Reconciling Observations of Sequence-specific Conformational Propensities with the Generic Polymeric Behavior of Denatured Proteins

Radii of gyration of denatured proteins vary with chain length and are insensitive to details of amino acid sequence. Observations of sequence independence in polymeric properties conflict with results from spectroscopic experiments, which suggest the presence of sequence-specific residual structure in denatured states. Can we reconcile the two apparently conflicting sets of observations? To answer this question, we need knowledge of the ensemble of conformations accessible to proteins in good solvents. The excluded-volume limit provides an ideal mimic of polymers in good solvents. Therefore, we attempt to solve the "reconciliation problem" by simulating conformational ensembles accessible to peptides and proteins in the excluded-volume limit. Analysis of these ensembles for a variety of polypeptide sequences leads to results that are consistent with experimental observations of sequence-specific conformational preferences in short peptides and the scaling behavior of polymeric quantities for denatured proteins. Reconciliation in the excluded-volume limit comes about due to a tug of war between two factors, namely, minimization of steric overlap and the competing effects of conformational entropy. Minimization of steric overlap promotes chain stretching and leads to experimentally observed sequence-dependent preferences for locally extended segments such as polyproline II helices, beta-strands, and very short stretches of alpha-helix. Conformational entropy opposes chain stretching, and the calculated persistence length for sequence-dependent conformational preferences is less than five amino acids. This estimate does not vary with amino acid sequence. The short persistence lengths lead directly to experimental observations of generic sequence-independent behavior of radii of gyration for denatured proteins.