Statistical thermodynamics of helix–coil transitions in biopolymers

Abstract

Helical conformations, such as the α-helix in polypeptides and the double helix in DNA, are common structural elements in biopolymers. As the temperature is raised or the pH is changed to extremes of acidity or alkalinity, the helix becomes disordered into a random coil state. The helix–coil transition has been extensively studied, both experimentally and theoretically, as a model for conformational transitions in biopolymers and as a way to obtain information about the intermolecular forces which stabilize biopolymer structure. We develop three theoretical treatments that describe the helix–coil transition with increasing degrees of detail and rigor: the all-or-none model, the zipper model, which allows initiation of the helix only once along the polymer chain, and the matrix model, which places no restrictions on helix–coil junctions. The matrix model is mathematically similar to the familiar one-dimensional Ising model.