Statistical Mechanics of Helix-Coil Transitions in Biological Macromolecules

The polypeptide backbone, common to the macromolecular chains of proteins, represents a highly co-operative system which is essentially one dimensional and, therefore, tractable theoretically. The complete partition function for such a system can be evaluated with the aid of either the method of Lagrangian multipliers or the method of steepest descents. When this is done a transition from alpha helix to random coil is obtained as a result. When the effects of solvation are included, the transition may or may not be inverted depending upon the nature of the solvent. The quantitative theoretical results for dilute solutions are in agreement with the available experimental data on the reversible ``denaturation'' of synthetic polypeptides. The separation of the two strands of the double-stranded helix of the genetic material, desoxyribonucleic acid, can be treated with the same theoretical method.