local hexagonal packing is also consistent with $^{\mathrm{1}}$H-MAS NMR spectra of the L$_{\mathrm{o}}$ phase, NMR diffusion experiments, and small angle X-ray- and neutron scattering. The balance of cholesterol-rich to local hexagonal order is proposed to control the partitioning of membrane components into the L$_{\mathrm{o}}$ regions. The latter have been frequently associated with formation of so-called rafts, platforms in the plasma membranes of cells that facilitate interaction between components of signaling pathways. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L18.00005: Why Hydrophilic Water can Permeate Hydrophobic Interior of Lipid Membranes Baofu Qiao, Monica Olvera de la Cruz Water molecules as well as some small molecules have long been found to be able to diffuse across lipid membranes. Such permeation is of significant biological and biotechnological importance. For instance, the permeation of water across lipid membrane plays a important role in regulating ionic concentrations inside of cells. Such water permeation without the assistance of proteins embedded in membranes has been found to be a energetically unfavorable process. We, for the first time, explicitly depict the driving force for such an energetically unfavorable process. Atomistic molecular dynamics simulations are employed to investigate water diffusion in both liquid-crystalline and ordered gel phases of membranes containing zwitterionic DPPC or anionic DLPS lipid. The membrane conformation is calculated to have a critical role in water permeation, regardless of the type of lipid. The fluctuations in the potential energy are found to have a significant, if not the exclusive, role in the transportation of water across lipid membranes. Our results are also informative for the diffusion of small molecules of CO$_2$, O$_2$ and drug molecules, the absence of diffusion of ions, and the diffusion of water into the hydrophobic pores of carbon nanotubes. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L18.00006: Dynamic Morphologies of Microscale Droplet Interface Bilayers Charles Collier, Jonathan Boreyko, Prachya Mruetusatorn, Stephen Sarles, Douglas Hayes Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers ($\mu $DIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. \textit{(1) Buckling and Fission}: \quad When forming $\mu $DIBs using lipids initially in the oil, lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. \textit{(2) Uniform Shrinking}: When using lipids initially in the aqueous phase to form $\mu $DIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. \textit{(3) Stretching and Unzipping}: Finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L18.00007: Tube formation in fluid membranes Tao Zhang, Rastko Sknepnek, Jennifer Schwarz, Mark Bowick Consider a point force pulling on a fluid membrane. As the magnitude of the force increases, there is a first-order shape transition from nontubular to tubular with a force barrier in between. Motivated by tube formation in endocytosis in yeast, we generalize this problem by including additional force components and steric interactions. Both new ingredients are a consequence of the underlying actin cytoskeletal network, which exerts active forces on the cell membrane to deform it into a tube. We study this generalized problem using variational and Monte Carlo methods in order to quantify endocytosis in yeast. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L18.00008: Effect of micellar topology on shear rheology Simon Rogers, Michelle Calabrese, Norman Wagner Micellar topology affects the nonlinear shear rheology of self-assembled surfactant solutions. Segmental alignment of wormlike micelles (WLMs) exhibiting varying degrees of branching was investigated under shear in the flow-gradient and flow-vorticity planes using new small angle neutron scattering (SANS) sample environments. The degree of branching in mixed cationic/anionic surfactant (CTAT/SDBS) WLMs is controlled via the addition of the hydrotropic salt sodium tosylate. Shear-induced segmental alignment of the micelles is characterized by spatially-resolved flow-small angle neutron scattering (flow-SANS). Our ability to resolve structural projections in both planes provides insight to branch behavior and kinematics under shear flows. Local segmental orientation and alignment in the flow-gradient plane is a non-monotonic function of branching level and radial position. Alignment in the flow-gradient plane is significantly higher than that observed in the flow-vorticity plane, suggesting that branches may simultaneously migrate into the vorticity direction and inhibit spatially localized flows. Shear and normal stresses calculated from micellar alignment using the stress-SANS law are favorably compared with rheological measurements under identical conditions. The results provide evidence for the effects of micellar topology on the nonlinear shear rheology of WLM solutions. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L18.00009: Lipid Biomembrane in Ionic Liquids Brian Yoo, Benxin Jing, Jindal Shah, Ed Maginn, Y. Elaine Zhu Ionic liquids (ILs) have been recently explored as new ``green'' chemicals in several chemical and biomedical processes. In our pursuit of understanding their toxicities towards aquatic and terrestrial organisms, we have examined the IL interaction with lipid bilayers as model cell membranes. Experimentally by fluorescence microscopy, we have directly observed the disruption of lipid bilayer by added ILs. Depending on the concentration, alkyl chain length, and anion hydrophobicity of ILs, the interaction of ILs with lipid bilayers leads to the formation of micelles, fibrils, and multi-lamellar vesicles for IL-lipid complexes. By MD computer simulations, we have confirmed the insertion of ILs into lipid bilayers to modify the spatial organization of lipids in the membrane. The combined experimental and simulation results correlate well with the bioassay results of IL-induced suppression in bacteria growth, thereby suggesting a possible mechanism behind the IL toxicity. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L18.00010: Ca$^{2+}$ induced changes in PIP$_{2}$ containing membranes at physiological concentrations Martin Forstner, Adolphe Badiambile, Ian McCabe Phosphoinositides (PIPs) play a crucial role in many cellular processes such as calcium release, exocytosis or endocytosis. In order to specifically regulate these functions PIPs must segregate in pools at the plasma membrane. A possible mechanism that could induce and regulate such organization of phosphoinositides is their interaction with bivalent cations. Using Langmuir monolayers, we investigated the effect of calcium and magnesium on the surface pressure-area/lipid isotherm of monolayer of phosphatidylinositol, phosphatidylinositol bisphosphate, dioleoylphosphatidylglycerol and palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The observed decrease of area per lipid, i.e. the increase in aggregation, is mostly dependent on the lipid's head group charge but ion specific. In addition, we discuss changes in free energy and compressibility of these monolayer-ion systems. Furthermore, a series of experiments were conducted on supported lipid bilayers containing physiological quantities of PIP$_{2}$. Fluorescence correlation spectroscopy was used to study the response of the PIP$_{2}$ to changes [Ca$^{2+}$ ]. 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. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L18.00011: Neutron Scattering Function for Branched Worm-Like Micelles Gregory Beaucage, Karsten Vogtt Micellar solutions can display a wide range of phase structure as a function of counter ion content, surfactant concentration, and the presence of ternary components. Under some conditions extended cylindrical structures are produced that display chain persistence and a scaling regime reminiscent of polymers coils. These worm-like micelles (WLMs) can form branched, chain structures, for instance at relatively high salt concentrations. The rheology of these branched WLMs is strongly dependent on migration of the branch points, and the dynamics of branch formation and removal. A scattering model that can quantify the branching density, branch length, branch functionality and the hyperbranch (branch-on-branch) content of these polymer-like structures will be presented. Data from several WLM systems will be explore using this new model. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L18.00012: Fragmentation and wetting of spherical micelles in confined flow Mona Habibi, Colin Denniston, Mikko Karttunen We use coarse-grained molecular-dynamics (MD) simulations to study the structural and dynamical properties of surfactant micelles under Poiseuille-like flow in a nano-confined geometry. The effect of flow, confinement, and wetting on spherical micelles of sodium dodecyl sulfate (SDS) is explored when the micelle is forced through a channel slightly smaller than its equilibrium size. Inside the channel, the micelle may fragment into smaller micelles. We demonstrate that in addition to the flow rate, the wettability of the channel surface dictates whether the micelle fragments and determines the size of daughter micelles. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L18.00013: Structure and flow properties of micelle-nanoparticle solutions from Molecular Dynamics simulations Radhakrishna Sureshkumar, Subas Dhakal, Abhinanden Sambasivam In aqueous media, cationic surfactant molecules spontaneously self-assemble into diverse morphologies depending upon temperature, surfactant concentration and solution ionic strength. Spherical, cylindrical and long ($\sim $ microns) flexible wormlike structures with or without branches with distinct rheological properties are observed. Inclusion of nanoparticles (NPs) provides additional means to manipulate structure and create active ``nano-fluids'' that respond to optical, magnetic or electrical stimuli. We study self-assembly, dynamics and rheology of such fluids using coarse-grained Molecular Dynamics simulations in presence of explicit solvent and salt. Specifically, we will discuss the mechanisms underlying fascinating phenomenology observed experimentally such as the pronounced non-monotonic dependence of the zero shear viscosity on salt/NP concentration, shear-induced structure formation, and isotropic to nematic transitions. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L18.00014: Nonperturbative Renormalization Group Approach to Polymerized Membranes Karim Essafi, Jean-Philippe Kownacki, Dominique Mouhanna Membranes or membrane-like materials play an important role in many fields ranging from biology to physics. These systems form a very rich domain in statistical physics. The interplay between geometry and thermal fluctuations lead to exciting phases such flat, tubular and disordered flat phases. Roughly speaking, membranes can be divided into two group: fluid membranes in which the molecules are free to diffuse and thus no shear modulus. On the other hand, in polymerized membranes the connectivity is fixed which leads to elastic forces. This difference between fluid and polymerized membranes leads to a difference in their critical behaviour. For instance, fluid membranes are always crumpled, whereas polymerized membranes exhibit a phase transition between a crumpled phase and a flat phase. In this talk, I will focus only on polymerized phantom, {\it i.e.} non-self-avoiding, membranes. The critical behaviour of both isotropic and anisotropic polymerized membranes are studied using a nonperturbative renormalization group approach (NPRG). This allows for the investigation of the phase transitions and the low temperature flat phase in any internal dimension $D$ and embedding $d$. Interestingly, graphene behaves just as a polymerized membrane in its flat phase. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L18.00015: Soap film as a 2D system: Diffusion and flow fields Skanda Vivek, Eric Weeks \\ We use microrheology to measure the 2D (interfacial) viscosity of soap films. Microrheology uses the diffusivity of tracer particles suspended in the soap film to infer viscosity. Our tracer particles are colloids of diameters d = 0.10 and 0.18 microns. We measure the interfacial viscosity of soap films ranging in thickness from 0.1 to 3 microns. The thickness of these films is measured using the infrared absorbance of the water based soap films. From film thickness, viscosity of the fluid used to make the film and particle diffusivity, we can infer the interfacial viscosity due to the surfactant layers at the film/air interfaces. We find positive constant interfacial viscosities for thin films (h/d $<$ 5), within error. For thicker films, we find negative viscosities, indicating 3D effects begin to play a role, as air stresses become less important. The transition from 2D to 3D properties as a function of h/d is sharp at about h/d=6. Additionally, we measure larger length scale flow fields from correlated particle motions and find good agreement with what is expected from the theory of 2D fluids for all our films. In conclusion, single particle diffusion shows a sharp transition away from 2D like behavior as h/d increases, but the long-range flow fields still act as 2D. [Preview Abstract] |
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