ABSTRACT

This chapter applies the partition function formalism to analyze the multistate conformational transitions of linear polymers in general, and the polypeptide α-helix-to-coil transition in particular. This α-helix–coil transition shows positive cooperativity, where structure formation within each repeating unit (individual residues) promotes structure formation in neighboring subunits. One type of protein conformational transition involving an extended structure is α-helix melting in peptides. The α-helix is a common element of protein structure, and was proposed from simple bonding considerations by Pauling before high-resolution protein structures had been determined. The α-helix is a common secondary structure element in folded, globular proteins. In most cases, these helices are stabilized by contacts with other parts of the proteins, since when these sequences are excised, they typically lose much of their helical structure. The zipper model is useful for analyzing the helix–coil transition of homopolymers with high cooperativity, it not applicable to heteropolymers, and gives a poor approximation when cooperativity is modest.