STABILITY OF ALPHA-HELICES

This chapter focuses on the mechanism of helix formation in an isolated peptide and the factors that determine the stability of a peptide helix. Helix propensities are considered together with N-cap and C-cap propensities, because measurement of helix propensities requires knowing values of the N-cap and C-cap propensities, and vice versa. The chapter considers side-chain interactions: these include both the interaction of a charged side chain with the helix macrodipole and specific interactions between a particular pair of side chains, such as ion pair and H-bond interactions. Measurement of these interactions is of interest for two reasons: their values are needed to relate the stability of a peptide helix to its amino acid composition and sequence; and peptide helices provide one of the best systems, and probably the most sensitive system, for quantifying the energetics of side-chain interactions. It also considers briefly the present status of the Chou-Fasman hypothesis and the relation between the mechanism of α-helix formation in peptides and proteins. It is necessary to use helix-coil transition theory to understand the populated intermediates and to analyze the energetics of helix formation. The two closely related theories of α-helix formation are the Zimm-Bragg theory and the Lifson-Roig theory. © 1995 Academic Press Inc.