Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F11: Focus Session: Active Soft Matter I - Transport, Biomimetics and Dynamic Response |
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Sponsoring Units: DPOLY GSNP DBIO Room: 203 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F11.00001: Polymer Prize Break |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F11.00002: Cytoskeletal organization by motor and polymerization forces Invited Speaker: Gijsje Koenderink Cells need to constantly change their change to perform vital functions, such as growth, division, and movement. Dysregulation of cell shape can have severe consequences such as cancer. Our goal is to resolve physical mechanisms that contribute to cell shape control. For this purpose, we study simplified experimental model systems reconstituted from purified cellular components. In this talk, I will give two examples of our recent work. The first example concerns active contractility of the actin cortex, which lies underneath the cell membrane and drives shape changes by means of myosin motors. Using in vitro models, we studied how myosin motors and actin filaments collectively self-organize into force-generating arrays. I will show that motors contract actin networks only above a sharp threshold in crosslink density. We discovered that right at this threshold, the motors rupture the network into clusters that exhibit a broad distribution of sizes, as expected in filamentous networks near a percolation threshold. The second example I will discuss concerns cell shape polarization directed by interactions between the actin and microtubule (MT) cytoskeletons. A prominent example is the guidance of MT growth along F-actin bundles towards specific targets, i.e. focal adhesions. It has been suggested that MT end-tracking proteins ($+$TIPs) that also bind F-actin are responsible for this process. We built an in vitro system involving a simplified actin-MT crosslinker molecule and could show that the interaction between MT ends and actin is sufficient to capture and re-direct MT growth along actin bundles. By keeping MT growth tightly coupled to F-actin, this mechanism allows linear arrays of actin bundles to act as templates for MT organization. Instead, when interacting with single actin filaments, MT ends become the dominant organizing factor, exerting forces that align, pull and even transport actin filaments in the direction of MT growth. We conclude that actin and MTs can influence each other's organization through coupling by $+$TIP proteins. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F11.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F11.00004: Stress activated contractile wavefronts in the mechanically-excitable embryonic heart Kevin Chiou, Stephanie Majkut, Dennis Discher, Tom Lubensky, Andrea Liu The heart is a prime example of a robust, active system with behavior--the heart beat--that is extraordinarily well timed and coordinated. For more than half a century, electrical activity induced by ion release and diffusion has been argued to be the mechanism driving cardiac action. But recent work indicates that this phenomenon is also regulated by mechanical activity. In the embryonic avian heart tube, the speed of the contractile wavefront traversing the heart tube with each beat is measured to be a monotonic, linear function of tissue stiffness. Traditional electrical conduction models of excitation-contraction cannot explain this dependence; such a result indicates that the myocardium is mechanically excitable. Here, we extend this work by using experimental observations of stiffness-dependent behavior in isolated cardiomyocytes as an input to study contractile wavefronts in the tissue as a whole. We model the heart tube as an active, overdamped elastic network where the primary stress mediator is the extracellular matrix. Using this simple model, we explain experimental observations of the systolic wave and predict qualitatively new behavior. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F11.00005: Photo-induced Mass Transport through Polymer Networks Yuan Meng, Mitchell Anthamatten Among adaptable materials, photo-responsive polymers are especially attractive as they allow for spatiotemporal stimuli and response. We have recently developed a macromolecular network capable of photo-induced mass transport of covalently bound species. The system comprises of crosslinked chains that form an elastic network and photosensitive fluorescent arms that become mobile upon irradiation. We form loosely crosslinked polymer networks by Michael-Addition between multifunctional thiols and small molecule containing acrylate end-groups. The arms are connected to the network by allyl sulfide, that undergoes addition-fragmentation chain transfer (AFCT) in the presence of free radicals, releasing diffusible fluorophore. The networks are loaded with photoinitiator to allow for spatial modulation of the AFCT reactions. FRAP experiments within bulk elastomers are conducted to establish correlations between the fluorophore's diffusion coefficient and experimental variables such as network architecture, temperature and UV intensity. Photo-induced mass transport between two contacted films is demonstrated, and release of fluorophore into a solvent is investigated. Spatial and temporal control of mass transport could benefit drug release, printing, and sensing applications. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F11.00006: Characterization of Particulate Matter Transport across the Lung-Surfactant Barrier using Langmuir Monolayers Jeremy Eaton, Michael Dennin, Alex Levine, Steven George We investigate the transport of particulate matter acros the lung using a monolayer of bovine lung surfactant tagged with NBD in conjunction with alveolar lung cells below the air-water interface. The monolaye dynamically compressed and expanded to induce phase transitions as well as buckling and folding. Polystyrene spheres ranging from 20 to 500 nm in diameter were tagged with fluorescent molecules and deposited on the monolayer. We will present results of preliminary studies of the transport of beads from the air-water surface to the lung cells through the monolayer. Characterization of the transfer will focus on differential fluorescence microscopy to distinguish uncoated beads from beads from beads coated with surfactant monolayers. The presence or absence of surfactant associated with the beads provides insight into potential transfer mechanisms and will serve as an input into models of the bead transfer. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F11.00007: Activating membranes Ananyo Maitra, Pragya Srivastava, Sriram Ramaswamy, Madan Rao We formulate a hydrodynamic theory of a fluid membrane coupled to a bulk medium comprising treadmilling filaments endowed with active stresses and show that active membrane dynamics [Phys. Rev. Lett \textbf{84}, 3494 (2000)] and spontaneous shape oscillations emerge from this description. We also consider membrane instabilities and patterns induced by the presence of filaments with polar orientational correlations in the tangent plane of the membrane. The dynamical features we predict should be seen in a variety of cellular contexts involving the dynamics of the membrane-cytoskeleton composite and cytoskeletal extracts coupled to synthetic vesicles. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F11.00008: Protein-Polyelectrolyte Coacervation: Morphology Diagram, Binding Affinity, and Protein Separation David Hoagland, Xiaosong Du, Paul Dubin For aqueous mixtures of negatively charged polysaccharide, hyaluronic acid (HA), and globular protein, either bovine serum albumin (BSA) or beta-lactoglobulin (BLG), a pH-ionic strength (I) morphology diagram, with regions of homogeneous solution, soluble complex, coacervation, precipitation, and redissolution, was developed by pH titrations performed at fixed I. The systems are models for coacervation, or liquid-liquid phase separation, between flexible and compact solutes of opposite charge. Protein charge here is tuned by pH, and titration keeps the mixtures close to equilibrium. At high I, only homogeneous solution is observed, as true at high and low pH. Diagrams for the proteins differ because HA affinity for BSA is higher than for BLG, traced to BSA's greater charge patchiness and higher net charge; isothermal solution titration calorimetry finds a factor of two difference in binding energy. Dependences of transition pH on protein charge Z and solution I offer additional insights into interactions underlying morphology transitions. At optimal conditions, the affinity disparity is sufficient to achieve highly selective BSA coacervation in a 1:1 protein mixture, suggesting coacervation to separate similar proteins under mild, non-denaturing conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F11.00009: Spontaneous motion and deformation of a droplet driven by chemical reaction Natsuhiko Yoshinaga Spontaneous motion has been attracting lots of attention in last decades in nonlinear and nonequilibrium physics partially for its potential application to biological problems such as cell motility. Recently several model experiments showing spontaneous motion have been proposed in order to elucidate underlying mechanism of the motion. The systems in these works consist of relatively simple ingredients, for instance oil droplets in water, but nevertheless the results show rich motion and deformation of the droplet. Importantly, the system breaks symmetry and chooses one direction of motion. In this work, we theoretically derive a set of nonlinear equations exhibiting a transition between stationary and motile states starting from advection-reaction-diffusion equation driven away from an equilibrium state due to chemical reactions. A particular focus is on how hydrodynamic flow destabilizes an isotropic distribution of a concentration of chemicals. We also discuss a shape of the droplet. Due to self-propulsive motion and flow around the droplet, a spherical shape becomes unstable and it elongates perpendicular to the direction of motion. This fact would imply that the self-propulsion driven by chemical reaction is characterized as a pusher in terms of a flow field. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F11.00010: Structural transitions in helical polymers Matthew Williams, Michael Bachmann Helical structures, as well as more complex tertiary structures, made up of helixes are relevant in biological systems. We perform generalized-ensemble Monte Carlo simulations to examine homopolymer models which include a torsional potential energy associated with each bond. With the inclusion of a torsional potential, helical structures emerge and can contort to form a variety of tertiary structural phases. We explore the two-dimensional space, parametrized by temperature and torsional energy scale, to map helical structures and to locate structural transitions. We see transitions occur between helical and non-helical secondary structures and also between various tertiary structures. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F11.00011: Peptidyl Materials Formed Through Click Chemistry Enhanced Coiled-Coil Interactions Kenneth Koehler Biologically derived materials offer a level of sophistication synthetically fabricated materials have only attempted to mimic. This level of complexity may be found in materials such as peptides. Implementing new theory and modeling, peptides with the propensity to form coiled-coil (CC) bundles were designed and synthesized. Through the use of this \textit{de novo} approach, modeling allowed prediction of the feasibility to include non-natural amino acids conducive to click chemistry into the peptide. Amino acids showcasing thiol or alkyne functionalities were considered owing to the ability of these moieties to participate in the thiol-ene and copper click reactions respectively. Once synthesized, the peptides decorated with these clickable motifs were placed in solution and allowed to self-assemble into CC's. CD spectroscopy and DLS experiments confirmed the formation and assembly of CC's. Click reactions were then incited to link the CC assemblies together and form a network with predictable dimensionality and pore size between CC bundles. To incite network formation, click reactions between CC side chain residues and suitably functionalized crosslinkers were implemented. The linking of coiled-coils and material formation were assessed using DLS and TEM. [Preview Abstract] |
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