Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B45: Focus Session: Structure and Dynamics of Biomembranes II |
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Sponsoring Units: DBIO Chair: Mu-Ping Nieh, University of Connecticut Room: Hilton Baltimore Holiday Ballroom 4 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B45.00001: Effect of Protein Crowding: Multivalent Protein Binding Induces a New Phase State in Lipid Membranes Invited Speaker: Tonya Kuhl It is well known that lipid membrane properties change as a function of composition and phase state, and that protein-lipid interaction can induce changes in the membrane's properties and biochemical response. This talk demonstrates that multivalent binding of proteins to putative membrane receptors can induce structure changes and a new phase state in lipid membranes. These molecular level changes are precisely characterized using grazing incidence X-ray diffraction. Protein binding is shown to perturb lipid packing within lipid monolayers and bilayers resulting in topological defects and the emergence of a new orientationally textured lipid phase. In bilayers this altered lipid order is transmitted from the receptor laden exterior membrane leaflet to the inner leaflet, representing a potential mechanism for lipid mediated outside-in signaling by multivalent protein binding. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B45.00002: Stabilization of composition fluctuations in mixed membranes by hybrid lipids Samuel Safran, Benoit Palmieri A ternary mixture model is proposed to describe composition fluctuations in mixed membranes composed of saturated, unsaturated and hybrid lipids. The asymmetric hybrid lipid has one saturated and one unsaturated hydrocarbon chain and it can reduce the packing incompatibility between saturated and unsaturated lipids. A methodology to recast the free-energy of the lattice in terms of a continuous isotropic field theory is proposed and used to analyze composition fluctuations above the critical temperature. The effect of hybrid lipids on fluctuations domains rich in saturated/unsaturated lipids is predicted. The correlation length of such fluctuations decreases significantly with increasing amounts of hybrids even if the temperature is maintained close to the critical temperature. This provides an upper bound for the domain sizes expected in rafts stabilized by hybrids, above the critical temperature. When the hybrid composition of the membrane is increased further, a crossover value is found above which ``stripe-like'' fluctuations are observed. The wavelength of these fluctuations decreases with increasing hybrid fraction and tends toward a molecular size in a membrane that contains only hybrids. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B45.00003: Self-assembly of colloidal rafts Prerna Sharma, Thoams Gibaud, Andrew Ward, Zvonimir Dogic Interactions between nanometer-sized particles or molecules suspended in a bulk fluid are well understood. However, when such particles are embedded in a membrane, the inter-particle potential is significantly modified by membrane mediated forces and gives rise to novel phase behavior. Visualizing and manipulating such inclusions in a lipid bilayer is difficult due to the nanometer length scales involved. Here, we use a model system of micron sized colloidal membranes doped with molecules shorter or longer than that of the bulk. Surprisingly, the dopant molecules form self-limited finite size clusters. These clusters further self-organize into a wide variety of higher order structures such as hexagonal and square lattice arrays, lamellar patterns and saddle shaped surfaces. Understanding the phase behavior and measuring repulsive forces between such clusters may have implications for the similar mechanisms that operate in conventional lipid bilayers. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B45.00004: Mechanism of lipid bilayer penetration by mixed monolayer-protected gold nanoparticles Reid Van Lehn, Prabhani Atukorale, Randy Carney, Francesco Stellacci, Darrell Irvine, Alfredo Alexander-Katz Recently, gold nanoparticles (AuNPs) protected by a binary mixture of hydrophobic and hydrophilic alkanethiol ligands were observed to spontaneously penetrate cellular membranes via a non-specific mechanism. Penetration was observed even at low temperatures and in the presence of endocytotic inhibitors, implying that AuNPs crossed the membrane by a non-endocytotic process. Furthermore, penetration was shown to depend on the amphiphilicity and nanoscale morphology of the protecting monolayer. In this work, we use a variety of simulation techniques to elucidate the mechanism of lipid bilayer penetration and compare our results to experiments with lipid vesicles. We show that these AuNPs can stably embed within lipid bilayers by ``snorkeling'' charges out of the bilayer core; the stability of such a state is a function of particle size, the composition of the protecting monolayer, and other environmental conditions. We use detailed simulations to analyze structural changes in the surrounding lipids and show that the energy barrier for embedding is considerably reduced in the presence of bilayer defects. We expect that these results will enable the design of novel drug delivery carriers and biosensors. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B45.00005: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 12:39PM - 12:51PM |
B45.00006: Dynamics and Self-Assembly of Nanoparticles on Biomembranes Rupak Bhattacharya, Vishal Maingi, Subbarao Kanchi, Bagul Rahul Suresh, N Jayaraman, Prabal Maity, K.G Ayappa, Jaydeep Basu We have recently been investigating the diffusion mediated self-assembly of various types of Dendrimers on supported DMPC lipid bilayer. Atomic Force Microscopy is used to study the pattern formation for PETIM dendrimers of different core composition as well as of generations. Extensive studies have been carried out using different concentration and different packing of lipid molecules constituting the lipid bilayer. Interestingly Oxygen Core dendrimer forms regular circular patterns on membranes whereas the Nitrogen Core dendrimer do not. A fully atomistic Molecular Dynamics simulation with implicit water clearly shows the evidence of domain formation for O-core dendrimers on bilayer, which is absent in the other one. Different generation for Oxygen core dendrimers forms patterns with a pore inside. The reduction of the diameter of these patterns with decreasing packing of lipid molecules indicates the possible role of lipid molecules in aggregation process. Further study using Confocal Fluorescence Correlation Spectroscopy is underway to correlate this type of membrane mediated pattern formation with underlying lipid diffusion. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B45.00007: Cholesterol Translocation in a Phospholipid Membrane Amit Choubey, Rajiv Kalia, Noah Malmstadt, Aiichiro Nakano, Priya Vashistha Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes, and controlling intracellular transport and signal transduction. Using all-atom molecular dynamics and the parallel replica approach, we study the process of CHOL interleaflet transport (flip-flop) in a dipalmitoylphosphatidycholine (DPPC)--CHOL bilayer, the effect of this process on mechanical stress across the bilayer, and the role of CHOL in inducing molecular order in the respective bilayer leaflets. The simulations are carried out at physiologically relevant CHOL concentration (30{\%}), temperature 323 K and pressure 1 bar. CHOL flip-flop events are observed with a rate constant of 3$\times $10$^{4}$ s$^{-1}$. Once a flip-flop event is triggered, a CHOL molecule takes an average of 73 nanoseconds to migrate from one bilayer leaflet to the other. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B45.00008: Simulating liquid-liquid phase separation and lipid transport on the Anton special purpose machine Edward Lyman, Logan Sandar, Alexader Sodt, Richard W. Pastor We present simulation data for a bilayer composed of a ternary mixture of cholesterol, dioloeoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine. The composition is chosen to be in the two-phase region and the temperature in the vicinity of the miscibility transition. Using the Anton special purpose computer to generate continuous trajectories longer the ten microseconds---which admits complete mixing of the lipids---we observe robust liquid-liquid phase coexistence. The time-and ensemble-averaged mean squared displacement (MSD) displays anomalous scaling on timescales less than 50 nsec and normal diffusion on longer timescales. The short-time anomalous scaling is explained by a mode-coupling argument[Flenner et al Phys Rev E 79:011907(2009)]. The per-lipid MSD's suggest that a few lipids remain associated with the liquid ordered domain for the duration of the simulation, suggesting a possible mechanism for anomalous transport on experimentally accessible timescales. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B45.00009: Interaction of Ionic Liquids with Lipid Biomembrane: Implication from Supramolecular Assembly to Cytotoxicity Benxin Jing, Nan Lan, Y. Elaine Zhu An explosion in the research activities using ionic liquids (ILs) as new ``green'' chemicals in several chemical and biomedical processes has resulted in the urgent need to understand their impact in term of their transport and toxicity towards aquatic organisms. Though a few experimental toxicology studies have reported that some ionic liquids are toxic with increased hydrophobicity of ILs while others are not, our understanding of the molecular level mechanism of IL toxicity remains poorly understood. In this talk, we will discuss our recent study of the interaction of ionic liquids with model cell membranes. We have found that the ILs could induce morphological change of lipid bilayers when a critical concentration is exceeded, leading to the swelling and tube-like formation of lipid bilayers. The critical concentration shows a strong dependence on the length of hydrocarbon tails and hydrophobic counterions. By SAXS, Langmuir-Blodgett (LB) and fluorescence microscopic measurement, we have confirmed that tube-like lipid complexes result from the insertion of ILs with long hydrocarbon chains to minimize the hydrophobic interaction with aqueous media. This finding could give insight to the modification and adoption of ILs for the engineering of micro-organisms. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B45.00010: The Effect of Tension on Phase Transitions and Domains in Phospholipid Membranes Maria Santore, Dong Chen The relevance phase transitions in phospholipid membranes to the effect of confinement on phase transitions and to the structure-function relationship in biological membranes has driven decades of scientific study of the behavior of model membranes. A primary focus of these studies has been the impact of temperature. We argue here, however, that tension can have a profound impact on transitions, suppressing domain formation, or shifting the nature of the domains themselves. While Clausius-Clapeyron predicts depression of a melting transition as small as 1/3 C for every mN/m of applied tension, the presence of a triple point or similar features can lead to the formation of different domains altogether. We provide here dramatic demonstrations of these behaviors in the form of fluorescence microscopy images in systems with controlled tension. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B45.00011: Confinement of 5CB Between Lyotropic Bilayers Cory Dolbashian, Rizwan Mahmood, Tommaso Bellini, Noel Clark We report phase behavior of mixtures of 5CB (4-Cyano-4'-Pentyl-1, 1'-biphenyl), a calamitic thermotropic liquid crystal, with mixtures of the lyotropic double tailed cationic surfactant DDAB (diodecyldimethylammonium-bromide) and water. These mixtures had a fixed ratio of DDAB to water (75{\%} / 25{\%}) and 5CB concentrations ranging from 10{\%} to 85{\%}. Our preliminary phase diagram suggests transition from isotropic to lamellar phase having higher birefringence at higher DDAB concentration. We have also observed low vale of birefringence at lower DDAB concentration suggesting swelling of bilayers. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B45.00012: Bilayer thickness mismatch controls domain size in biomimetic membranes Frederick A. Heberle, Robin S. Petruzielo, Jianjun Pan, Paul Drazba, Norbert Ku\v{c}erka, Robert F. Standaert, Gerald W. Feigenson, John Katsara In order to promote functionality, cells may alter the spatial organization of membrane lipids and proteins, including separation of liquid phases into distinct domains. In model membranes, domain size and morphology depend strongly on composition and temperature, but the physicochemical mechanisms controlling them are poorly understood. Theoretical work suggests a role for interfacial energy at domain boundaries, which may be driven in part by thickness mismatch between a domain and its surrounding bilayer. However, no direct evidence linking thickness mismatch to domain size in free-standing bilayers has been reported. We describe the use of Small Angle Neutron Scattering (SANS) to detect domains in simplified lipid-only models that mimic the composition of plasma membrane. We find that domain size is controlled by the degree of acyl chain unsaturation of low-melting temperature lipids, and that this size transition is correlated to changes in the thickness mismatch between coexisting liquid phases. [Preview Abstract] |
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