Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B41: Focus Session: Supramolecular Self-Assembly--Controlling Network and Gel Formation I |
Hide Abstracts |
Sponsoring Units: DPOLY DBP Chair: Aline Miller, University of Manchester Room: A115/117 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B41.00001: Bio-mimetic metal-ligand crosslinks yield self-healing polymer networks with near-covalent elastic moduli Niels Holten-Andersen, Matthew Harrington, Henrik Birkedal, Bruce Lee, Phillip Messersmith, Herbert Waite, Ka Yee Lee Growing evidence supports a load-bearing role for metal-polymer interactions in biological protein networks. In particular, the strength of the coordinate bonds in metal-ligand coordination complexes combined with their capacity to reform after breaking has been proposed as a source of the high toughness and potential self-healing in certain natural materials. Some of the highest stabilities among metal-ligand coordination complexes are found between Fe3+ and catechol ligands at alkaline pH where the tris-catecholato-Fe3+ stoichiometry prevails, yet the effect of such crosslinks on material properties has not been fully characterized due to the low solubility of Fe3+ at high pH. Inspired by the pH jump experienced by marine biomaterials during secretion, we have developed a simple method to control catechol-Fe3+ inter-polymer crosslinking via pH. The resulting gels display elastic moduli (G') that approach covalently crosslinked gels as well as self-healing properties. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B41.00002: Controlling Mechanical Properties of Bis-leucine Oxalyl Amide Gels William Chang, Daniel Carvajal, Kenneth Shull is-leucine oxalyl amide is a low molecular weight gelator capable of gelling polar and organic solvents. A fundamental understanding of self-assembled systems can lead to new methods in drug delivery and the design of new soft material systems. An important feature of self-assembled systems are the intermolecular forces between solvent and gelator molecule; by changing the environment the gel is in, the mechanical properties also change. In this project two variables were considered: the degree of neutralization present for the gelator molecule from neutral to completely ionized, and the concentration of the gelator molecule, from 1 weight percent to 8 weight percent in 1-butanol. Mechanical properties were studied using displacement controlled indentation techniques and temperature sweep rheometry. It has been found that properties such as the storage modulus, gelation temperature and maximum stress allowed increase with bis-leucine oxalyl amide concentration. The results from this study establish a 3-d contour map between the gelator concentration, the gelator degree of ionization and mechanical properties such as storage modulus and maximum stress allowed. The intermolecular forces between the bis-leucine low molecular weight gelator and 1-butanol govern the mechanical properties of the gel system, and understanding these interactions will be key to rationally designed self-assembled systems. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B41.00003: 4D Structural Dynamics of Sheared Collagen Networks Richard Arevalo, Daniel Blair, Jeffrey Urbach Soft biopolymer networks undergo substantial bulk stiffening when subject to shear strain. This nonlinear rheological signature has been observed for a wide range of semiflexible and stiff biopolymers, but the underlying geometric fiber rearrangements have not been measured and the resulting stress propagation through the network has not been experimentally assessed. We apply steady shear strains to collagen gels adhered to a thin elastic polyacrylamide gel substrate embedded with fluorescent displacement markers, while simultaneously imaging the three-dimensional network with a coupled confocal-rheometer. We observe dramatic network realignment towards the shear gradient driven by the nonaffine stretching, buckling, and rotation of constituent fibers and simultaneously measure stress inhomogeneities at the collagen-polyacrylamide interface. These observations elucidate the physical mechanisms governing strain-stiffening and our recent observation of the system-size dependence of this effect. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B41.00004: Molecular Dynamics Modeling of Actin Network Formation Ronald Pandolfi, Peter Becich, Lam Nguyen, Linda Hirst Actin filaments are ubiquitous and critical in cellular functions. The polymer protein F-actin is a semi-flexible filament that forms networks in the presence of binding proteins (i.e. $\alpha$-actinin, filamin, fascin). Molecular dynamics modeling and simulation of the formation of these networks has revealed the dependence of network structure on the ratio of G-actin monomers to cross-linkers, cross-linker shape, and filament length. In this study we focus on the effects of filament length on the assembled system. Comparative experimental work informs the accuracy of the modeled systems. Fourier analysis of the simulated networks allows quantitative characterization of the network structure. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B41.00005: Polyelectrolyte-Surfactant Complexes: A New Class of Organogelators Kevin Cavicchi, Yuqing Liu, Gustavo Guzman Polyelectrolyte-surfactant complexes (PE-SURFs) are a class of polymers generated by neutralizing a polyelectrolyte with an oppositely charged surfactant. It has been found that PE-SURFs composed of polystyrene sulfonate and long chain alkyl dimethyl amines act as good organogelators for a range of hydrophobic, organic solvents. Thermo-reversible organogels are formed by heating and cooling PE-SURF/solvent solutions. The gel transition temperature is influenced by the degree of polymerization, the length of the alkyl side-chain, the solubility parameter of the solvent, and the concentration of the gelator. Freeze-drying and scanning electron microscopy characterization of the resultant xerogels shows the formation of rod- and plate-like network morphologies depending on the system parameters. This behavior is consistent with gelation driven by the self-assembly of the amphiphilic PE-SURFs into micellar networks. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B41.00006: Self-Assembly of DNA--Block Copolymer Micelles Wei Qu, Xuan Jiang, Hai-Quan Mao, Erik Luijten Cationic--hydrophilic block copolymers have been developed as a potential carrier for use in gene delivery, displaying good transfection efficiency and biocompatibility. The cationic blocks effectively condense the DNA into a core surrounded by a protective and stabilizing corona formed by the hydrophilic blocks [Jiang \emph{et al.}, J. Control.\ Release \textbf{122} 297--304 (2007)]. Although the DNA condensation induced by the cationic blocks can be understood from energetic considerations, the formation of micelles with distinct morphologies is more complicated, as it involves several competing interactions. We employ computer simulations to model this interplay of driving forces. By correlating our simulation results with experimental observations, we provide an understanding of the self-assembly process and determine the key structural and experimental parameters that influencing the morphology of the DNA--block copolymer micelles. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B41.00007: LL37-DNA complexes and auto-immune diseases Fan Jin, Lori K. Sanders, Wujing Xian, Michel Gilliet, Gerard C. L. Wong LL37 is an alpha-helical host defense peptide in humans. Recent work has shown that Toll-like receptor-9 (TLR9), an intracellular receptor in plasmacytoid dendritic cells (pDCs) of the immune system that normally responds to pathogen nucleic acids, can be pathologically triggered by self DNA in the form of DNA-LL37 complexes. Synchrotron small-angle x-ray scattering (SAXS) measurements reveal an unanticipated form of self-assembly between DNA and this positively charged macroion. We examine the generality of this with other macroions, and propose a new geometric criterion for immune cell activation. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B41.00008: Stiffness of DNA nanotubes: insights for the design of dsDNA materials Paul Weitekamp, Daniel Schiffels, Alex Iteen, Deborah Fygenson DNA is increasingly used as a material in the design and construction of elaborate structures with nanoscale precision and functionalities. Whether self-assembled from tiles of short, synthetic oligomers or woven from purified genomic strands, most DNA nanostructures are based on parallel arrays of double-stranded DNA (dsDNA) held together by Holliday junction-like cross-links. There is considerable evidence that the double-helices thus intertwined are largely B-form in structure, but the mechanical integrity of the resulting nanostructures has gone largely unexplored. Here we present a systematic study of the stiffness of DNA nanotubes varying parameters such as helix number, cross-link density and strand complexity. We find stiffness is a useful reporter of structural quality for nanotubes and extract design principles for optimizing mechanical integrity of dsDNA materials. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:27PM |
B41.00009: Peptide assemblies: from cell scaffolds to immune adjuvants Invited Speaker: This talk will discuss two interrelated aspects of peptide self-assemblies in biological applications: their use as matrices for regenerative medicine, and their use as chemically defined adjuvants for directing immune responses against engineered antigens. In the first half of the presentation, the design of peptide self-assemblies as analogues for the extracellular matrix will be described, with a focus on self-assemblies displaying multiple different cell-binding peptides. We conducted multi-factorial investigations of peptide co-assemblies containing several different ligand-bearing peptides using statistical ``design of experiments'' (DoE). Using the DoE techniques of factorial experimentation and response surface modeling, we systematically explored how precise combinations of ligand-bearing peptides modulated endothelial cell growth, in the process finding interactions between ligands not previously appreciated. By investigating immune responses against the materials intended for tissue engineering applications, we discovered that the basic self-assembling peptides were minimally immunogenic or non-immunogenic, even when delivered in strong adjuvants. -But when they were appended to an appropriately restricted epitope peptide, these materials raised strong and persistent antibody responses. These responses were dependent on covalent conjugation between the epitope and self-assembling domains of the peptides, were mediated by T cells, and could be directed towards both peptide epitopes and conjugated protein antigens. In addition to their demonstrated utility as scaffolds for regenerative medicine, peptide self-assemblies may also be useful as chemically defined adjuvants for vaccines and immunotherapies. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B41.00010: Multiscale peptide self-assembly Justin Barone, Devin Ridgley Here, we demonstrate a hierarchical peptide self-assembly process from the nanometer to the micrometer scale. The process begins by mixing a short hydrophobic peptide and a longer $\alpha$-helix peptide. Cross-$\beta$ nanostructures spontaneously form that then aggregate into nanometer fibrils and then micron-sized fibers. FT-IR and Raman spectroscopy show unraveling of $\alpha$-helices and packing of aliphatic side groups as the major events leading to $\beta$-sheet and large fiber formation. A thermodynamic model is presented that uses conformational change and hydrogen bond formation to describe free energy change. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B41.00011: Intermolecular Hydrogen Bonding in Peptide and Modified Jeffamine Organogels Daniel Savin, Adam Richardson In these studies, we present two systems whereby supramolecular assembly results in rigid organogels. First, a series of AB diblock copolymers consisting of poly(Lysine(Z)) (P(Lys(Z)) blocks were synthesized and found to form stable, rigid organogels in THF (ca. 1 - 1.5 wt.\% solutions) and chloroform at room temperature. In these systems, the protecting group on the P(Lys) side-chains remains intact and gel formation results from the assembly of the solventphobic P(Lys(Z)) chains through intermolecular beta-sheet formation. The non-peptide block was found to have an effect on organogel properties due to interfacial frustration, which disrupts H-bonding. Second, Jeffamine polymers were modified in a facile way to incorporate intermolecular H-bonding groups to yield networks able to gel various solvents as well as mineral and canola oil. We present the physical and rheological properties of the organogels produced. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B41.00012: Designing responsive peptide hydrogels using peptide-responsive polymer conjugates Alberto Saiani, Anton Maslovskis, Aline Miller Self-assembly represents a simple and efficient route to the construction of large, complex structures. Peptide self-assembly in particular offers the possibility to design new functional bio-materials that find application in drug delivery and tissue engineering. The $\beta $-sheet motif is are of particular interest as short peptides can be designed to form $\beta $-sheet rich fibres that entangle and consequently form hydrogels. These hydrogels can be functionalised using specific biological signals and can also be made responsive through the use of enzymatic catalysis and/or conjugation with responsive polymers. In this presentation we will focus on the design of the latter using peptide-responsive polymer conjugates. The main objective is to create hydrogels possessing an internal transition resulting from the conjugation with the responsive polymer in the gel state that can be used as a trigger for example the release of a drug. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B41.00013: Stimuli-Responsive Peptide-based Triblock and Star Copolymers Jacob Ray, Sandeep Naik, Ashley Johnson, Jack Ly, Daniel Savin Stimuli-responsive copolymers demonstrate diverse aggregation behavior in aqueous solution. In general, the molecular architecture and the balance of hydrophilic and hydrophobic volumes influence morphology. This study involves polypeptide-based ABA linear triblock and AB2 star copolymer (which structurally resemble phospholipids) amphiphiles. Model systems for this study are poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblocks and poly(L-glutamate) (PE) based star copolymers. Extensive studies with KPK systems have resulted in morphological transitions by modifying pH, and we hypothesize that a change in individual chain conformation is the driving force for these transitions. Preliminary results for PE-based star copolymers with various hydrophobic moieties suggest polymersome (vesicle) formation. Light scattering (dynamic and static) and TEM were used to determine aggregate size and morphology as a function of pH; furthermore, circular dichroism (CD) spectroscopy was used to measure helix-to-coil transitions of the polypeptide blocks. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700