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Self-assembly, stability studies and computer modeling of ultra thin protein-polyelectrolyte multilayer films

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Keywords
  • Chemistry
  • Analytical|Chemistry
  • Polymer
Disciplines
  • Biology
  • Chemistry

Abstract

This thesis examines stability, protein structure, orientation and utility of ultrathin layer-by-layer self-assembled protein-polyelectrolyte films using spectroscopic as well as electrochemical techniques. Computer simulations were used to understand the physical interactions driving the multilayer film formation and to study their local structure. ^ Cross-linking of myoglobin promoted by 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide within films of polystyrene sulfonate after layer-by-layer self-assembly provided remarkable stabilization. Crosslinking greatly improved adhesion of the films to fused silica slides and allowed extensive optical studies over a wide pH range. Circular dichroism and visible absorbance spectra showed that Mb retained its native conformation when films were placed in solutions of pH as low as 2 and up to pH 11. Linear dichroism revealed an average orientation of the Mb iron heme cofactors of 58° to the film normal. High concentrations of urea did denature the protein in the films, however. At pH 1, Mb in solution is fully unfolded, but retained considerable α-helical content in the crosslinked films. Both the polyion film environment and crosslinking seem to play roles in stabilizing protein secondary structure and function at low pH. Crosslinked Mb-polyion films on pyrolytic graphite electrodes were used in strongly acidic solutions for the electrochemical catalytic reduction of trichloracetic acid, hydrogen peroxide, and oxygen. The pH-dependent catalytic reduction of trichloracetic acid was faster in 0.1 M HCl than in the medium pH range. ^ Molecular dynamics simulations of polyelectrolyte multilayering on a charged spherical particle revealed that the sequential adsorption of oppositely charged flexible polyelectrolytes proceeds with surface charge reversal, and highlighted electrostatic interactions as the major driving force of layer deposition. Far from being completely immobilized, multilayers feature a constant surge of chain intermixing during the deposition process, consistent with experimental observations of extensive interlayer mixing in these films. The formation of multilayers as well as the extent of layer intermixing depends on the degree of polymerization of the polyelectrolyte chains and the fraction of charge on its backbone. The presence of ionic pairs between oppositely charged macromolecules forming the layers seems to play an important role in stabilizing the multilayer film. ^ The ability to control nanoscale structure and order of multilayer films is essential for surface patterning and templating. The last part extends the molecular dynamics simulations of multilayer assembly to simulate the alternate adsorption of ‘protein-like’ colloidal particles with oppositely charged polyelectrolytes onto a solid planar surface. Layer-by-layer assembly of structurally rigid colloidal particles and polyelectrolytes was performed on a solid planar surface formed by hexagonally packed particles. Multilayer build-up was achieved in the case of fully charged colloidal particles. However, spherical-shaped model protein constructed by utilizing the charged residues of lysozyme (PDB structure) at pH 7 did not yield multilayer formation indicating a necessity to decrease the pH in order to increase the net charge on the protein. ^

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