Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V31: Bioploymers: Molecules, Solutions and Networks II |
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Sponsoring Units: DPOLY DBP Chair: Jay Tang, Brown University Room: LACC 503 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V31.00001: Membrane Transport Mechanisms Mihail Mihailescu, Anna Balazs One of the most important functions of biological membranes is regulating the passage of various chemical components in and out of a cell. The large variation in the physical characteristics of components that traffic through the membrane hints at the diversity of the membrane behavior. As opposed to small molecule transport, the transport of large molecules, such as bio-polymers, often involves a cooperative dynamic behavior that leads to formation and closure of a hole in the membrane. Dynamic simulations of the cooperative phenomena within the framework of self-consistent theory of polymeric systems allow us to monitor the phases of the transport process and to identify its macroscopic characteristics. In this study, we employ numerical simulations of the transport of a target molecule through a model membrane and identify the dependence of the transport process on microscopic properties of both the target molecule and the polymeric components of the membrane. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V31.00002: Polynucleotide Adsorption onto Negatively Charged Surfaces Hao Cheng, Joseph A. Libera, Kai Zhang, Michael J. Bedzyk, Monica Olvera de la Cruz Though DNA adsorption onto negatively charged surfaces such as mica in divalent salt solution has long been studied by atomic force microscope (AFM), the adsorption process is poorly understood. We develop a mean filed theoretical model which includes the pH effect on surface charge density and also the reaction between divalent ions and silanol groups at silica surface. When polynucleotides approach the nearly neutralized surface, the electrostatic field variation promotes reactions and results in polynucleotide adsorption. The equilibrium constant difference of the second reaction for varied divalent metal ions is the main reason for why not all the divalent counterions can mediate DNA adsorption onto like charged surfaces. The variation of polynucleotide adsorption amount with divalent salt concentration and pH value is consistent with what were found in AFM study. In our experiment, we use long-period X-ray standing wave (XSW) to measure in situ mercurated polynucleotide adsorption onto a negatively charged silica amorphous surface in ZnCl$_{2}$ solution. The most advantage of XSW measurement is that it can show atoms distribution quantitatively in the direction perpendicular to the surface. Our calculation results are in excellent agreement with the quantitatively experimental results. More generally, the effects of polyelectrolyte line charge density and monovalent salt on adsorption are also analyzed theoretically. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V31.00003: Characterization of Surface-Tethered Particles by TIRFM Arivalagan Gajraj, Seth Blumberg, Matthew Pennington, Jens-Christian Meiners Tethered particle experiments track the Brownian motion of a microsphere to obtain information about intra-molecular processes involving the tethering biopolymer. While these experiments are very powerful techniques that yield insight into intra-molecular dynamics, accurate quantitative analysis can be a limiting factor. For instance, most of these experiments suffer from incomplete information about the out of plane trajectory of the microsphere. Also, tethered-particles generally exhibit a large variation in behavior from molecule to molecule. Further complications can arise from electrostatic and hydrodynamic interactions of the surface with the microsphere. To address these complications we have extensively characterized the temporal and spatial trajectories of DNA tethers obtained from a stroboscopically illuminated TIRF microscope. To eliminate visual bias, we have developed automatic acquisition and selection criteria. Our results permit a comparison to theoretical models for tethered particle behavior and allow a more sophisticated understanding of large- scale biopolymer conformations such as those associated with DNA looping. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V31.00004: DNA monolayers: Charging behavior and capacitance response Rastislav Levicky, Gang Shen, Youlei Weng Surface-immobilized DNA films present versatile experimental models for investigating organization and properties of charged polymers at solid-liquid interfaces. In this work, end-tethered layers of DNA chains are prepared on metal (gold) supports and the response of the modified surface to a sinusoidal voltage applied between the surface and bulk solution is monitored using electrochemical impedance spectroscopy (EIS). The current- voltage behavior, expressed as an interfacial impedance, is determined as a function of ionic strength, counterion valency, and chain surface density. The response is found to reflect local conditions within the DNA brush. At lower ionic strengths, the response is insensitive to bulk salt conditions, but at higher ionic strengths brush behavior is increasingly influenced by added salt. Relative to monovalent salt, presence of higher valency counterions further suppresses brush response to solution conditions. In order to independently determine DNA surface coverage, a quantitative X-ray photoelectron spectroscopy method has been developed. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V31.00005: Water at a Janus Interface: An Exception to a Basic Assumption of Rheology Yingxi Elaine Zhu, Steve Granick Understanding water at interfaces is important in fields ranging from biology to geology, yet the details are disputed. Using a surface forces apparatus, we have studied the mechanical response of molecularly-thin films of water confined between a hydrophobic surface on one side and a hydrophilic surface on the other side (a Janus interface) to shear. It is usually assumed that when a fluid is sheared, the observed shear force should be invariant to which surface is translated relative to the other. Surprisingly, we found that depending on which side of the interface was sheared, the nanorheological properties of water differ significantly. When the hydrophobic side was sheared, we observed time-averaged G'($\omega )$=G''($\omega )$ accompanied by enormous fluctuations. In contrast, when the hydrophilic surface was sheared, we observed a terminal relaxation time -- classical Newtonian behavior. The identification of distinct rheology asymmetry suggests tentatively that the response to shear in these nanometer-thick fluids is controlled by in-plane diffusion rather than exchange of water molecules between surface and bulk, and provides a unifying perspective from which to analyze fluid viscosity in nanofluidic channels. In preliminary experiments, we have also studied hydrophobic surfaces mixed with variable amounts of polar functionality. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V31.00006: Light Regulated Anchoring of Biomolecules Via Photoactive Polyelectrolytes Jason Benkoski, Aldo Jesorka, Fredrik Hook Measuring the biological activity of proteins is critical for the development of advanced pharmaceuticals, which often target specific transmembrane proteins. The development of suitable measurement techniques is limited, however, by the need to host membrane proteins within a lipid matrix. We have therefore developed tethered vesicle assemblies which not only allow for undisrupted hosting of membrane proteins, but are also compatible with a battery of surface sensitive measurement techniques. The assemblies are comprised of three parts: a supported phospholipid bilayer (SPB), DNA oligomers tagged with a lipophilic polymer that becomes hydrophilic when exposed to UV light, and small unilamellar vesicles (SUVs). These three components provide a nonfouling surface, a mobile tether which connects the SPB to the SUVs, and a protein hosting matrix, respectively. We demonstrate that individual binding events can be monitored through the observation of vesicle agglomeration and reduced diffusivity as monitored by fluorescence microscopy. We further demonstrate that such assemblies can be released from the surface by exposing the photoresponsive tethers to UV light. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V31.00007: Amphiphilic Diblock Copolypeptides that Controllably Self-Assemble into Hydrogels and Vesicles Lisa Pakstis, Andrew Nowak, Eric Holowka, Jeffery Thompson, Timothy Deming, Darrin Pochan Diblock copolypeptides consisting of a hydrophilic lysine (K) block and a hydrophobic leucine (L) block assemble into stiff, porous hydrogels at low volume fractions of polymer ( $<$ 0.5 wt \%) or vesicle assemblies depending on the polymer molecular weight. In both the hydrogels and vesicles, nanoscale assembly is dictated by the alpha helical secondary structure of the leucine block whereas any hierarchical, microscale assembly is controlled through the assembly environment and molecular design. In the hydrogels, laser scanning confocal and cryogenic transmission electron microscopies and ultra small angle neutron scattering data revealed the formation of membranes on the nanoscale that interconnect to create a porous network on the nano- and microscale. Block copolymer relative composition and molecular weight can be changed to controllably alter the bulk network moduli. Decreasing the degree of polymerization \~100 resulted in the break-up of interconnected membranes for network structure and the consequent formation of vesicles. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V31.00008: Design of artificial proteins to incorporate non-biological cofactors Ting Xu, Shixin Ye, Joe Strzalka, Sophia Wu, Andrey Tronin, Michael Therien, J. Kent Blasie The design of artificial proteins with synthetic non-biological cofactors could lead to peptide based~systems with novel properties not exhibited by biological systems. Extended pi-electron~systems were designed~to exhibit selected NLO responses and light-induced electron transport. Amphiphilic 4-helix bundle peptides have been designed to selectively incorporate electron donor-acceptor prosthetic groups within both the hydrophilic and hydrophobic domains. The binding between a series of non-biological metalloporphyrin cofactors and the designed amphiphilic 4-helix bundles was studied. Incorporation of the non-biological cofactors into the 4-helix bundle did not change the secondary structure of the proteins. The binding selectivity depends on hydrophobicity and steric hindrance of the cofactors. Artificial proteins monolayer, both the apo- and holo-form, can be oriented vectorially at the air/water interface. This development may potentially lead to `bio-inspired' materials with novel electron transfer properties. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V31.00009: On the Structure of Gum Arabic in Aqueous Solution Yael Dror, Yachin Cohen, Rachel Yerushalmi-Rozen Gum arabic (GA), a natural composite polysaccharide derived from exudates of \textit{Acacia senegal} and \textit{Acacia seyal} trees, is commonly used in food hydrocolloids. It was shown to effectively disperse carbon nanotubes in water. GA consists mainly of a highly branched polysaccharide and a protein-polysaccharide complex (GAGP) as a minor component. In this work the microstructure of the gum in water was studied by small angle x-ray and neutron scattering combined with cryo-transmission electrons microscopy. An intricate structure is revealed, composed of many spheroidal polysaccharide aggregates and a small amount of large coils of GAGP. Inter-aggregate correlations result in a scattering peak, the spacing of which exhibits a -1/3 power-law dependence on concentration, and which diminishes with increased ionic strength. Changes in the conformation of the large GAGP coils can be followed at very low scattering vectors (q). A coil to rod transition with decreasing concentration is indicated by a change from -2 to -1 in the power-law q-dependence of the scattering intensity. It is suggested that the concentration of the GA in solution affects the structural correlations between the polysaccharide and the GAGP complex, and thus may also affect the surface activity of the gum. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V31.00010: Superlubricity of a natural polysaccharide from the alga Porphyridium sp. Delphine Gourdon, Qi Lin, Emin Oroudjev, Helen Hansma, Jacob Israelachvili Using a surface forces apparatus we have studied the adhesive and lubrication forces of mica surfaces separated by a molecularly-thin, sub-nanometer, film of a high molecular weight (2.6 MDa) naturally occurring anionic polysaccharide adsorbed from aqueous solution. The adhesion and friction forces of the biopolymer were monitored as a function of time, shearing distance and driving velocity under a large range of compressive loads. Although the thickness of the confined biopolymer was $<$1 nm, the friction was ultra-low (coefficient of friction =~0.015) at pressures up to 100 atm and over 4 decades of velocity with no wear. Complementary atomic force microscopy imaging in solution shows that the biopolymer adsorbs well to the mica surface but remains mobile and easily dragged upon shearing. The good adsorption of this polysaccharide to negatively charged surfaces, its low friction, its robustness (high-load carrying capacity and wear protection), as well as the weak (logarithmic) dependency of the friction on the sliding velocity make it, or this class of polyelectrolytes, excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the tribological properties of this polysaccharide will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V31.00011: Dynamical and physical changes of chitosan solutions during storage Nir Kampf, Ellen J. Wachtel, Anton Zilman, Jacob Klein, Noah Ben-Shalom Considerable attention has been paid to the biological properties and potential uses of chitosan and its derivatives in medical applications, industrial food processing, and agriculture. Whereas industrial users of chitosan do not usually further purify and manipulate the chitosan prior to use, in many scientific reports further cleaning as well as size fractionation of chitosan solutions do take place in order to meet the requirements of the analytical procedures and instrumentation. In the scientific publications concerning chemical or physical characteristics of chitosan, the so called "steady state" of the free polymeric solution is assumed to be achieved rapidly. In our study, we found that even up to 500 h the hydrodynamic properties continue to change. It was observed using both capillary and cone and plate viscometers, that the viscosity of unfractionated and fractionated chitosan at various polymer concentrations, molecular weights and degrees of deacetylation, \underline {decreased} significantly during the storage period. These changes were also confirmed by dynamic light scattering measurements, which could be interpreted in terms of a decrease in the average hydrodynamic radius of the chitosan particles. The addition of salt to the chitosan solution was also studied. We supported our findings by theoretical modelling based on the transition from rigid rods into flexible polymeric chains. Our findings are of importance not only as a fundamental phenomenon but also to a large variety of applications, as the conformational state is known to affect chitosan reactivity, efficiency and shelf-life. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V31.00012: Inhomogeneity of Type I Collagen Gels Olga S. Latinovic, H. Daniel Ou-Yang This paper reports a study of inhomogeneity of the gels by comparing the structural and micromechanical properties of Type I Collagen solutions. Using thermal fluctuations and forced oscillations of the particles embedded in the gel matrix, we use oscillating optical tweezers to determine local viscoelastic response of the matrix in the vicinity of the probe particle. Phase contrast and confocal fluorescence microscopy studies reveal phase separation in the gels with sparse and dense regions of fibril network. The inhomogeneity persists through the sol-gel transition. This study establishes correlation between the structural inhomogeneity and micromechanical properties in terms of local network density. [Preview Abstract] |
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V31.00013: Dynamics of Polyalanine in Water and in Glycerol Alper Buldum, Sharon R. Stefanovic Organic solvents such as glycerol and trehalose protect many organisms from dehydration and freezing. In an attempt to utilize this natural effect, solvents like these would be useful in the pharmaceutical industry. Here we present molecular dynamics simulations of a polyalanine polypeptide solvated in water and in glycerol. Many aspects of the dynamics of the polyalanine $\alpha$-helix are investigated and important viscosity and hydrogen bond effects on the fluctuations are discussed. Also considered are the effects on diffusion and radial distribution of a solvent upon addition of a polyalanine $\alpha$-helix. The understanding of the helix dynamics in different solvents may assist the understanding of the dynamics of entire proteins and their preservation in organic solvents. [Preview Abstract] |
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V31.00014: Blends of cysteine-containing proteins Justin Barone, Walter Schmidt Many agricultural wastes are made of proteins such as keratin, lactalbumin, gluten, and albumin. These proteins contain the amino acid cysteine. Cysteine allows for the formation of inter-and intra-molecular sulfur-sulfur bonds. Correlations are made between the properties of films made from the proteins and the amino acid sequence. Blends of cysteine-containing proteins show possible synergies in physical properties at intermediate concentrations. FT-IR spectroscopy shows increased hydrogen bonding at intermediate concentrations suggesting that this contributes to increased physical properties. DSC shows limited miscibility and the formation of new crystalline phases in the blends suggesting that this too contributes. [Preview Abstract] |
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