Helix-coil transitions in nonpolar peptides

A Monte Carlo simulation of the folding and unfolding of a two-dimensional model polypeptide, which has alpha-helical motifs, in nonpolar side chains were carried out. The terms that approximate the effect of the hydrophobic interaction in a solvent are phenomenologically included. Only three states were considered for each unit: extended with hydrogen bonds, turns and coils. We investigated the nature of hydration in the assembly process of alpha-helical stretches. The conversion exhibited transitions between three states: random coils (C) at a high temperature, helix bundles (B) at a medium temperature, and alpha-helices (H) without lateral hydrophobic bonds at a low temperature. Small hydrophobic interactions result in only the H-C transition lacking the B state. On the other hand, as the hydrophobic effect strengthens, the B state appears at a medium temperature. The B-C and B-H transitions appear as an all-or-none-type transition and a gradually diffused type transition, respectively. This problem is related to protein folding. In the simulation performed in this study, we discuss the cold denaturation of proteins from the viewpoint of B-H transitions.