Browsing by Author "Carol Hall, Committee Member"

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Design and Interfacial Activity of Copolymers with Controlled Monomer Sequence Distributions
(2004-01-13) Semler, James Joseph; Jan Genzer, Committee Chair; Carol Hall, Committee Member; Harold Ade, Committee Member; Peter Kilpatrick, Committee Member; Richard Spontak, Committee Member
We study the bulk and interfacial behavior of A-B copolymers. Emphasis is placed upon addressing the role of the monomer sequence distribution in A-B copolymers as it pertains to the copolymer's mobility in confined geometries and its ability to recognize chemical patterns on surfaces. Monte Carlo simulations are used to study the ability of block (A-b-B) and alternating (A-alt-B) copolymers to recognize chemical patterns on flat, impenetrable surfaces comprising two distinct chemical sites, C and D. The copolymer adsorption is driven by the repulsion between A and B segments along the copolymer chain and the attraction between B segments and D sites on the surface. The principle parameters that govern the ability of A-b-B and A-alt-B copolymers to recognize surface patterns are: the strength of the interaction between B segments and D surface sites, the A-B monomer sequence distribution, and the size and spatial distribution of adsorbing D sites. Our simulations reveal that both A-b-B and A-alt-B copolymers are capable of recognizing surface patterns and increasing the B-D attraction enhances the partitioning of A and B segments at the surface. Commensurability between the copolymer's monomer sequence distribution and the size and spatial distribution of the surface heterogeneities is also found to affect the ability of A-b-B and A-alt-B copolymers to recognize surface chemical patterns. When the adsorbing domain size exceeds the size of the copolymer's parallel component to the radius of gyration, A-b-B copolymers are found to transfer the surface pattern into the bulk with high fidelity. A-alt-B copolymers, however, are able to replicate the surface pattern into the bulk material when heterogeneous domain sizes are much smaller. We introduce a novel 'coloring' scheme to synthesize polystyrene-polybromostyrene (PS-co-PBrS) copolymers with statistically random (r-(PS-co-PBrS)) and random-blocky (b-(PS-co-PBrS)) monomer sequence distributions. Our results show that r-(PS-co-PBrS) and b-(PS-co-PBrS) copolymers with equivalent bromine content possess different intrinsic viscosities and radii of gyration. We attribute this behavior to the ability of b-(PS-co-PBrS) coils to form globular structures in toluene where PBrS forms a dense core and PS remains predominantly in a loose corona. This behavior is in contrast to that of r (PS co-PBrS) coils where both the PBrS and PS are homogeneously distributed. The interfacial behavior of the random and blocky copolymers is also found to differ. Specifically, thin films of r-(PS-co-PBrS) deposited on top of flat silica substrates covered with a semifluorinated self-assembled monolayer are found to dewet at a faster rate than b-(PS-co-PBrS) of comparable thickness at the same T−Tg, where Tg is the bulk glass transition temperature of the PS-co-PBrS copolymer. To our knowledge, this is the first experimental evidence that supports claims from computational studies arguing that the sequence distribution of random copolymers affects the chain's mobility on a surface. Molecular insights into the 'coloring' reaction are provided by Monte Carlo simulations of the experimental reaction scheme. The probability of chemically altering expanded homopolymer coils is found to be equal for all units along the length of the chain. In contrast, 'coloring' of collapsed homopolymer coils reveals that the probability of modification is widely distributed. These results further support our claim that copolymers with random and random-blocky monomer sequence distributions can be synthesized by 'coloring' expanded and collapsed homopolymer coils, respectively.
Kinetic Folding Studies of Apaf-1 CARD and Procaspase-3
(2008-11-13) Milam, Sara Lis; Carol Hall, Committee Member; William Miller, Committee Member; A. Clay Clark, Committee Chair; Michael Goshe, Committee Member
Apoptosis regulates the balance between cell growth and death. Apoptosis consists of two main signaling pathways, extrinsic and intrinsic. We have examined the kinetic folding mechanism of two proteins, Apaf-1 CARD and procaspase-3, which play a major part in these signaling cascades. The caspase recruitment domain (CARD) of Apaf-1 (apoptotic protease activating factor) is the 97 amino acid N-terminal domain involved in protein-protein interactions, allowing for the activation of procaspase-9. Apaf-1 CARD consists of six antiparallel Î±-helices arranged in a Greek key topology. Single and sequential mixing stopped-flow studies showed that Apaf-1 CARD folds and unfolds rapidly and suggest a folding mechanism that contains parallel channels with two unfolded conformations folding to the native conformation. KINSIM simulations show that a slow folding phase is described by a third conformation in the unfolded ensemble that interconverts with one or both unfolded species. Overall, the native ensemble is formed rapidly upon refolding. Procaspase-3, like all other caspases, exists in the cell as an inactive zymogen and is the final step in the apoptotic pathway. Once activated it cleaves numerous substrates, leading to the dismantling of the cell. The refolding pathway of homodimeric procaspase-3 is complex, consisting of multiple monomeric intermediates with a slow rate of dimerization. The refolding and unfolding burst phase revealed multiple species formed within milliseconds of folding. Kinetic data support the hypothesis of two native conformations, one of which is enzymatically active. Collectively, these results demonstrate that dimerization is an important aspect in both folding and activation of procaspase-3. Overall, the kinetic folding data for Apaf-1 CARD and procaspase-3 provide an improved picture of the function and regulation of apoptosis. These studies propose new targets for therapeutic design to combat diseases associated with apoptosis.
Meniscus-Directed Assembly of Biologically Active Coatings of Cells, Microparticles, and Nanoparticles
(2009-04-23) Stamm, Lindsey Brooks Jerrim; Orlin Velev, Committee Chair; Anne Lazarides, Committee Member; Jan Genzer, Committee Member; Christine Grant, Committee Member; Carol Hall, Committee Member
Convective assembly principles and techniques were used in two complementary studies for depositing close packed yeast-coated surfaces and gold nanoparticle wires. Convective assembly at high volume fraction was used for the rapid deposition of uniform, close-packed coatings of Saccharomyces cerevisiae onto glass slides. A computational model was developed to calculate the thickness profiles of such coatings for various experimental conditions. Both experimentation and numerical simulations demonstrated that the deposition process is strongly affected by the presence of sedimentation. The deposition device was inclined to increase the uniformity of the coatings by causing the cells to sediment toward the three-phase contact line. In accordance with the simulation, the experiments showed that both increasing the angle of the device and decreasing the angle between the slides increased the uniformity of the deposited coatings. Finally, the Ã¢â‚¬Å“convective-sedimentationÃ¢â‚¬Â assembly method was used to deposit composite coatings of live cells and large latex particles as an example of biologically active composite coatings. These coatings were allowed to proliferate and demonstrate a proof-of-concept of a self-cleaning surface. Two methods were developed for the deposition of micro- and nanoparticles into linear assemblies that could be used in biosensors and biomaterials. In capillary-guided deposition, a capillary is withdrawn across a wettable substrate, resulting in the assembly of a particle line. We characterized the effects of particle concentration and withdrawal speed and correlated them to structure of the deposited assemblies. The particles are assembled into one of three different structures, depending on the particle volume fraction and deposition speed. We demonstrate that the metallic nanoparticle lines are Ohmically conductive. Using wedge-templated deposition, linear assemblies were deposited from sessile droplets on moderately hydrophobic surfaces. The particles convectively assemble at the freely-receding three-phase contact line and are pulled into a line against the wedge. The deposited lines can be long and narrow with a few breaks or significantly wider and shorter but unbroken. These methods could be used for engineered patterning of nanoparticle structures on surfaces.
Single-File and Anomalous Diffusion in Porous Carbons.
(2010-08-04) Moore, Joshua; Keith Gubbins, Committee Chair; Donald Brenner, Committee Member; Carol Hall, Committee Member; Orlin Velev, Committee Member