Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z14: Equilibrium Self-Assembly |
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Sponsoring Units: DFD Chair: James Gilchrist, Lehigh University Room: 315 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z14.00001: Para-, ferro- and antiferro-magnetic order in beta-sheet tapes of oligopeptides Sara Jabbari-Farouji, Paul van der Schoot Beta-sheet-forming peptides give rise to self-assembled hierarchical structures such as tapes, ribbons and fibrils, which at sufficiently high concentrations form nematic liquid crystalline solutions and gels. Applications of these novel materials are found in nanotechnology, medicine and personal care products. Such aggregates not only appear in the context of desirable biomaterials but also in pathological self-assembly of mis-folded proteins, forming aggregates such as ``amyloids''. Recently a theoretical model was developed to understand the properties of these self-assembling structures [1]. The question which arises is what happens if we mix different peptide species varying e.g. in length or interaction energy. Do they mix in self-assembled structures or form separate ones? This is of crucial importance as most of industrially produced materials are not monodisperse. To model the simplest polydisperse system, we apply two-component self-assembled Ising model, in which three energy scales are involved. We show that depending upon the relative values of these energy scales and concentrations of the two components, different morphologies of tapes consisting of both components are formed exhibiting paramagnetic, ferromagnetic or antiferromagnetic order. [1] A. Aggeli, et al; PNAS \textbf{2001, }98, 11857 [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z14.00002: Self-assembly induced protein crystallization Hongjun Liu, Sanat Kumar, Jack Douglas The strongly anisotropic nature of inter-protein interactions naturally leads them to self-assemble into structures mirroring the symmetry of the inter-protein potential. Self-assembly is a thermodynamically distinct phenomenon from phase separation, and we consider whether it can play a direct role in nucleating protein crystallization. Previous simulations and measurements have established that protein clusters formed below the critical point for liquid-liquid phase separation ($T_c$) can facilitate crystal nucleation. However, recent experiments have indicated the existence of clustering-induced protein nucleation even for $T > T_c$, where phase separation does not exist. Here we simulate a minimal model of patchy particles and indeed find that transient clusters formed through self-assembly (even above $T_c$) can nucleate crystal growth. Importantly, the self-assembled clusters help to select the symmetry of the resulting crystal growth. In contrast, protein crystallization for $T < T_c$ does not have this directing influence, and polycrystalline growth forms, such as spherulites, are then prevalent. Our simulations suggest that self-assembly directed crystallization might be common in protein solutions and that this process is relevant for understanding protein crystallization polymorphism. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z14.00003: Mechanisms for semi-flexible filament self-assembly: an experimental and simulation study Lam Nguyen, Wei Yang, Steve Acquah, Harold Kroto, Linda Hirst The self-assembly of semi-flexible filaments, such as F-actin, in the presence of cross-linkers has been studied experimentally and via molecular dynamics simulation. Several imaging techniques including fluorescence and electron microscopy have been used to elucidate the structural properties of formed bundles and networks of filaments. With the help of simulation we are able to observe the dynamical process of the self-assembly and study the driving forces behind filament aggregation. The roles of different parameters such as cross-linker density and filament length have been investigated, determining the assembled system properties. We find both of these parameters to play a key role in the final structure formation. Understanding the mechanism for the self-assembly of these semi-flexible filaments will be very useful in the application of developing a new class of biological materials. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z14.00004: Computer simulations of the self-assembly of chiral superstructures from rigid achiral constituents Christopher Hixson, Fangyong Yan, David Earl We present the results of computer simulations of an achiral rigid bent-core model system at a range of temperatures and densities. We observe nematic and smectic phases, but more interestingly observe chiral micelles and columns at lower densities. The origin of these chiral features are explored using minimization techniques and parallel tempering searches. We show that chiral structures are minima of the potential energy surface. Additionally, we show that the addition of chiral dopant induces the system to order into a single twist direction.\footnote{Fangyong Yan, Christopher Adam Hixson, and David J. Earl, Phys. Rev. Lett. {\bf 101}, 157801 (2008)} [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z14.00005: Reversible pH-Induced Structural Transition in a Polyelectrolyte-Surfactant System: from Semi-flexible Rod to String of Spheres Viet Lam, Lynn Walker We have characterized a polyelectrolyte-surfactant system that forms stable rod-like aggregates in aqueous solution. While this structure is stable to most changes in solution condition, we have observed a reversible change in behavior with pH. This is due to a pH-induced structural transition from the original semi-flexible rod at neutral pH to a more flexible object at acidic conditions. A simple model of polyelectrolyte chain crossing multiple surfactant spherical micelles, or a string of spheres, has been proposed as the structure of the aggregates at low pH. This represents a novel rod-like nanoscale system that goes through a reversible gelation with pH, with possible use in oil drilling (matrix acidification aid), liquid flow control, or transport of hydrophobic materials. Here, we will present a simple model of the structural change and experimental justification. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z14.00006: Self-Assembly of Highly Segregating Diblock Copolymer in Solution Dilru Ratnaweera, Stephen Clarson, Dvora Perahia Solvent affinity drives the association of diblock co-polymers in selective solvents. The shape of the micelles is affected by the size of the blocks and their interaction with the solvent. Most experimental and theoretical studies have investigated solutions of diblocks with a relatively low incompatibility, requiring relatively large blocks to associate. The current work introduces a small angle neutron scattering study of a highly segregated diblock-copolymer, a trifluoro propylmethyl siloxane - polystyrene (PTFPMS-PS) in solutions of$ d$-toluene, a good solvent for the polystyrene. Studies were carried out over volume fractions of 0.1 to 0.5 of the fluorinated siloxane segment. The high degree of segregation results in association into star-like micelles with the fluorinated siloxane in the core and a swollen corona even at very low volume fractions of the fluorinated segments. The micelles exhibit unique temperature stability in comparison with aggregates formed by diblock-copolymers in a lower segregation regime. The detailed structure of these aggregates as a function of volume fraction and temperature will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z14.00007: Mesophases of soft-sphere aggregates Homin Shin, Gregory Grason, Christian Santangelo Soft spheres interacting via a hard core and purely repulsive shoulder self-assemble into clusters forming a variety of mesophases. We combine a mean field theory developed from a lattice model with a level surface analysis of the periodic structures of soft-sphere aggregates to study stable morphologies for a class of interaction potentials. The mean- field solution shows that the site occupation density and interparticle potential are self-consistently related to an ``effective field'' acting on each particle. In the strong segregation limit, the space group symmetry of possible aggregate structures associated with the spatially modulated field, together with a half-filling condition at the interface of morphology, allows us to produce a phase diagram including Lamella, Hexagonal-columnar, and BCC phases, and their inverse phases in the parameter space of chemical potential and interparticle potential. Finally, we discuss the finite- temperature corrections to strong segregation theory in terms of Sommerfeld-like expansion and how these corrections affect the thermodynamic stability of bicontinuous mesophase structures, such as gyroid. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z14.00008: Breaking it up: Simulations of micelle fission in explicit solvent Mikko Karttunen, Maria Sammalkorpi, Mikko Haataja We study self-assembly in micellar systems consisting of sodium dodecyl sulfate (SDS) using detailed 200-400 ns atomic scale molecular dynamics simulations. The simulations were done with explicit solvent, counterions and salt. We focus on the role of molecular level interactions driving self-assembly [1] and, in particular, show how micelle fission can be controlled using electrostatics. As our main result, we demonstrate the existence of a new fission pathway in charged micelles [2] and provide a physical explanation for it.\\[0.2cm] 1. M. Sammalkorpi, M. Karttunen, M. Haataja, J. Phys. Chem. B {\bf 111}, 11722 (2007).\\ 2. M. Sammalkorpi, M. Karttunen, M. Haataja, J. Am. Chem. Soc., in press. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z14.00009: Neutron Scattering Analysis of the Dynamics and Structure of Semiflexible, Self-Assembled Peptide Chain Networks and WormLike Micelles N. Wagner, M. Branco, D. Pochan, J. Schneider Self assembled peptide hydrogels are formed from synthetic $\beta $-hairpin peptides that undergo triggered self assembly to form a physically crosslinked network of entangled fibrils. Upon salt addition at pH 7.4, these peptides fold into a $\beta $-hairpin self-assemble to form a rigid hydrogel stabilized by non-covalent crosslinks. A single amino acid substitution is performed to change the charge on the peptide and greatly alter the rate of assembly. As a result, faster folding and self assembly kinetics are observed leading to more rigid gels. Transmission electron microscopy (TEM) and rheology demonstrate that the resultant, rigid networks of the semiflexible fibrils are composed of a bilayer of hairpins with a cross-sectional diameter of 3 nm, corresponding to the width of a folded peptide. Neutron spin echo (NSE) measurements show that the peptides can be modeled as semiflexible chains on lengthscales shorter than the characteristic mesh size. The chain diffusivity is reduced by the peptide substitution and this can be attributed to alteration of the electrostatic interactions between peptides in the fibril. Small angle neutron scattering (SANS) measurements show a transition from a cylindrical rod-like geometry to a more branched, fractal-like network topology upon amino acid substitution. These measurements explain the large increase in gel modulus observed upon amino acid substitution. These results facilitate the rational design of self-assembling peptide materials for biomaterial applications. NSE results for semiflexible wormlike micelles will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z14.00010: Chiral Self-Assembly of Rodlike Viruses Edward Barry, Zvonimir Dogic, Robert Meyer, Robert Pelcovits, Rudolf Oldenbourg The self-assembly of two dimensional achiral membranes which occurs in entropic mixtures of monodisperse colloidal rods and non-adsorbing polymers will be described. The colloidal nature of the rod/polymer model system enables us to simultaneously examine the behavior of self-assembled membranes at both the molecular and continuum lengthscales. Combining observations made at the very different lengthscales, we investigate how chirality frustrates assembly of achiral 2D membranes altogether, and instead drives the formation of very complex and highly regular chiral structures. Representative structures obtained through chiral self-assembly include: twisted ribbons, double helices, two dimensional analogs of a TGB phase, and regular arrays of pores embedded within a 2D membrane. [Preview Abstract] |
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