Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W30: Focus Session: Biopolymers at Interfaces |
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Sponsoring Units: DPOLY DBP Chair: Darrin Pochan, University of Delaware Room: Baltimore Convention Center 327 |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W30.00001: Studies in Biological-Materials Interfaces. Invited Speaker: The control of the physicochemical properties of surfaces in contact with biological systems represents a fundamental issue in many applications ranging from coatings to biotechnology and microelectronics. In particular, advances in biotechnology depend on the ability to fashion materials with precise control of feature size and functionality. This presentation focuses on issues of specific and non-specific binding and strategies being developed to control both. Examples of specific binding that enable investigation of cell function will be presented. The broader issue of non-specific binding and how it relates to fouling release will also be discussed in terms of surface structure. Both polar and non-polar surfaces have been investigated and each type shows promise for release specific biological systems. The identification of a ``universal'' surface for release of all biological systems remains elusive. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W30.00002: Universality Classes and Unusual Thermodynamics of Unbinding Transitions of Semi-flexible Polymers Confined to a Surface Leonardo Golubovic, Lianghui Gao We theoretically address unbinding of semi-flexible polymers from long line-like attractive potential wells of various forms. These transition phenomena are seen in recent experiments with DNA adsorbed on microstructured supported cationic lipid membranes, and they provide a new way to stretch single DNA molecules [Hochrein, Leierseder, Golubovic, and Raedler, 2005]. For simple attractive potential wells (``rectangular wells'') the transition is of the second order. Heat capacity divergence however has a non-standard from, C$\sim $1/[$\vert $Tc - T$\vert $ log($\vert $Tc - T$\vert )$], marked by a logarithmic correction related to the fact that the probability to find the polymer within the well region vanishes as $\sim $1/ log($\vert $Tc-T$\vert )$ at the transition point. On the other hand, for attractive potential wells having a hard wall potential added on one side, the transition becomes a non-standard hybrid between the first and second order phase transitions: the probability to find the polymer within the well approaches a non-zero value as the transition is approached and then it discontinuously drops to zero (producing a latent heat consumption). However, interestingly, in addition to the latent heat consumption, an unusual heat capacity divergence (of the form C$\sim $1/$\vert $Tc - T$\vert$$^{1/2}$) also occurs as the polymer unbinding point is approached. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W30.00003: Polymer confinement and bacterial gliding motility Junhwan Jeon, Andrey Dobrynin Cyanobacteria and myxobacteria use slime secretion for gliding motility over surfaces. In cyanobacteria the slime is extruded from the nozzle-like pores of 14-16 nm outer diameter and approximately 7nm inner diameter located near the septa that separate the cells of a filament. The pores are inclined at an angle of 30-40 degrees relative to the cell axes, and are oppositely directed on both sides of the septum. Such pore orientation provides directionality for the slime secretion as well as cell motion. To understand the mechanism of gliding motion and its relation to slime polymerization, we have performed molecular dynamics simulations of a molecular nozzle with growing inside polymer chains. These simulations show that the compression of polymer chains inside the nozzle is a driving force for its propulsion. There is a linear relationship between the average nozzle velocity and the chain polymerization rate with a proportionality coefficient dependent on the geometric characteristics of the nozzle such as its length and friction coefficient. This minimal model of the molecular engine was used to explain the gliding motion of cyanobacteria and myxobacteria over surfaces. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W30.00004: Direct Observation of Biaxial Confinement of a Semi-flexible Filament in a Channel M.C. Choi We have studied the biaxial confinement of a semi-flexible filament in a channel by in situ video fluorescence microscopy*. As the channel width decreases, F-actin undergoes a transition from a 2D random regime to a 1D biaxially confined regime, leading to an increased effective persistence length. A theoretical calculation shows that the tangent-tangent correlation function in the confined regime shows a minimum, then reaches to a constant at long distances, indicating that confinement induces long-range order in a semi-flexible filament. The location of the minimum of the experimental correlation function is consistent with our theoretical calculation. This work was supported by KISTEP I-03-064, KISTEP IMT-2000-B3-2, MOHW 0405-MN01-0604-0007, NSF DMR 00-80034, 05-03347, 02-03755, 01-29804, NSF CTS-0404444, ONR N00014-05-1-0540, and DOE W-7405-ENG-36. M.C.Choi acknowledges partial support from the Korea Research Foundation Grant KRF-2005-214-C00202. *M.C.Choi et al., \textit{Macromolecules}, 38, 9882 (2005) [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W30.00005: Conformation of Lysozymes Confined to nano Particles Yuying Wei, R. Kenneth Marcus, Dvora Perahia Confinement of bio-molecules while retaining their activity is a key to many applications. The main challenge lies in the fact that when protein molecules interact with other particles they often lose their tertiary structure, resulting in irreversible reduction of their biological activity. The interfacial interactions of including direct chemical interactions, morphological factors, as well as adsorption under shear and hydrodynamic characteristics of flow next to the interfaces are among the factors that control the configuration. Using lysozyme as a model protein, the effects of physical absorption as a function of the topography of the confining surface from flat to curve with controlled roughness will be discussed. Atomic Force Microscopy together with small angle neutron studies correlated with measurements of retaining the degree of helicity in the system, provide a new insight into the factors that affect the conformational changes in protein upon confinement. While the chemical nature of the surface is an important parameter the topography of the surface determine many characteristics from the amount of absorption to distribution as well as the desoption of the protein. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W30.00006: Interaction forces and surface morphology of microtubule-associated protein tau Kenneth Rosenberg, Jennifer Ross, Eric Feinstein, Stuart Feinstein, Jacob Israelachvili The microtubule-associated protein tau exists in six isoforms due to alternative mRNA splicing and is localized in the axons of neuronal cells. These isoforms differ by the inclusion of 3 or 4 microtubule-binding imperfect repeat regions (31 aa each) at the C-terminal end of the protein or by 0, 1, or 2 N-terminal end inserts (29 aa each). Using a surface forces apparatus (SFA), we have measured the interaction forces as a function of distance between two symmetric layers of tau protein (all six isoforms) adsorbed onto mica. By comparing the interaction forces between the different isoforms , it is clear that the tau protein forms a brush-like layer on the mica surface which swells upon increasing ionic strength. Additionally, we have looked at an asymmetric system with one surface of tau opposite bare mica. In the asymmetric system of tau and an opposing mica surface, there is a $>$10$^{2}$ increase in the magnitude of this adhesive force suggesting that the tau-mica interaction is much more adhesive than the tau-tau interaction. These data clearly show that tau adsorbed onto mica acts as a spacer and due to the cross-bridging between the mica surfaces, provides a strong adhesion, as has been observed in vitro with microtubules. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W30.00007: Surface Plasmon Resonance Studies of Polysaccharide Self-Assembly on Cellulose Abdulaziz Kaya, Alan R. Esker, Wolfgang G. Glasser Wood is a multiphase material consisting of cellulose crystals embedded within a non-crystalline hetereopolysaccharide (hemicellulose) and lignin rich phase. The hierarchial arrangement of these three chief components in wood produces excellent properties like resistance to fracture and toughness. Through the study of polysaccharide self-assembly onto a model cellulose surface, further insight into the interactions between hemicelluloses and cellulose can be gained. In our study, we synthesized pullulan cinnamates with different degrees of substitution of cinnamoyl groups as a model for a hemicellulose with lignin-like moieties. Surface plasmon resonance measurements probe the self-assembly behavior of pullulan and pullulan cinnamate onto a cellulose coated gold surface. Our results suggest that pullulan does not adsorb onto the model cellulose surface, whereas pullulan cinnamate does. These preliminary results signify the important role that lignin-like substituents play on hemicellulose self-assembly onto cellulose surfaces. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W30.00008: Assembly artificial proteins and conjugated porphyrins for biomolecular materials Ting Xu, Joe Strzalka, Shixin Ye, Sophia Wu, Jiayu Wang, Thomas P. Russell, Michael Therien, J. Kent Blasie It is non-trivia to incorporate both the electron donor and acceptor in a controlled manner into amphiphilic 4-helix bundle peptides. Extended pi-electronÊsystems have been designed and tailored, with appropriate donors, acceptors and constituents, exhibit selected light-induced electron transport and/or proton translocation over large distances. We studied the binding between a series of conjugated porphyrins and the designed amphiphilic 4-helix bundles peptides at selected locations. Incorporation of the conjugated porphyrins into the 4-helix bundle did not interfere the protein secondary structure or the 4-helix bundle formation. The amphiphilic protein/cofactor complexes have good thermal stability.The artificial protein Langmuir monolayers, both the apo- and holo-form, can be oriented vectorially at the air/water interface upon compression. GID show a glass-like inter-bundle positional ordering in the monolayer plane. We will discuss the efforts on re-designing the artificial proteins to incorporate them into these nanoporous templates made from diblock copolymers . [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W30.00009: Structural Transitions of F-actin Polyelectrolyte Bundles in the Presence of Strongly Size-mismatched Cations Robert Coridan, Lori K. Sanders, Wujing Xian, Gerard C. L. Wong In the presence of multivalent cations, the polyelectrolyte F-actin exhibits the phenomenon of `like-charge attraction'. Simple divalent ions cause F-actin to form close-packed bundles with an interstitial 1-D density wave of ions along the length of the bundle. Lysozyme, a nonavalent (+9) cationic globular protein (45{\AA}x25{\AA}x25{\AA}) causes F-actin to form similar bundles, with a larger inter-actin distance and an incommensurate 1-D column of close-packed lysozyme along the three-fold tunnel within the bundle. Using genetically engineered lysozyme with different charges, we examine the competition of these cationic agents and their effect on F-actin bundle structure. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W30.00010: Defect Induced Morphologies of Biopolymer Bundles Ajay Gopinathan, Mark Henle, Uri Raviv, Daniel Needleman Bundles of stiff biopolymers, such as actin and microtubules, form important structural elements in the cell, including filopodia, microvili, cilia and contractile rings. These structures perform specific functions that rely crucially on their mechanical properties, which in turn depend on the internal organization of the bundles. Recent experiments on microtubule bundle formation in the presence of multivalent counterions [D. J. Needleman, \textit{et. al}, \textit{Proc. Natl. Acad. Sci.}, \textbf{101} 16099 (2004)] have observed that the bundles adopt static curved configurations whose wavelengths are several orders of magnitude less than their persistence length. In this talk, we show that these severe distortions can be explained by the presence of edge dislocation and twist defects, indicating that these defects could play a significant role \textit{in vivo}. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W30.00011: Polyamine Induced Bundling of F-actin Glenna Z. Sowa, David S. Cannell, Emil Reisler, Andrea J. Liu To better understand the mechanism of F-actin bundle formation, we have measured the phase boundary between isotropic F-actin and F-actin bundles as a function of polyamine concentration. F-actin was incubated with spermine or spermidine overnight, and the samples were spun at low speeds to separate bundles from unbundled F-actin. The relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis. With this approach, we have mapped the phase boundary between bundled F-actin and isotropic unbundled F-actin for two F-actin/polyamine linker systems. Surprisingly, the dependence of bundle formation on actin concentration is small to non-existent. At the actin concentrations we studied, actin tends to form bundles at or above a single linker concentration. In order to understand the interactions holding F-actin together in bundles, we used NMR to determine where the polyamines were with respect to the bundled and unbundled phases of actin. Surprisingly, the spermine and spermidine did not segregate with the bundled actin indicating that they do not bind to the actin strongly even though their addition to F-actin solutions induces bundle formation. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W30.00012: The fluctuating-rod limit of semiflexible biopolymers Ashok Prasad, Yukoh Hori, Jan\'{e} Kondev We study the mechanical properties of semiflexible polymers, such as DNA and actin, in the ``fluctuating rod'' limit. This limit is attained when the contour length of the polymer is comparable to its persistence length, or when thermal fluctuations have been smoothed out by a large applied force. In this limit, we compute the exact average end-to-end distance and shape of the polymer for boundary conditions that correspond to different single molecule stretching experiments. We consider both the case of a force applied at one end of the polymer, when the tension is uniform along the chain, and the case of an applied field, when the tension increases linearly. For the latter case, we derive the force-extension relation valid for a wide range of electric field strengths, which may be used to extract the effective charge density of actin in solution. We also show that the experimental condition of axis-clamping by a laser tweezer gives rise to a measurable effect on its force-extension properties. This calculation underscores the importance of taking the entropic effects of the boundary conditions into account in single molecule experiments. This work is supported by NSF DMR-0403997. JK is a Cottrell Scholar of Research Corporation. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W30.00013: Statistical and Mechanical Properties of Semiflexible Polymers in an External Field Ya Liu, Bulbul Chakraborty Semiflexible polymers such as the double-stranded DNA, are well described by the worm-like chain model originally proposed by Kratky and Porod (Rec. Trav. Chim. 68, 1106, (1949)). Recent work has focused on understanding statistical properties such as their end-to-end distribution function (J.Chem.Phys 121, 6064 (2004), PRE 71, 031803 (2005)) and their mechanical properties in response to a stretching force or the electric field (PRE 72, 041918 (2005)). The problem becomes very complicated unless the long-chain or rod-like-chain approximations for the persistence length are made. Self-avoidance effects are always neglected even for long chain in two dimensions and for confined polymers where these effects could become important. We make use of the Bond Fluctuation Algorithm (Macromolecules 21, 2819(1988)) to study the behavior of semiflexible polymers for all persistence lengths and investigate the relationship of their shape to the persistence length, the chain length and the external field. We will compare our results for the extension of a polymer under a constant stretching with analytical results in weak and strong force limit (PRE 72, 041918 (2005)). This work has been supported by NSF-DMR 0403997. [Preview Abstract] |
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