Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z50: Membranes, Micelles and Vesicles |
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Sponsoring Units: GSOFT Chair: Haskell Taub, University of Missouri, Columbia Room: 218 |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z50.00001: Dynamic Heterogeneity in Lipid Structures Christina Othon, Neda Dadashvand We have characterized the temperature and pressure dependent scaling of dynamic heterogeneity in a homogenous liquid phase of a lipid monolayer using time-resolved fluorescence anisotropy (TRFA) microscopy. Rotational diffusion is far more sensitive to highly correlated motions than translational diffusion due to the enhanced influence of nearest neighbor interactions. Highly correlated motion results in regions of high-density, low mobility lipids, and low-density, high mobility lipids; and are observed as the bimodal distribution of rotational correlation times. For biological lipid membranes the presence of highly correlated motion will greatly influence the rates of protein sorting and self-assembly, as particles suspended in the fluid can become kinetically trapped. Rotational diffusion timescales ($\sim$ ns) are far shorter than the lifetime of dynamic clusters and lipid raft-like structures ($\sim$ 10 $\mu $s), and thus the distribution of rotational correlation times can provide critical insight into the presence of these structures. We have characterized rotational dynamic distributions for a variety of phosphocholine moieties, and found dynamics consistent with highly correlated motion. Using the proximity to the phase transition, and the scaling of the temperature dependence of the heterogeneity we apply theoretical models developed for other condensed matter systems help us define limits on the size and lifetime of dynamic clusters in lipid structures. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z50.00002: Interplay between group function of kinesin based transport and lipid bilayer mobility. Joseph Lopes, Linda Hirst, Jing Xu Motor proteins, discovered in recent decades, are important building blocks to life. These molecular machines transport cargo and although indispensable~to cell function, are not well understood at present. Single kinesin transport properties have been documented, but their group function remains unknown. In this project, the properties of kinesin-based transport by multiple motors are investigated in-vitro to establish a link between travel distance and lipid diffusion in the vesicle membrane. In the experiments, silica beads coated in a supported lipid membrane and giant lipid vesicles are transported along a microtubule by embedded kinesin motors. In an alternate geometry, this system can be ~inverted, whereby motors are bound to a surface of a lipid bilayer and ~microtubules are deposited. ~We have characterized motor function with respect to the fluidity of the membrane. To measure the diffusion properties of different membranes, planar lipid bilayers are prepared on silica slides and supported by bovine serum albumin protein. To establish a diffusion constant at room temperature for the lipid membrane we use the FRAP technique (fluorescence recovery after photobleaching). Using this method we can investigate if there is any interplay between group travel function and membrane fluidity. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z50.00003: Nanoscopic Dynamics of Phospholipid Based Unilamellar Vesicles: Effect of Phase Transition and Addition of Melittin Peptide and Cholesterol V. K. Sharma, E. Mamontov, D. B. Anunciado, H. O'Neill, V. Urban Dynamics of DMPC phospholipid in unilamellar vesicles (ULV) has been investigated using quasielastic neutron scattering (QENS) techniques and reported here. Effect of addition of melittin and cholesterol on the dynamics of the lipid molecules in the ULV is also investigated. For DMPC ULV, a sharp fall in the elastic scan intensity is observed at 296 K, which is an indication of a solid gel to fluid phase transition. The addition of cholesterol or melittin inhibits this steep fall indicating that these molecules do have an influence on the main phase transition of DMPC ULV. QENS experiments have been carried out on DMPC ULV solution in the presence and absence of these additives at 280 K, in the solid gel phase, and at 310K, where lipids are in the fluid phase. The data analysis clearly shows the presence of two distinct motions: lateral and internal motions of the DMPC monomer. Both lateral and internal motions are found to be affected by the main phase transition. The addition of cholesterol or melittin influences the dynamics significantly in a different way, depending on the phase of lipid bilayers and the nature of additives. Effect of phase transition and additives on the dynamics of lipid in ULV will be discussed in details. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z50.00004: A Molecular Understanding of the Toxic Interactions of Ionic Liquids Towards a Lipid Biomembrane Brian Yoo, Edward Maginn, Yingxi Zhu There is a growing urgency to understand the toxicity of ionic liquids (ILs) due to their potential leakage into aquatic environment via aqueous waste streams in many large-scale commercial applications. Better understanding in the molecular interactions of ILs, primarily those in the popular imidazolium-class, with biological systems can serve as a physical foundation for their future design into ecologically benign ones. Here we investigate the toxic interaction of IL aqueous solutions with a supported lipid bilayer as a model cell membrane, using a combined experimental (fluorescence microscopic measurements) and multiscale simulation-based analysis. Both experimental and computer simulation studies have shown that the interactions of ILs with a supported lipid bilayer can lead to the insertion of ILs into the lipid bilayer, causing biomembrane morphological changes into multilayers, fibers, and/or vesicles with a strong dependence on the alkyl side chain length of IL cations. Using atomistic and coarse grained simulations, we have examined the potential of mean force of IL upon approaching a lipid bilayer and resulting changes in the mechanical compliance of lipid bilayer induced by IL interactions. We find that the resulting IL-lipid bilayer complexes can be strongly dependent on the ILs' ability to form cationic micelles. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z50.00005: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z50.00006: Curvature coupling influences lipid phases and morphologies on substrates Jukka M\"a\"att\"a, Sampsa Vierros, Maria Sammalkorpi The shape of a lipid aggregate contributes to many crucial biological processes like motility, fission, fusion and trafficking. At a molecular scale, the shapes lipids self-assemble in aqueous solution are coupled to molecular spontaneous curvature. At interfaces, the interface imposes an additional external constraint, which influences the assembled morphology. In this study, we examine the coupling between molecular spontaneous curvature and imposed external curvature in self-assembling lipid systems via molecular simulations. We map the adsorption morphologies for lipids of varying curvature as a function of substrate characteristics; the external curvature imposed by the substrate acts as a control factor in determining the morphology. In two-component systems external curvature introduced by the substrate can result in phase separation. We compare and connect our findings with experimental findings on lipid adhesion and adsorption morphologies. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z50.00007: Interaction measurement of particles bound to a lipid membrane Raphael Sarfati, Eric Dufresne The local shape and dynamics of the plasma membrane play important roles in many cellular processes. Local membrane deformations are often mediated by the adsorption of proteins (notably from the BAR family), and their subsequent self-assembly. The emerging hypothesis is that self-assembly arises from long-range interactions of individual proteins through the membrane's deformation field. We study these interactions in a model system of micron-sized colloidal particles adsorbed onto a lipid bilayer. We use fluorescent microscopy, optical tweezers and particle tracking to measure dissipative and conservative forces as a function of the separation between the particles. We find that particles are driven together with forces of order 100 fN and remain bound in a potential well with a stiffness of order 100 fN/micron. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z50.00008: Effects of water models and simulation system size on dynamic heterogeneity of single component lipid membranes Younghoon Oh, Jeongmin Kim, Bong June Sung Biological membranes are composed of various different types of molecules and their composition is usually spatially heterogeneous. Recently it has been reported that the dynamics of lipids could also become spatially heterogeneous, where fast and slow regions of lipids could coexist. A recent simulation study showed that the diffusion of lipids even in single component lipid membranes could be spatially heterogeneous in liquid-ordered phase at sufficiently low temperature. [1] On the other hand, in the liquid-disordered phase at relatively high temperature, the dynamics of lipids was homogeneous in membranes. In this work, we systematically investigate the diffusion of lipids in single component DPPC lipid bilayers by employing three different water models (Big Multipole Water, Polarizable MARTINI and LJ MARTINI) and three different simulation cell sizes (L $=$ 5nm, 10nm and 20nm). We find that even though the liquid-disordered to liquid-ordered phase transition occurs at different temperatures for different water models, the diffusion of lipids become spatially heterogeneous in liquid-ordered phases for all three different force fields. Reference [1] F. W. Starr, B. Hartmann, J. F. Douglas, Soft Matter, 10, 3036 (2014) [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z50.00009: Modulation of MscL activity in droplet interface bilayers through tailored interfacial mechanical properties Joseph Najem, Eric Freeman, Sergei Sukharev, Donald Leo MscL, a large-conductance mechanosensitive channel, is an osmolyte release valve that aids bacteria in surviving hypo-osmotic shocks. The large scale of its tension-driven opening transition makes it a strong candidate to serve as a transducer in stimuli-responsive biomolecular materials. In the previous work, a V23T mutant of MscL produced a reliable activation in a droplet interface bilayer (DIB) with applied axial droplet compression. Near the maximal compression, the aqueous droplets deform and the resulting increase in surface area leads to an increase in tension in the water-lipid-oil interface. This increase in tension is the product of the relative change in the droplet surface area and the elastic modulus of the DPhPC monolayer (120 mN/m). Here, we study the interfacial properties of the droplets as a way for modulating the activity of the embedded MscL channels. This is accomplished through varying mixtures of diphytanoyl phospholipids. The results show that gating probability of MscL in DIBs increases when lipids with a higher elastic modulus are used or when cholesterol is added to the monolayer. Moreover, an intrinsic electrical bias inside the lipid membrane is created when having DPhPC lipids with higher dipole in one droplet and DOPhPC characterized with lower dipole, in the other. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z50.00010: Structural dynamics of surfactant solutions in planar extensional flow Binbin Luo, Wesley Burghardt We report in situ x-ray scattering investigation of the structure of aqueous surfactant solutions in planar extensional flow. Samples were studied in a cross-slot stagnation flow cell fed by a syringe pump using a highly collimated synchrtron x-ray beam that provides for spatially resolved measurements of fluid structure in the stagnation region of the flow. Prior attempts to use planar stagnation flows for either x-ray or neutron scattering employed low-aspect ratio flow geometries in which the kinematics are dominated by parasitic velocity gradients along the incident beam direction. In contrast, our cross-slot flow cell employs an aspect ratio of 5:1, providing a much more ideal two-dimensional extensional flow field in the stagnation region. This device has been used to study two different surfactant systems, one a wormlike micelle solution at high salt concentration which exhibits rheology similar to that of entangled polymers. Here the focus is on the degree of micelle orientation produced as a function of extension rate. We have also studied a system that forms lamellar ordering. In addition to induced alignment of the mesophase structure, it is also possible to interrogate flow-induced changes in lamellar d-spacing in this material. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z50.00011: Toward a unified view of the structure and dynamics of water associated with single-supported zwitterionic and anionic membranes Zachary Buck, Andrew Miskowiec, Helmut Kaiser, Gavin King, Haskell Taub, Flemming Hansen, Madhusudan Tyagi, Souleymane Diallo, Eugene Mamontov, Kenneth Herwig High-resolution quasielastic neutron scattering was used to investigate the diffusive motion of water associated with single-supported bilayers of the zwitterionic lipid DMPC [1] and the anionic lipid DMPG [2]. The temperature dependence of the elastically-scattered neutron intensity from these samples indicates a series of freezing and melting transitions of the hydration water which differ greatly depending on the charge state of the lipid [2]. We interpret these distinct transitions as evidence of different types of water common to the two membranes: bulk-like water probably located above the membrane and two types of confined water in closer proximity to the lipid head groups. The temperature dependence of the diffusion coefficient of the hydration water determined for both membranes supports the interpretation of distinct water types each with its characteristic translational diffusion rate. Although sharing water types, the two membranes differ greatly in the temperature range over which their water freezing and melting transitions occur. [1] M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [2]$^{\mathrm{\thinspace }}$A.$^{\mathrm{\thinspace }}$Miskowiec \textit{et al}., Europhys. Lett. \textbf{107}, 28008 (2014). [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z50.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z50.00013: Observation of Iron Specific Interaction with a Charge Neutral Phospholipid Wenjie Wang, Honghu Zhang, Shuren Feng, Josue San Emeterio, Ivan Kuzmenko, Marit Nilsen-Hamilton, Surya Mallapragada, David Vaknin Using surface sensitive X-ray scattering and spectroscopic techniques we show that phosphatidyl choline (PC) head groups attract positively charged iron ions and complexes even at pH values that are lower than 3. DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) is a zwitterionic lipid typically used as a model system for cell membranes. Within a large pH range (3 -11), it carries a negative charge on the phosphate group and a positive charge on the quaternary ammonium cation, thus appears charge neutral. Further lowering the pH, i.e. adding a proton to the phosphate group, results in a positively charged headgroup. Surprisingly, we detect significant enrichment of iron at the interface of the DPPC monolayer and the aqueous subphase with the pH maintained at 3 or even lower. With a supposedly charge neutral or even positive surface, the observation of surface bound, charge positive iron ions or iron hydroxides is counter-intuitive and suggests iron-specific interaction with the phospholipid headgroup, which is not governed by electrostatic interaction. The effect of the integration of Mms6, a membrane protein that promotes the formation of magnetic nanocrystals, into the DPPC monolayer will also be discussed. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z50.00014: Coarse grained molecular simulations of melting kinetics of DPPC vesicles Lara A. Bolling-Patel, James T. Kindt Phase transitions in unilamellar vesicles are of particular interest as the increase in permeability of lipid bilayers around the main phase transition temperature makes them candidates for drug encapsulation and temperature-responsive delivery. We study the transition between the gel and fluid phases of a unilamellar vesicle of MARTINI coarse grain DPPC lipids with a diameter of 40 nm following temperature jumps from 280 K to temperatures near the transition temperature of 295 K. At 290 K and 295 K vesicles show single exponential melting kinetics in qualitative agreement with the early stages of melting measured in IR temperature-jump experiments. These trajectories exhibit partial melting over 500 ns, accompanied by a decrease in the number of gel domains from 8 domains in the initial faceted structure to 5 and 3 respectively. Melting at 295 K results in a shape change to an asymmetric structure that appears to be transforming into an oblate solid. Complete melting is seen for temperature jumps to 300 K and 310 K, in which cases vesicles undergo shape transitions into prolate dumbbell shapes. The shape changes that accompany the phase transition indicate that the phase transition kinetics are correlated to changes in curvature. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z50.00015: Formation and fluctuations of domains at the edges of membranes self-assembled from bidisperse filamentous phages Jerome Fung, Zvonimir Dogic When a non-adsorbing polymer is added to a colloidal suspension of $\sim$1-$\mu$m-long, $\sim$7-nm-diameter chiral filamentous phages, the resulting depletion attraction between the phages can cause them to self-assemble into monolayer membranes. Here we consider membranes assembled from mixtures composed of approximately 20\% 1.3-$\mu$m-long M13K07 phages and 80\% 880-nm-long fd-Y21M phages. At sufficiently high polymer concentrations, we observe the formation of domains rich in M13K07 within a single membrane. Depending on the handedness of the tilt at the membrane edge, these domains can either coat the membrane edge in a uniformly thick layer or protrude into the membrane bulk in arch-like threads. We probe the free energy of these domains by measuring the thermal fluctuations of the shape of their edges and relate our observations to the underlying chiral interactions between the phages. [Preview Abstract] |
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