Simple model for the helix-random coil transition in polypeptides.

Abstract

A simple model for the helix—random coil transition in polypeptides is constructed. Only the limiting case of a sharp transition is considered; however, the model could easily be extended to treat the actual case of a diffuse transition. The formulation of the problem given by Lifson and Roig is used; the main innovation is the introduction of a model interaction with a small number of parameters. For transitions occurring near room temperature, the transition temperature and thermodynamic functions are given in terms of only two parameters, each of which has a simple physical interpretation. The parameters are treated as temperature independent. The model describes in a natural way systems in which both normal and inverted transitions occur. In a normal transition, the helix is the phase stable below the transition temperature; in an inverted transition, it is the phase stable above the transition temperature. In the latter case, the model also describes a reversion of the helix to coil form at elevated temperatures. On the basis of experimental data, the parameters of the theory and the thermodynamic functions are calculated for two systems, one having a normal, and one an inverted transition. The interpretation of the theory in terms of the nature of the polymer—solvent interaction is discussed. The relationship of our work to that of Gibbs and DiMarzio is discussed.