The mechanism of helical transition of proteins by organic solvents

This paper describes a theory for the mechanism of three-state transition of proteins which is often observed in aqueous organic cosolvent systems, i.e., from the native, via intermediate to helical forms. The first transition, accompanied by changes in the tertiary and/or secondary structures, was explained by larger bindings of the organic solvent molecules to the intermediate than to the native state; the second transition, resulting in changes mainly in the secondary structure, i.e., helical transition, was explained by less hydration sites for the helical state. Computer simulations of the transition were carried out using plausible values for the number of alcohol and water binding sites of proteins as well as for the equilibrium constant of the transitions in the absence of cosolvent. A reasonable agreement with the experimental transitions was observed. The stronger effect of alcohols with longer alkyl chains was explained by their greater binding to nonpolar groups and their larger exclusion from peptide groups.