Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z41: Focus Session: Biopolymers II: Phase Behavior, Rheology, and Mechanics |
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Sponsoring Units: DPOLY Chair: Joseph Lott, University of Southern Mississippi Room: 214A |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z41.00001: Chirality-selected phase behavior in ionic polypeptide complexes Invited Speaker: Matthew Tirrell We demonstrate that chirality determines the phase state of polyelectrolyte complexes formed from mixing dilute solutions of oppositely charged polypeptides. In these systems, the physical state of the resultant complex is determined by the combination of electrostatic and hydrogen bonding interactions. The formation of fluid complexes occurs when at least one of the polypeptides in the mixture is racemic, which disrupts backbone hydrogen bonding networks. Pairs of purely chiral polypeptides, of any sense, form compact, fibrillar solids with a $\beta $-sheet structure on mixing. Analogous behavior occurs in micellar cores formed from polypeptide block copolymers with polyethylene oxide, where microphase separation into discrete, self-assembled aggregates with either solid or fluid cores, and eventually into ordered phases at high concentrations, is possible. Chirality is an exploitable tool for manipulating material properties in systems based on polyelectrolyte complexation. Its role in these systems gives insight into polyelectrolyte complex phase behavior more broadly. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z41.00002: Biomimetic Coacervate Environments for Protein Analysis Sarah Perry, Patrick McCall, Samavayan Srivastava, David Kovar, Margaret Gardel, Matthew Tirrell Living cells have evolved sophisticated intracellular organization strategies that are challenging to reproduce synthetically. Biomolecular function depends on both the structure of the molecule itself and the properties of the surrounding medium. The ability to simulate the in vivo environment and isolate biological networks for study in an artificial milieu without sacrificing the crowding, structure, and compartmentalization of a cellular environment, represent engineering challenges with tremendous potential to impact both biological studies and biomedical applications. Emerging experience has shown that polypeptide-based complex coacervation (electrostatically-driven liquid-liquid phase separation) produces a biomimetic microenvironment capable of tuning protein biochemical activity. We have investigated the effect of polypeptide-based coacervates on the dynamic self-assembly of cytoskeletal actin filaments. Coacervate materials are able to directly affect the nucleation and assembly dynamics. We observe effects that can be attributed to the length and chemical specificity of the encapsulating polypeptides, as well as the overall crowded nature of a polymer-rich coacervate phase. Coacervate-based systems are particularly attractive for use in biochemical assays because the compartmentalization afforded by liquid-liquid phase separation does not necessarily inhibit the transport of molecules across the compartmental barrier. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z41.00003: Molecular Rigidity and Entropy-Enthalpy Compensation in DNA Hybridization Jack Douglas, Fernando Vargas-Lara Entropy-enthalpy compensation (EEC) is a general and relatively poorly understood pattern in the energetic parameters governing both binding constants and relaxation processes in condensed matter. After defining the basic phenomenology, we focus on how polymer additives, chain confinement, chain length variation affect a well-studied molecular binding process, the hybridization of duplex DNA. Our study is based on a coarse-grained model of DNA that does treat water explicitly. We find that both crowding due to polymer additives and geometrical confinement lead to a change of the effective chain rigidity and that changes in DNA generally lead to a pattern entropy-enthalpy compensation in the DNA association similar to experimental observations. Modulation of the rigidity of binding specifies by constraints associated with chain structure or environmental conditions can greatly influence both the location and cooperativity of molecular binding transition and the relative enthalpy and entropy contributions to the free energy of binding. Entropy-enthalpy compensation arises in numerous synthetic and biological molecular binding processes and we suggest that that changes in molecular rigidity might provide a common explanation of this ubiquitous phenomenon. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z41.00004: Spider Silks-Biomimetics Beyond Silk Fibers: Hydrogels, films {\&} Adhesives from Aqueous Recombinant Spider Silk dopes: A Synchrotron X-Ray Nano-Structural Study Sujatha Sampath, Justin Jones, Thomas Harris, Randolph Lewis With a combination of high strength and extensibility, spider silk's (SS) mechanical properties surpass those of any man made fiber. The superior properties are due to the primary protein composition and the complex hierarchical structural organization from nanoscale to macroscopic length scales. Considerable progress has been made to synthetically mimic the production of fibers based on SS proteins. We present synchrotron x-ray micro diffraction (SyXRD) results on new fibers and gels (hydrogels, lyogels) from recombinant SS protein water-soluble dopes. Novelty in these materials is two-fold: water based rather than widely used HFIP acid synthesis, makes them safe in medical applications (replacement for tendons {\&} ligaments). Secondly, hydrogels morphology render them as excellent carriers for targeted drug delivery biomedical applications. SyXRD results reveal semi-crystalline structure with ordered beta-sheets and relatively high degree of axial orientation in the fibers, making them the closest yet to natural spider silks. SyXRD on the gels elucidate the structural transformations during the self-recovery process through mechanical removal and addition of water. Studies correlating the observed structural changes to mechanical properties are underway. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z41.00005: Seeing believes: Watching entangled sculpted branched DNA in real time Ah-Young Jee, Juan Guan, Kejia Chen, Steve Granick The importance of branching in polymer physics is universally accepted but the details are disputed. We have sculpted DNA to various degrees of branching and used single-molecule tracking to image its diffusion in real time when entangled. By ligating three identical or varying length DNA segments, we construct symmetric and asymmetric ?Y? branches from elements of lambda-DNA with 16 um contour length, allowing for single-molecule visualization of equilibrium dynamics. Using home-written software, a full statistical distribution based on at least hundreds of trajectories is quantified with focus on discriminating arm-retraction from branch point motion. Some part of our observations is consistent with the anticipated ?relaxation through arm retraction? mechanism but other observations do not appear to be anticipated theoretically. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z41.00006: DNA electrophoresis in tri-block copolymer gels---experiments and Brownian dynamics simulation Ling Wei, David H. Van Winkle The mobility of double-stranded DNA ladders in Pluronics\textregistered P105, P123 and F127, was measured by two-dimensional gel electrophoresis. Pluronics\textregistered are triblock copolymers which form gel-like phases of micelles arranged with cubic order at room temperature. A 10 base pair and a 25 base pair DNA ladder were used as samples in gel electrophoresis. The monotonically decreasing mobility with increasing length observed in the agarose separations is not observed in separations in Pluronics\textregistered . Rather, a complicated dependence of mobility on DNA length is observed, where mobility vs. length increases for short DNA molecules then decreases for longer molecules. There is also a variation of mobility with length correlated to the micelle diameter. Brownian dynamics simulations of a discrete wormlike chain model were performed to simulate short DNA molecules migrating in free solution and in a face-centered cubic matrix. By incorporating hydrodynamic interactions, the trend of simulated length-dependent mobility qualitatively agrees with experimental measurements. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z41.00007: Injectable Self-Assembling Peptide Hydrogel: Effects of Hydrophobic Drug Encapsulation and Delivery Jessie Sun, Brandon Stewart, Alisa Litan, Sigrid Langhans, Joel P. Schneider, Darrin J. Pochan We successfully encapsulated and continuously delivered a hydrophobic drug over the course of a month at effective, significant concentrations in a beta-hairpin peptide network that self-assembles into a shear-thinning injectable solid with immediate rehealing behavior. The peptidic network of the hydrogel is a result of the entangled and branched fibrillar nanostructure. This nanostructure protects the hydrophobic drug in an aqueous environment, while still maintaining original hydrogel network structures and properties. The characterization of the location and effect of the drug on the overall hydrogel properties over time are important to understand for future encapsulations of similarly hydrophobic payloads. The characterization techniques used to better understand the release and properties of the drug-gel constructs include rheology, small angle x-ray and neutron scattering, and in vitro methods. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z41.00008: Anomalous diffusion dynamics of associating artificial proteins in hydrogels Shengchang Tang, Muzhou Wang, Bradley Olsen Associative polymer gels have attracted a great deal of interest as responsive materials and biomaterials; while a great deal is known about their mechanical properties, knowledge about self-diffusion in these materials is still limited. Using coiled-coil proteins as a model associative polymer system where the number of stickers per polymer and molar mass of chains between stickers are exactly defined, we investigate self-diffusion in associative polymer hydrogels using forced Rayleigh scattering on the length scales ranging from 0.3 to 50 $\mu $m. Although the presence of associative groups reduces the rate of diffusion, ``superdiffusive'' scaling is observed for the first time up to a length scale of 10 $\mu $m. Fickian diffusion is recovered at larger length scales. The anomalous diffusion strongly depends on the temperature and the hydrogel concentration. We propose a simple two state model to capture the interplay between the diffusion of the proteins and the association of the coiled-coil segments. The model is able to capture both the anomalous regime and the Fickian regime, and provides estimation of the apparent diffusivities and the dissociation rates of the coiled-coil domains. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z41.00009: Physics of soft hyaluronic acid-collagen type II double network gels Svetlana Morozova, Murugappan Muthukumar Many biological hydrogels are made up of multiple interpenetrating, charged components. We study the swelling, elastic diffusion, mechanical, and optical behaviors of 100 mol\% ionizable hyaluronic acid (HA) and collagen type II fiber networks. Dilute, 0.05-0.5 wt\% hyaluronic acid networks are extremely sensitive to solution salt concentration, but are stable at pH above 2. When swelled in 0.1M NaCl, single-network hyaluronic acid gels follow scaling laws relevant to high salt semidilute solutions; the elastic shear modulus $G'$ and diffusion constant $D$ scale with the volume fraction $\phi$ as $G'\sim \phi^{9/4}$ and $D \sim \phi^{3/4}$, respectively. With the addition of a collagen fiber network, we find that the hyaluronic acid network swells to suspend the rigid collagen fibers, providing extra strength to the hydrogel. Results on swelling equilibria, elasticity, and collective diffusion on these double network hydrogels will be presented. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z41.00010: Viscoelastic Characterization of Gels at Metal-Protein Interfaces Elizabeth Martin, Kenneth Shull The interfacial gelation of proteins at metallic surfaces was investigated with an electrochemical quartz crystal microbalance (QCM). When Cr electrodes were corroded in proteinaceous solutions, it was found that gels will form at the Cr surfaces if molybdate ions are also present in the solution. A similar film will form on Cr when the proteins are replaced with a poly(allylamine) polyelectrolyte, suggesting that the gelation is due to a cross-linking reaction between the protein amine groups and the molybdate ions. Further, a method was developed to characterize the viscoelastic properties of thin polymeric films in liquid media using the QCM as a high frequency rheometer. By measuring the frequency and dissipation at multiple harmonics of the resonant frequency, the viscoelastic phase angle, density \textemdash modulus product, and mass per unit area of a film can be determined. The method was applied to characterize the protein films, demonstrating that they have a phase angle near 80$^{\circ}$ and a density \textemdash modulus product of $\sim$10$^{7}$ Pa-g/cm$^{3}$. Data imply that the gels are comprised of a weak proteinaceous network and exhibit similar mechanical properties as solutions containing 50 wt\% protein. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z41.00011: Investigation of Monodisperse Dendrimeric Polysaccharide Nanoparticle Dispersions Using Small Angle Neutron Scattering John Atkinson, Jonathan Nickels, Erzsi Papp-Szabo, John Katsaras, John Dutcher Phytoglycogen is a highly branched polysaccharide that is very similar to the energy storage molecule glycogen. We have isolated monodisperse phytoglycogen nanoparticles from corn and these particles are attractive for applications in the cosmetic, food and beverage, and biomedical industries. Many of these promising applications are due to the special interaction between the nanoparticles and water, which results in: (1) high solubility; (2) low viscosity and high stability in aqueous dispersions; and (3) a remarkable capacity to sequester and retain water. Our rheology measurements indicate that the nanoparticles behave like hard spheres in water, with the viscosity diverging for concentrations \textgreater 25{\%} (w/w). Because of this, aqueous suspensions of phytoglycogen provide an ideal platform for detailed testing of theories of colloidal glasses and jamming. To further explore the interaction of the phytoglycogen particles and water, we have performed small angle neutron scattering (SANS) measurements on the Extended Q-Range SANS (EQ-SANS) diffractometer at the Spallation Neutron Source at Oak Ridge National Laboratory. Measurements performed on phytoglycogen dispersions in mixtures of hydrogenated and deuterated water have allowed us to determine the particle size and average particle spacing as a function of the phytoglycogen concentration in the limits of dilute and concentrated dispersions. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z41.00012: The mechanical properties of p-granule components Louise Jawerth, Shambaditya Saha, Marcus Jahnel, Frank Juelicher, Anthony Hyman We study the major constituents of liquid droplets called ``p-granules''. During the one-cell stage of Caenorhabditis elegans development, these p-granules preferentially form on the posterior side of the cell and remain there preferentially as the cell divides leading to the majority of droplets remaining in one of the daughter cells. This process repeats during subsequent cell divisions with, again, the majority of droplets being maintained in one cell. When this process concludes and the worm continues to develop, the cell containing the p-granule droplets will eventually become the gonad of the adult worm. Previous work has suggested that the spatial segregation of p-granules that occurs at each stage is the result of a phase separation analogous to classic liquid-liquid demixing. We study this process under various buffer conditions in vitro by using a model system consisting of purified p-granule components. We find that these form liquid droplets under physiological conditions which resemble p-granules. We further report on the physical properties of these liquids and how they compare to their in vivo counterparts. In particular, we measure their surface tension and viscosity. We find surface tensions around 10-7 N/m and viscosities much higher than that of water. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z41.00013: The spatial response of nonlinear strain propagation in response to actively driven microspheres through entangled actin networks Tobias Falzone, Savanna Blair, Rae Robertson-Anderson The semiflexible biopolymer actin, a ubiquitous component of nearly all biological organisms, plays an important role in many mechanically-driven processes such as muscle contraction, cancer invasion and cell motility. As such, entangled actin networks, which possess unique and complex viscoelastic properties, have been the subject of much theoretical and experimental work. However, due to this viscoelastic complexity, much is still unknown regarding the correlation of the applied stress on actin networks to the induced filament strain at the molecular and micro scale.~Here, we use simultaneous optical trapping and fluorescence microscopy to characterize the link between applied microscopic forces and strain propagation as a function of strain rate and concentration. Specifically, we track fiduciary markers on entangled actin filaments before, during and after actively driving embedded microspheres through the network. These measurements provide much needed insight into the molecular-level dynamics connecting stress and strain in semiflexible polymer networks. [Preview Abstract] |
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