Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W20: Invited Session: Physics of Biomacromolecules |
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Sponsoring Units: DPOLY Chair: Patrick Underhill, Rensselaer Polytechnic Institute Room: Ballroom B |
Thursday, March 5, 2015 2:30PM - 3:06PM |
W20.00001: Electrostatic self-assembly of biomolecules Invited Speaker: Monica Olvera De La Cruz Charged filaments and membranes are natural structures abundant in cell media. In this talk we discuss the assembly of amphiphiles into biocompatible fibers, ribbons and membranes. We describe one- and two-dimensional assemblies that undergo re-entrant transitions in crystalline packing in response to changes in the solution pH and/or salt concentration resulting in different mesoscale morphologies and properties. In the case of one-dimensional structures, we discuss self-assembled amphiphiles into highly charged nanofibers in water that order into two-dimensional crystals. These fibers of about 6 nm cross-sectional diameter form crystalline arrays with inter-fiber spacings of up to 130 nm. Solution concentration and temperature can be adjusted to control the inter-fiber spacings. The addition of salt destroys crystal packing, indicating that electrostatic repulsions are necessary for the observed ordering. We describe the crystallization of bundles of filament networks interacting via long-range repulsions in confinement by a phenomenological model. Two distinct crystallization mechanisms in the short and large screening length regimes are discussed and the phase diagram is obtained. Simulation of large bundles predicts the existence of topological defects among bundled filaments. Crystallization processes driven by electrostatic attractions are also discussed. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:42PM |
W20.00002: Theory and Computational Design of Protein Materials Invited Speaker: Jeffery Saven Protein design opens routes to arrive at novel molecules, materials and nanostructures. Recent theoretical methods can identify the properties of amino acid sequences consistent with desired structures and functions. Such methods leverage concepts from statistical mechanics and address the structural complexity of proteins and their many possible amino acid sequences. Computationally designed protein-based systems have been experimentally realized to encapsulate nonbiological cofactors and assemble into predetermined crystalline structures. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 4:18PM |
W20.00003: Self-assembly of Amyloid Fibrils in One, Two and Three Dimensions Invited Speaker: Raffaele Mezzenga Amyloid fibrils are protein aggregates, which occur in-vivo in the case of neurodegenerative diseases and in-vitro in the design of advanced functional materials of relevance in nanotechnology and nanosciences. At length scales above the well-established atomistic fingerprint of amyloid fibrils, these colloidal aggregates exhibit mesoscopic properties comparable to those of natural polyelectrolytes, yet with persistence lengths several orders of magnitude beyond the Debye length. This intrinsic rigidity, together with their chiral, polar and charged nature, provides these systems with some unique physical behavior in one, two and three dimensions. In this talk I will discuss our current understanding on the mesoscopic properties of amyloid fibrils at the single molecule level, the implication of their semiflexible nature on their liquid crystalline properties, and I will illustrate how this information proves useful in understanding their collective behavior in bulk and when adsorbed at liquid interfaces. By the careful exploitation of the physical properties of amyloid fibrils, the design of advanced materials with unprecedented physical properties becomes possible, and I will give a few examples on how these systems can ideally suit the design of biosensors and biomaterials. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:54PM |
W20.00004: Protein-engineered block-copolymers as stem cell delivery vehicles Invited Speaker: Sarah Heilshorn Stem cell transplantation is a promising therapy for a myriad of debilitating diseases and injuries; however, current delivery protocols are inadequate. Transplantation by direct injection, which is clinically preferred for its minimal invasiveness, commonly results in less than 5{\%} cell viability, greatly inhibiting clinical outcomes. We demonstrate that mechanical membrane disruption results in significant acute loss of viability at clinically relevant injection rates. As a strategy to protect cells from these damaging forces, we show that cell encapsulation within hydrogels of specific mechanical properties will significantly improve viability. Building on these fundamental studies, we have designed a reproducible, bio-resorbable, customizable hydrogel using protein-engineering technology. In our Mixing-Induced Two-Component Hydrogel (MITCH), network assembly is driven by specific and stoichiometric peptide-peptide binding interactions. By integrating protein science methodologies with simple polymer physics models, we manipulate the polypeptide chain interactions and demonstrate the direct ability to tune the network crosslinking density, sol-gel phase behavior, and gel mechanics. This is in contrast to many other physical hydrogels, where predictable tuning of bulk mechanics from the molecular level remains elusive due to the reliance on non-specific and non-stoichiometric chain interactions for network formation. Furthermore, the hydrogel network can be easily modified to deliver a variety of bioactive payloads including growth factors, peptide drugs, and hydroxyapatite nanoparticles. Through a series of \textit{in vitro} and \textit{in vivo} studies, we demonstrate that these materials may significantly improve transplanted stem cell retention and function. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:30PM |
W20.00005: New frontiers in single polymer dynamics Invited Speaker: Charles Schroeder Single molecule techniques allow for the direct observation of polymer dynamics under highly non-equilibrium conditions. Until recently, however, these methods have been largely confined to linear semi-flexible DNA molecules as ``model'' polymer chains. This talk will show recent work from our group in extending the field of single polymer dynamics to new materials, including branched polymers and truly flexible polymer chains. In this way, we explore new questions in classical polymer physics such as the role of architecture, topology, and backbone flexibility on chain dynamics at the molecular level. Recently, we used single molecule methods to directly visualize comb-shaped DNA polymers. Macromolecular DNA combs are synthesized utilizing a hybrid enzymatic-synthetic approach, wherein chemically modified DNA branches and DNA backbones are generated in separate polymerase chain reactions, followed by graft-onto reactions via ``click'' chemistry. This method allows for the synthesis of dual-color DNA combs, such that the backbone and side branches can be tracked independently using single molecule fluorescence microscopy. In this way, we study the dynamic properties of single comb polymers under flow, including conformational and stretching dynamics for highly branched chains and polymer relaxation following cessation of flow. In related work, we also study the dynamics of flexible polymer chains using fluorescently-labeled single stranded DNA. We observe that truly flexible polymers exhibit key differences in dynamics compared to semi-flexible DNA. Overall, our work highlights the ways in which single molecule methods can be brought to bear on fundamental problems in polymer physics. [Preview Abstract] |
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