Peptide conformations in alcohol and water: Analyses by the reference interaction site model theory

It is experimentally known that alcohol induces peptides to form a-helix structures much more than water. Though the a-helix structure formed is independent of the alcohol species, degree of the induction increases as bulkiness of the hydrocarbon group in an alcohol molecule increases. In this article we investigate conformations of peptides (Met-enkephalin and the C-peptide fragment of ribonuclease A) in methanol, ethanol, and water using the reference interaction site model theory. Molecular models are employed fur the solvents. Our theoretical results show the following. Alcohol indeed facilitates peptide molecules to form the secondary structures with intramolecular hydrogen bonds such as the a-helix. In alcohol a solvophobic atom of a peptide is less solvophobic than in water while a solvophilic atom is less solvophilic. The solvation free energy in alcohol thus becomes considerably less variable against conformational changes than in water, with the result that the conformational stability in alcohol is governed by the conformational energy. The peptide molecule tends to take a conformation with the lowest conformational energy such as the a-helix, which is independent of the alcohol species. Moreover, these trends are enhanced as bulkiness of the hydrocarbon group in an alcohol molecule increases. In the text, the microscopic origin of the differences between alcohol and water in solvation of peptide molecules, which cannot be obtained by analyses treating the solvent as a dielectric continuum, is discussed in detail.