Expansion
and Internal Friction in Unfolded Protein
Chain
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Abstract
Similarities
in global properties of homopolymers and unfolded
proteins provide approaches to mechanistic description of protein
folding. Here, hydrodynamic properties and relaxation rates of the
unfolded state of carbonmonoxide-liganded cytochrome <i>c</i> (cyt-CO) have been measured using nuclear magnetic resonance and
laser photolysis methods. Hydrodynamic radius of the unfolded chain
gradually increases as the solvent turns increasingly better, consistent
with theory. Curiously, however, the rate of intrachain contact formation
also increases with an increasing denaturant concentration, which,
by Szabo, Schulten, and Schulten theory for the rate of intramolecular
contact formation in a Gaussian polymer, indicates growing intramolecular
diffusion. It is argued that diminishing nonbonded atom interactions
with increasing denaturant reduces internal friction and, thus, increases
the rate of polypeptide relaxation. Qualitative scaling of the extent
of unfolding with nonbonded repulsions allows for description of internal
friction by a phenomenological model. The degree of nonbonded atom
interactions largely determines the extent of internal friction