Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y41: Focus Session: Biopolymers I: Biohybrids, Biointerfaces, and Modeling |
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Sponsoring Units: DPOLY Chair: Bradley D. Olsen, Massachusetts Institute of Technology Room: 214A |
Friday, March 6, 2015 8:00AM - 8:36AM |
Y41.00001: Multiscale modelling of polymers at soft-bio interface Invited Speaker: Paola Carbone The behaviour of polymers at a liquid/liquid interface has become increasingly technologically important in recent years. For example, many of the self-assembly processes involving macromolecules occur at such interfaces and one of the most common chemical processes used to produce polymer nanoparticles $-$the solvent displacement method$-$ involves the diffusion of the polymer chains from a good solvent, where the polymer initially dissolves, to a non-solvent where the nanoparticles are formed. Finally, polymer-based drug nanocarriers (either nanoparticles or micelles) are becoming increasingly popular in drug delivery and their behaviour at fluid interfaces (such as a lipid/water boundary) should be properly understood in order to predict their biological activity. Here we show how using a multiscale approach it is possible to gain a detailed picture of the thermodynamic stability of homo- and co-polymers at fluid interfaces spanning from universal rules valid to any polymer systems at high dilution to the specific cases of amphiphilic linear and branched polymers. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y41.00002: Self-Assembly of DNA--Graft Copolymer Nanoparticles Zonghui Wei, Yong Ren, John-Michael Williford, Hai-Quan Mao, Erik Luijten Self-assembled DNA--copolymer nanoparticles are promising gene delivery systems due to their high biocompatibility. Notably, such nanoparticles can exhibit a variety of morphologies. Previously, we demonstrated that the nanoparticle shape can be tuned through variation of solvent polarity in a solution of DNA and block copolymers. Moreover, we confirmed that this shape can influence transfection efficiency [1]. In terms of ease of manufacturing as well as tunability of the system, it is important to explore the possibility of employing other types of condensing agents. Here, we report on the use of polyelectrolytes with grafted PEG side chains, which offer facile synthesis and an additional control parameter in the form of grafting density. Via a combination of experiments and molecular dynamics simulations we demonstrate that a high degree of shape control of the micellar nanoparticles can indeed be achieved through variation of the density and length of the grafted side chains. [1] Jiang, X.; Qu, W.; Pan, D.; Ren, Y.; Williford, J. M.; Cui, H. G.; Luijten, E.; Mao, H. Q. Advanced Materials \textbf{25}, 227--232 (2013). [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y41.00003: Monodisperse dendrimeric phytoglycogen nanoparticles in water act as hard sphere colloidal dispersions John Dutcher, Erzsi Papp-Szabo, Carley Miki Phytoglycogen is a highly branched polysaccharide that is very similar to the energy storage molecule glycogen. We have isolated monodisperse phytoglycogen nanoparticles from corn and these particles are attractive for applications in the cosmetic, food and beverage, and biomedical industries. Many of these promising applications are due to the special interaction between the nanoparticles and water, which results in: (1) high solubility; (2) low viscosity and high stability in aqueous dispersions; and (3) a remarkable capacity to sequester and retain water. We have used cone-and-plate and concentric cylinder rheometry to measure the dependence of the zero shear viscosity of aqueous dispersions of phytoglycogen on the phytoglycogen concentration. We find that the nanoparticles behave like hard spheres in water, with the viscosity diverging for volume fractions very close to that corresponding to randomly packed hard spheres. This simple system provides an ideal platform for detailed testing of theories of colloidal glasses and jamming. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y41.00004: Tracing lipids and their association with keratin in the adhesive gecko setae by NMR Spectroscopy Dharamdeep Jain, Alyssa.Y. Stark, Peter.H. Niewiarowski, Toshikazu Miyoshi, Ali Dhinojwala Numerous examples exist in nature where the coexistence of lipids and keratin is prominent. Examples include cell membranes, epidermis, avian feathers, wool, insect cuticle and the adhesive hairy features known as ``setae'' on the gecko toe. Until recently the setae were only considered to be composed of keratinous material. Given the prevalence of lipid-keratin associations in nature however, it is perhaps not surprising that phospholipids were found in the setae, and interestingly, in the form of a footprint after a gecko moves along a surface. However, the organization and the molecular-level behavior of lipids and keratin in the setae is still not known. Here, we demonstrate the use of NMR spectroscopy to detect lipids and understand their association with keratin in the molts termed as ``sheds'' from the toe pad and the non-adhesive regions of the epidermal skin. Our results show a distribution of similar lipids in both the skin and toe shed but with different dynamics at a molecular level. The study can help us understand the system better both biologically and for the design of better synthetics, but our findings may also have a larger impact on the recurring observations of lipids in many popular biomaterials and biological systems. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y41.00005: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y41.00006: Deposition and Grafting of Collapsed Elastin-Like Co-Polypeptides on Silicon Robin Mays, Julie Albert, Sarah MacEwan, Michael Dickey, Ashutosh Chilkoti, Jan Genzer Protein-based polymers offer the potential for responsive, bio-compatible, well-defined (molecular weight and sequence) systems. Elastin-like peptides (ELPs) are well suited for solution-based biomedical applications, including drug delivery and biomolecular purification. In order to use ELPs for stimuli-responsive surfaces, a detailed understanding of deposition conditions and behavior is instrumental. We grafted diblock ELPs with lower critical solution temperature (LCST) behavior to silicon surfaces. We synthesized 33kDa ELPs through genetic expression in bacteria with recombinant DNA technology. The diblock copolypeptides we used have a hydrophobic block (VPGVG) and a hydrophilic block (VPGSG), with each block having a different LCST. These diblock ELPs form micelles in solution when heated above the transition temperature of the hydrophobic block. We can graft either fully solvated ELPs or micellar ELP structures to an EDC/NHS activated surface. Our findings indicate a surprising stability of ELP aggregation on surfaces. We investigated the effects of time, temperature, and grafting block on the morphology, thickness, and water contact angle of our surfaces. Using atomic force microscopy, we studied the morphology of the deposited ELPs both in air and water. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y41.00007: Nanostructure Formation in Fusion Protein Block Copolymers Containing A Globular Protein Block Bradley Olsen, Guokui Qin, Matthew Glassman, Christopher Lam, Dongsook Chang, Eric Schiable, Alexander Hexemer Fusion proteins provide an elegant method for the synthesis of precisely defined block copolymers, where the use of molecular biology techniques enables monodisperse synthesis, precise control over block length and arrangement, and the incorporation of complex folded chain shapes and biofunctional structures. Here, we show that a block copolymer that contains a globular protein block and a coil-like protein block can self-assemble into a nanostructured material despite the chemical similarity between the two halves of the molecule. Using model polymers composed of the red fluorescent protein mCherry and an elastin-like polypeptide (ELP), the phase behavior of a simple linear fusion is shown to resemble that of protein-polymer conjugates. Molecular biology also enables the preparation of well-controlled double tailed fusion structures, and the self-assembly of these molecules shows that the chain topology of the fusion protein has a large impact on its self-assembly. At the same molar mass and composition, the double tailed structures self-assemble at a lower concentration and exhibit a greater number of order-order transitions than their single tailed counterparts. In addition, the double tailed fusions retained a higher fraction of functional protein in the final material. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y41.00008: Tailoring selectivity and flux in interior functionalized peptide nanotubes through self-assembly Sinan Keten, Luis Ruiz Self-assembly of cyclic peptide nanotubes (CPNs) in polymer thin films has opened up the possibility to create separation membranes with tunable nanopores that can differentiate molecules at the sub-nanometer level. Recent studies have demonstrated that the interior chemistry of the CPNs can be tailored by inserting functional groups in the nanopore lumen (mCPNs) (Hourani et al. JACS, 2011). Through theory and multi-scale MD simulations, here we explain how the stacking ordering of binary mixtures of functional CPs can be prescribed using the entropic elasticity of conjugated polymers (Ruiz {\&} Keten, Soft Matter, 2014). The linear self-similar coarsening growth mechanism, kinetic trapping and its effects on stacking sequences will be demonstrated (Ruiz {\&} Keten, J. Phys. Chem. Lett., 2014). Building on these insights, we present a new approach to addressing the challenge of boosting water flux and ion selectivity simultaneously, specifically by inserting two different types of functional groups in the lumen, and by controlling CP stacking order. Simulations elucidate how functional groups inspired from biological amino acids influence the transport of water and ions through mCPNs. We find mixing functional groups to tune partial charge distributions and pore size can be used to boost flux and selectivity (Ruiz {\&} Keten, Nanoscale, 2014). Our computational thought experiments lay the foundation for bioinspired principles to discover artificial nanochannels for separation applications. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y41.00009: Coarse-grained modelling of RNA Petr Sulc, Flavio Romano, Thomas Ouldridge, Jonathan Doye, Ard Louis We present a new, nucleotide-level model for RNA, oxRNA, based on the coarse-graining methodology recently developed for the oxDNA model of DNA. The model is designed to reproduce structural, mechanical and thermodynamic properties of RNA, and the coarse-graining level aims to retain the relevant physics for RNA hybridization and the structure of single- and double-stranded RNA. In order to explore its strengths and weaknesses, we test the model in a range of nanotechnological and biological settings. Applications explored include the folding thermodynamics of a pseudoknot, the formation of a kissing loop complex, the unzipping of a hairpin motif, and the thermodynamics and kinetics of RNA strand-displacement reaction. We argue that the model can be used for efficient simulations of the structure of systems with thousands of base pairs, and for the assembly of systems of up to hundreds of base pairs. The source code implementing the model is released for public use at dna.physics.ox.ac.uk. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y41.00010: Rationally Designed Random Heteropolymer Surfactants for the Encapsulation and Stabilization of Proteins in Organic Solvents Brian Panganiban, Baofu Qiao, Mona Obadia, Monica Olvera de la Cruz, Eric Drockenmuller, Ting Xu Stabilizing proteins in organic solvents can provide opportunities to overcome challenges in many areas, such as biosynthetic catalysis of hydrophobic substrates and biomimetic materials. Reverse micelles have been used to encapsulate proteins in organic solvents; however, currently-used small molecule surfactants are insufficient in both stabilizing native protein conformation and allowing for the retention of inherent protein functionality for extended periods of time. These surfactants are often quite dynamic and cannot completely suppress organic solvent penetration, resulting in protein denaturation. To address this pitfall, we report a new class of random heteropolymer surfactants that anchor to the protein surface through multiple non-covalent, complimentary interactions. These newly designed polymeric surfactants can effectively increase the retention of activity of several proteins in organic solvent in comparison to both a small molecule surfactant and an amphiphilic diblock copolymer. The modularity of this design process has the potential to be translated to a variety of proteins that can provide an enhanced platform for applications that include molecular recognition, catalysis, nanoscale assemblies, and medical therapeutics. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 11:00AM |
Y41.00011: Binding and protonation of polypeptides and proteins in pH responsive gels Invited Speaker: Igal Szleifer The binding and adsorption of polypeptides and proteins in pH sensitive gels is the result of the interplay between specific and non-specific interactions, protonation state and conformational properties of the proteins and the network. In this work we will present our recent predictions on the (non-trivial) adsorption of polypeptides and proteins on polyacid hydrogels. In particular we study the adsorption of polyhistidine and lysozyme on polyacrylic acid gels. We will show the qualitatively different response of the hydrogel to the adsorption of these different species and the large changes that occur in the acid base equilibrium within the hydrogel compared to the bulk solution. Of particular interest is how different amino acids within the same proteins show very different responses to pH changes, even though their bulk solution behavior is similar. The importance of charge regulation through acid-base equilibrium in biopolymers in general and its dependence on environmental conditions will be discussed. [Preview Abstract] |
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