Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P13: Membranes: General, Surface, Biological |
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Sponsoring Units: DFD Chair: Haskell Taub, University of Missouri Room: B112 |
Wednesday, March 17, 2010 8:00AM - 8:12AM |
P13.00001: Studies of the Temperature-Dependent Structure of DMPC Bilayer Lipid Membranes by Atomic Force Microscopy A. Miskowiec, M. Bai, H. Taub, F.Y. Hansen We are using Atomic Force Microscopy (AFM) to characterize the structure and topography of single-supported bilayer lipid membranes to complement quasielastic neutron scattering investigations of the membrane dynamics. To investigate the effect of different membrane-substrate interactions, samples of hydrated DMPC bilayer membranes have been fabricated on four different supports: 1) a bare SiO$_{2}$-coated Si(100) wafer; 2) a SiO$_{2}$-coated Si(100) wafer preplated with a monolayer of the pure alkane $n$-C$_{36}$H$_{74}$ in which the molecules are aligned with their long axis parallel to the SiO$_{2}$ surface; 3) an underlying DMPC membrane itself supported on a SiO$_{2}$ surface; and 4) a SiO$_{2}$-coated Si(100) wafer covered with a polyethylenimine (PEI) cushion. Above room temperature, our AFM images show a decrease in the DMPC membrane thickness with increasing temperature consistent with chain-melting transitions of the lipid tails. The onset temperature at which the membrane thickness begins to decrease and the temperature at which its thickness saturates both decrease with weaker binding to the support and with a greater level of hydration. [Preview Abstract] |
Wednesday, March 17, 2010 8:12AM - 8:24AM |
P13.00002: Phospholipid-coated microbubbles: the acoustic signature of monolayer buckling Valeria Garbin, Marlies Overvelde, Jeroen Sijl, Benjamin Dollet, Nico de Jong, Detlef Lohse, Michel Versluis In medical ultrasound imaging, the echo of the blood pool is enhanced using ultrasound contrast agents. The contrast agent suspension consists of microbubbles (1 to 5 $\mu$m in radius) of an inert gas coated with a phospholipid monolayer. We characterize the changes in microbubble dynamics due to the coating, through combined micromanipulation by means of optical tweezers and ultra-high speed imaging at 15 million frames per second with the Brandaris 128 camera. The experiments reveal that buckling of the phospholipid monolayer increases the non-linear response of the contrast agent bubbles at low acoustic pressure. [Preview Abstract] |
Wednesday, March 17, 2010 8:24AM - 8:36AM |
P13.00003: Coarse-grain simulation studies of lipid vesicles Robin Selinger, Jun Geng, Jonathan Selinger We model the shape evolution of an initially spherical lipid vesicle when the lipid bilayer undergoes a transition from an untilted phase to a tilted phase. Our coarse-grain model is a generalization of an approach due to Lykotrafetis, Zhang, Suresh and Li [preprint], and is efficient enough to allow simulation of an entire vesicle in three dimensions. Topological defects are generated during the phase transformation with a total topological charge of +2 as required by the Gauss-Bonnet theorem. These defects couple to the curvature of the membrane. We explore the resulting complex shape evolution and compare to both theoretical predictions and experimental observations. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P13.00004: Role of Collective Degrees of Freedom in Formation and Disintegration of Spherical Micelles Dmitry Kopelevich, Yong Nam Ahn, Gunjan Mohan Dynamics of self-assembly and structural transitions in amphiphilic systems play an important role in various technological and biological processes. We recently demonstrated that even such a simple process as addition of a single surfactant molecule to a micelle involves a complex interplay between micellar and monomer configurations. In this talk, we present a quantitative model for collective dynamics of these degrees of freedom during the monomer addition and removal. This is accomplished by reconstruction of a multi-dimensional free energy landscape of the system and identification of the minimal energy path (MEP) on this landscape. Although analysis of MEP allows us to identify collective degrees of freedom relevant to the monomer addition and removal, MEP alone is not sufficient to adequately describe these processes. Comparable time-scales of several independent degrees of freedom during non-adiabatic stages of these processes imply that the system dynamics cannot be described by a quasi-one-dimensional motion along MEP. Therefore, we solve a multi-dimensional Langevin equation to correctly describe the non-adiabatic system dynamics. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P13.00005: Critical capsule deformation in several linear flows, modeled using the Immersed Boundary Method Alex Szatmary, Charles Eggleton Elastic capsules are exposed to a variety of flows in microfluidic devices. Capsules can deform continuously to bursting when exposed to sufficiently intense flows. Here, this critical behavior is modeled for several linear flows, including Couette flow, and plane, axisymmetric, and biaxial extensional flows, as well as superpositions of these. Several membrane constitutive equations are used to model behavior of a variety of capsules and biological cells. Computational modeling results using the immersed boundary method are reported. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P13.00006: Manipulation, stability, and controlled release of micelles in AC-electric fields Victoria Froude, Y. Elaine Zhu In this work, we explore the rich AC-electrokinetic effects to manipulate micelles of varied chemical structures and examine their stability in response to applied AC-electric fields. We investigate the AC-field induced transport and instability of sodium dodecyl sulfate (SDS) micelles and cetyl trimethylammonium bromide (CTAB) micelles tagged with various hydrophilic and hydrophobic fluorescent probes by using fluorescence correlation spectroscopy (FCS) at a single-molecule resolution. Micelle concentration and dielectrophoresis (DEP) mobility are examined over a broad range of AC-field frequency from 1 KHz -10 MHz and amplitude from 5V-10V. We observe a strong AC-frequency dependence of micelle concentration between two microelectrodes, from which the DEP crossover frequency switching between the positive and negative DEP response is determined. Surprisingly, we also observe the AC-field induced instability of the micelle structure and the resultant release of fluorescent probes at a characteristic low AC-field frequency of about 1-10 kHz for specific probes in SDS micelles, which could have a potential application for controlled drug release by AC-electric fields. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P13.00007: Investigating bile salt aggregation using coarse-grained molecular dynamics simulations Ana Vila Verde, Daan Frenkel Bile salts are necessary for fat digestion due to their unusual surfactant properties: they assemble into small, polydisperse micelles and easily form mixed micelles with poorly soluble amphiphiles. Understanding these properties requires molecular scale information about bile salt micelles, something challenging to obtain experimentally but amenable to computational modeling. To address this issue we build a coarse-grained model of bile salts. We investigate their aggregation behavior through molecular dynamics simulations in a grand-canonical parallel tempering scheme. We validate our model against available solubility and light scattering data. Our results indicate that at physiological bile salt and counter ion concentrations, bile salts pack in many different orientations in pure bile micelles, contrary to standard surfactants. This feature may be physiologically relevant, allowing bile salts to solubilize the heterogeneous blends of fats typical of digestion. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P13.00008: Mechanical properties of giant folds in a Langmuir monolayer Thomas Boatwright, Jeffrey Yu-Chieh Yang, Alex J. Levine, Michael Dennin We study the mechanical properties of giant folds in a catanionic monolayer at the air water interface. The system of study is a dioctadecyldimethylammonium bromide (DODAB) and sodium dodecyl sulfate (SDS) monolayer which folds upon compression in a Langmuir trough. Carboxylate-coated polystyrene beads (1~micron~diameter) are attached to the monolayer in order to track its displacement with epifluorescence microscopy and particle image velocimetry. This analysis yields a measurement of the velocity of the monolayer around the fold. The quantities of monolayer material entering and leaving the fold are recorded as well. Maximum material velocities and fold depths are found to be on the order of 0.1 mm/s and 1 mm, respectively. Analysis also reveals that the unfolded material displacement follows a characteristic curve. Mechanical properties of the monolayer are also probed with optical tweezer microrheology. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P13.00009: Lipid/water system with varying charge densities at the interface monitored by sum-frequency vibrational spectroscopy Woongmo Sung, Sangjun Seok, Doseok Kim Lipids having negatively- and positively charged headgroups were mixed together and spread on water to make Langmuir monolayers with interface charge densities controlled at will. These systems were then monitored with surface-selective sum-frequency vibrational spectroscopy. Sum-frequency signal from the interfacial water molecules changed sensitively with the composition of lipids in the mixture, reflecting the electric field induced by the lipid headgroups. By comparing the interference patterns between CH stretch vibration peaks of the lipid molecules and OH stretch vibration peaks of the water molecules in the sum-frequency spectra, the change in the polar ordering of the interfacial water molecules was monitored. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P13.00010: Attractive amphiphilic polymer layers form amorphous membranes Ho Cheung Shum, Jerome Bibette, David Weitz Amphiphilic polymer molecules becomes attractive and form aggregates in a poor solvent. By confining two layers of attractive polymer layers at neighboring interfaces, we form a thin amorphous membrane instead of aggregates. The rigidity of the membrane is shown to be controlled by the magnitude of attractive interaction between the two layers. At very high attractive interactions, possibly glassy membranes are formed. We demonstrate these using microfluidics to form polymer vesicles with amorphous membranes. By measuring the energy of adhesion between the two layers, we propose a physical explanation behind the membrane formation process. The amorphous membranes are shown to have a similar structure as lipid bilayers, but with a significantly improved rigidity. Our system provides a simple way to look into membrane formation and suggests that the membrane rigidity is closely related to the interactions between the constituent molecules. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P13.00011: The Effects of Concentration and Temperature on Vesicle Adsorption and Bilayer Formation Kimberly Weirich, Jacob Israelachvili, Deborah Fygenson Supported lipid bilayers (SLBs) are pursued as thin surface coatings and as model systems in which to study membrane-bound processes. We investigate the adsorption of small unilamellar phospholipid vesicles onto glass and the subsequent formation of planar SLBs using temperature-controlled, time-resolved fluorescence microscopy. We report the effects of vesicle concentration and temperature on the time course of lipid adsorption. Our results suggest that isolated vesicle rupture is a rare event and that bilayer edge plays a key role in SLB formation. It enhances vesicle-surface affinity and promotes further rupture. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P13.00012: Coarse-graining, Electrostatics and pH effects in phospholipid systems Alex Travesset, Sweta Vangaveti We introduce a minimal free energy describing the interaction of charged groups and counterions including both classical electrostatic and specific interactions. The predictions of the model are compared against the standard model for describing ions next to charged interfaces, consisting of Poisson-Boltzmann theory with additional constants describing ion binding, which are specific to the counterion and the interfacial charge (``chemical binding''). It is shown that the ``chemical'' model can be appropriately described by an underlying ``physical'' model over several decades in concentration, but the extracted binding constants are not uniquely defined, as they differ depending on the particular observable quantity being studied. It is also shown that electrostatic correlations for divalent (or higher valence) ions enhance the surface charge by increasing deprotonation, an effect not properly accounted within chemical models. The model is applied to the charged phospholipids phosphatidylserine, Phosphatidc acid and Phosphoinositides and implications for different biological processes are discussed. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P13.00013: Measurement of red blood cell mechanics during morphological changes Gabriel Popescu, YongKeun Park, Catherine Best, Ramachandra Dasari, Michael Feld, Tatiana Kuriabova, Mark Henle, Alex Levine The human red blood cell (RBC) membrane, a fluid lipid bilayer tethered to an elastic 2D spectrin network, provides the principal control of the cell's morphology and mechanics. These properties, in turn, influence the ability of RBCs to transport oxygen in circulation. Current mechanical measurements of RBCs rely on external loads. Here we apply a Noncontact optical interferometric technique to quantify the thermal fluctuations of RBC membranes with 3 nm accuracy over a broad range of spatial and temporal frequencies. Combining this technique with a new mathematical model describing RBC membrane undulations, we measure the mechanical changes of RBCs as they undergo a transition from the normal discoid shape to the abnormal echinocyte and spherical shapes. These measurements indicate that, coincident with this morphological transition, there is a significant increase in the membrane's shear and bending moduli. This mechanical transition can alter cell circulation and impede oxygen delivery. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P13.00014: Molecular Transport through Flexible Membranes: Coupling between Solute Dynamics and Membrane Fluctuations Yong Nam Ahn, Young-Min Ban, Dmitry Kopelevich Mechanism of solute transport through self-assembled membranes, such as lipid bilayers and surfactant monolayers, is investigated. It is demonstrated that dynamics of the solute molecule significantly deviates from the Markovian Brownian motion. Specifically, the correlation time of the random force acting on the solute increases by two orders of magnitude within a very narrow (less than 1 nm wide) region within or near the membrane. We demonstrate that the slow fluctuations of the random force in this narrow region are caused by dynamic coupling of the solute transport with the membrane undulations. This coupling is the strongest near a free energy barrier for the solute transport through the membrane. Therefore, the coupling is expected to play significant role in the mass transport through a membrane. A stochastic model for the coupled solute-membrane dynamics is developed using results of molecular dynamics simulations. The observed mechanism appears to be very general and is expected to affect mass transport through other flexible membranes. [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P13.00015: Experimental and Numerical Investigation of the Equilibrium Geometry of Liquid Lenses J. C. Burton, F. M. Huisman, P. Alison, D. Rogerson, P. Taborek The equilibrium configuration of a non-wetted fluid/fluid/gas system takes the form of a floating liquid lens. We have computed the shapes of lenses for various liquid/liquid combinations in air for a wide range of droplet volumes by numerically solving the Young-Laplace equation, including the effects of gravity. The results of the calculations are compared to laser shadowography photographs of various alkane-water liquid lens systems, which were analyzed using basic ray-tracing to determine the lens profiles. Moir\'{e} imaging was also used to measure the deformation of the water interface due to the lens' presence. The agreement between experiment and theory is good for pure fluids. We also introduced a surfactant, dodecyltrimethylammonium bromide (DTAB), into the sub-fluid phase (water) at concentrations between 0 and 20 mmol/kg. In agreement with other experiments, we find a minimum contact angle at low concentrations corresponding to a pseudo-partial wetting transition of the alkane/water/surfactant system. [Preview Abstract] |
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