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Solution Properties of Synthetic Polypeptides. XI. Solvent Effect on Helix–Coil Transition in Polypeptides

A number of synthetic polypeptides are known to undergo “inverse” thermal helix–coil transitions in solvent mixtures containing an “active” solvent such as dichloroacetic acid (DCA). A statistical mechanical theory of the effects of the active component on the transition is developed here on the assumption that the active solvent exists in the form of a dimer and that the dimerized molecules react with pairs of CO and NH groups of randomly coiled peptide units. Expressions are derived for the equilibrium constant, u, of helix formation and the cooperativity parameter, σ, and they are used to analyze data obtained for poly(γ-benzyl-L-glutamate) and poly(β-benzyl-L-aspartate) in mixtures of DCA and ethylene dichloride as functions of temperature and solvent composition. The values of the transition parameters so determined make it possible to understand not only why and how the observed values of transition enthalphy ΔH and cooperativity parameter σ depend on temperature and solvent composition, but also the general features of inverse transitions in quantitative terms. An important conclusion is that σ is profoundly affected by the binding of the active solvent on the polypeptide chain.