Major objectives are to obtain a physical understanding of the forces stabilizing two-chain, alpha-helical coiled coils and of the kinetics and mechanism by which they form from separated, unfolded chains. Such understanding will be applicable not only to coiled coils themselves, an important structural class of protein found in muscle, cellular frameworks, bacterial cell walls, and DNA-binding proteins, but also to the more complex problem of folding in globular proteins. Coiled coils are chosen for study, so as to maximize interpretability of results. To that end, particular use is to be made of tropomyosin; relatively long, isolated segments of tropomyosin; short, synthetic peptides of sequences chosen from particular regions of the tropomyosin sequence; and polymeric, synthetic peptides with a repeating heptet amino acid sequence designed to mimic the sequence of coiled coils and made by bacterial expression of engineered genes. Physical techniques, particularly static and stopped-flow circular dichroism, high resolution 2-dimensional NMR, and solid-state NMR will be used to define thermodynamic and kinetic characteristics of coiled-coil folding and unfolding. These techniques in conjunction with theory will be used to determine such quantities as local helix-helix interaction free energies, homodimer vs. heterodimer preferences, the nature of the molecular conformational population, and the rate constants and activation parameters for chain exchange and chain reassociation.
