Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D48: Focus Session: Dynamically Bonded Soft Matter |
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Sponsoring Units: DBIO DPOLY Chair: Niels Holten-Andersen, Massachusetts Institute of Technology Room: 217C |
Monday, March 2, 2015 2:30PM - 2:42PM |
D48.00001: Adhesion of \textit{D. discoideum} on Hydrophobic Substrate Bret Flanders, Nicoleta Ploscariu Adhesion by amoeboid cells, such as \textit{D. discoideum}, is poorly understood but critical for other behaviors such as phagocytosis and migration. Furthermore, both leucocytes and breast cancer cells employ the amoeboid mode of movement at various points in their life-cycles. Hence, improved knowledge of amoeboid adhesion may lead to be new strategies for controlling other important cellular processes. This study regards adhesion by \textit{D. discoideum} on silanized glass substrates. Reflection interference contrast microscopy is used in conjunction with other methods to determine the contact angle, cell-medium interfacial energy, and adhesion energy of these cells. The contact angle of individual cells settling under gravity onto a substrate is observed to increase as the size of the contact patch increases. This behavior occurs on slower time-scales than expected for the settling of inert vesicles. The implications of this observation on the nature of the underlying forces will be discussed. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D48.00002: Energetic modeling and single-molecule verification of dynamic regulation on receptor protein diffusion by actin corrals and lipid raft domains receptor Chien Yu Lin, Jung Y. Huang, Leu-Wei Lo To faithfully estimate a signal that varies in both space and time, the optimization strategy used by a live cell is to organize a collection of distributed and mobile receptors into a mobile active clustering. However, living eukaryotic cells are highly heterogeneous and stochastically dynamic. It is therefore important to develop an energetic model based on fundamental laws to verify that the underlying processes are energetically favorable. We developed an energetic model based on the generalized Langevin equation and the Cahn-Hilliard equation to simulate the diffusive behaviors of receptor proteins in the plasma membrane with a hierarchical structure of actin corrals, lipid domains, and receptor proteins. Single-molecule tracking data of EGFR acquired on live HeLa cells agrees with the simulation results. We discovered that after ligand binding, EGFR molecules move into lipid nanodomains. The transition rates between different diffusion states of liganded EGFR molecules are regulated by the lipid domains. Our method captures both the sensitivity of single-molecule processes, statistic accuracy of data analysis, and the hierarchical structure of plasma membranes. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D48.00003: Possible Domain Formation In PE/PC Bilayers Containing High Cholesterol Matthew Hein, Fazle Hussain, Juyang Huang Cholesterol is a significant component of animal cell membranes, and its presence has the effects of not only adding rigidity to the lipid bilayer, but also leading to the formation of lipid domains. Two other lipids of interest are phosphatidylethanolamine (PE), which constitutes about 45 percent of the phospholipids found in human nervous tissues, and phosphatidylcholine (PC), which is found in every cell of the human body. The maximum solubility of cholesterol is the highest mole fraction of cholesterol that the lipid bilayer can retain, at which point cholesterol begins to precipitate out to form cholesterol monohydrate crystals. We have measured the maximum solubility of cholesterol in mixtures of 16:0-18:1PE and 16:0-18:1PC using a new light scattering technique, which utilizes the anisotropic nature of light scattering by cholesterol crystals. This new method is highly accurate and reproducible. Our results show that the maximum solubility of cholesterol increases linearly as a function of the molar ratio POPC/(POPE+POPC), which suggests possible domain formation in mixtures of PE and PC containing maximum amount of cholesterol. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D48.00004: Self-Healing of Polymer Networks with Reversible Bonds Invited Speaker: Michael Rubinstein Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory we study a simple model of autonomic self-healing of polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. We study the reaction kinetics of reversible bonds in this simple model and analyze the different stages in the self-repair process. The formation of bridges and the recovery of the material strength across the fractured interface during the healing period occur appreciably faster after shorter waiting time, during which the fractured surfaces are kept apart. We observe the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium (very low) density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess nonequilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface (formation of bridges) occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk. The model is extended to describe enhanced toughness of dual networks with both permanent and reversible cross-links. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D48.00005: Assembly of transmembrane proteins on oil-water interfaces Peter Yunker, Corey Landry, Shaorong Chong, David Weitz Transmembrane proteins are difficult to handle by aqueous solution-based biochemical and biophysical approaches, due to the hydrophobicity of transmembrane helices. Detergents can solubilize transmembrane proteins; however, surfactant coated transmembrane proteins are not always functional, and purifying detergent coated proteins in a micellar solution can be difficult. Motivated by this problem, we study the self-assembly of transmembrane proteins on oil-water interfaces. We found that the large water-oil interface of oil drops prevents nascent transmembrane proteins from forming non-functional aggregates. The oil provides a hydrophobic environment for the transmembrane helix, allowing the ectodomain to fold into its natural structure and orientation. Further, modifying the strength or valency of hydrophobic interactions between transmembrane proteins results in the self-assembly of spatially clustered, active proteins on the oil-water interface. Thus, hydrophobic interactions can facilitate, rather than inhibit, the assembly of transmembrane proteins. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D48.00006: Bio-Inspired Composite Interfaces: Controlling Hydrogel Mechanics via Polymer-Nanoparticle Coordination Bond Dynamics Niels Holten-Andersen In soft nanocomposite materials, the effective interaction between polymer molecules and inorganic nanoparticle surfaces plays a critical role in bulk mechanical properties. However, controlling these interfacial interactions remains a challenge. Inspired by the adhesive chemistry in mussel threads, we present a novel approach to control composite mechanics via polymer-particle interfacial dynamics; by incorporating iron oxide nanoparticles (Fe3O4 NPs) into a catechol-modified polymer network the resulting hydrogels are crosslinked via reversible coordination bonds at Fe3O4 NP surfaces thereby providing a dynamic gel network with robust self-healing properties. By studying the thermally activated composite network relaxation processes we have found that the polymer-NP binding energy can be controlled by engineering both the organic and inorganic side of the interface. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D48.00007: Photo-induced Reshuffling of Covalent Networks for Shape Actuators Mitchell Anthamatten, Yuan Meng Photo-responsive allyl sulfide linkages within a polymer network can undergo addition fragmentation chain transfer (AFCT), in the presence of free radicals, to cause bond reshuffling. This phenomenon is employed to program a single-phase, two-way shape actuator that is thermal-responsive, even without an applied external load. Semicrystalline poly(caprolactone) networks containing allyl sulfide linkages are melted, strained to various elongations (hundreds of percent), and irradiated. Light causes a cascade of AFCT events, resulting in rupture of some network strands, configurational relaxation of dangling ends, and reformation of network bonds. After irradiation, the resulting double networks assume a mechanical state-of-ease and chains are under permanent configurational bias; when cooled, they crystallize in a preferred direction leading to fully reversible shape actuation. The mechanism of shape actuation is investigated using a combination of calorimetry and X-ray scattering. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:54PM |
D48.00008: TBD |
Monday, March 2, 2015 4:54PM - 5:06PM |
D48.00009: Mussel-inspired reversible metal-coordinate bonds as a pathway towards temporal control over the mechanical hierarchy of soft materials Scott Grindy, Robert Learsch, Niels Holten-Andersen Dynamic, reversible crosslinks have been shown to specifically control the mechanical properties of a wide variety of mechanically tough and resilient biomaterials. Here, we show that reversible histidine-metal ion interactions, long thought to contribute to the strong mechanical properties and self-healing nature of mussel byssal threads, can be used to control and engineer the hierarchical mechanical properties of model polyethylene glycol hydrogels orthogonally from the spatial structure of the material. We delve into the physics underlying these types of materials to properly understand how to explicitly engineer the mechanical properties of tough soft materials by utilizing their temporal hierarchy. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D48.00010: Active Dynamic Frictional Probes Joshua Steimel, Juan Aragones, Alfredo Alexander-Katz In biological systems there are a myriad of interactions occurring instantaneously and these interactions can vary drastically in the strength of the interaction, the speed at which this interaction occurs, and the duration of the interaction. When multiple interactions occur any of these factors can determine which particular interaction is dominant. However, currently it is extremely difficult to measure binding affinity, K$_{\mathrm{on}}$, and K$_{\mathrm{off}}$ rates in a relatively high throughput manner. Here we propose a novel and versatile system that will be able to detect differences in binding affinity of wide range of transient interactions and will be able to extract the relevant time scales of these interactions. Our system will utilize ferromagnetic particles that can be easily functionalized with a receptor of interest and the substrate will be coated in the corresponding ligand. A rotating magnetic field will cause particles, henceforth referred to as rollers, to rotate and this rotational motion will be converted into translational motion via the effective frictional force induced by interaction that is being probed. By measuring the translation of the rollers to a baseline, where only hydrodynamic friction occurs, we can measure the relative strength of the interactions. We can also potentially measure kinetic information by changing the frequency at which the magnetic field rotates, since changing the frequency at which the bead rotates is akin to changing the time allowed for bond formation. We will measure a wide range of interaction including ionic, metal-ion coordination, IgG-Protein A complex, and biotin-streptavidin complex. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D48.00011: Magnesium Dependence of the RNA Free Energy Landscape Ryan Hayes, Jeffrey Noel, Ana Mandic, Paul Whitford, Karissa Sanbonmatsu, Udayan Mohanty, Jos\'e Onuchic The RNA free energy landscape is highly sensitive to ionic concentrations, and especially to Mg$^{2+}$, as most RNA tertiary structure will not form in the absence of Mg$^{2+}$. At physiological concentrations, the energy landscape must be smooth and funneled to fold on biological time scales, but changes in ionic concentration may affect the relative stability of alternative states. We perturb a structure-based model, which captures the funneled nature of the energy landscape, to include electrostatic effects. Our model includes explicit Mg$^{2+}$ and screening by implicit KCl. A dynamic model for the local competition between Manning condensed Mg$^{2+}$ and KCl is introduced, which makes the model more broadly applicable and transferable than a previous static model. We use the excess Mg$^{2+}$ ions associated with the RNA ($\Gamma_{2+}$) to test the model. $\Gamma _{2+}$ is an ideal metric because it is closely related to the Mg$^{2+}$-RNA interaction free energy, and is easily measurable in both experiment and simulation. The model captures intermediate states of a small pseudoknot missed by models without electrostatics. [Preview Abstract] |
Monday, March 2, 2015 5:30PM - 5:42PM |
D48.00012: Exploiting Dynamic Bonds in Polymer-grafted Nanoparticle Networks to Create Mechanomutable, Reconfigurable Composites Anna C. Balazs, Matthew J. Hamer, Balaji V.S. Iyer, Victor V. Yashin Via a new dynamic, three-dimensional computer model, we simulate the tensile deformation of polymer-grafted nanoparticles (PGNs) that are cross-linked by labile bonds, which can readily rupture and reform. For a range of relatively high strains, the network does not fail, but rather restructures into a stable, ordered structure. Within this network, the reshuffling of the labile bonds enables the formation of this new morphology. The studies reveal that the appropriate combination of stress-responsive hybrid materials and applied stress can yield distinct opportunities to dynamically switch between different structures, and thus, the properties of the material. Thus, the results provide guidelines for designing mechano-responsive hybrid materials that undergo controllable structural transitions through the application of applied forces. [Preview Abstract] |
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