| Abstract: | Trifluoroethanol, benzyl alcohol, and n-butanol bind to the peptide and acelyl CO groups of poly-O-acetyl-L -hydroxyproline in dichloromethane via hydrogen bonds. The binding aflinity decreases from trifhioroelhanol to n-buitanol. For the acelyl CO groups the binding does not depend on the conformation of the polymer but for the peptide CO groups the binding constants are larger by a factor of two to five time when it is in the helix II conformation (all peptide bonds trans) than when it assumes the helix I conformation (all peptide bonds cis). This preference is explained by the higher accessibility of the peptide CO groups in the II helix. The small additional energy which results from the preferential binding is sufficient, to induce a complete I → II transition because of the very high cooperativily of the system. The quantitative dependence of the equilibrium constant s for the propagation step of the transition on solvent composition (ratio of trifluoroethanol or benzyl alcohol to n-butanol) is derived from the binding data. It agrees satisfactorily with the empirical relation obtained from a best fit to transition curves of Ganseret al. The I ? II conversion of poly-L -proline is therefore an example of a conformational transition whose solvent dependence can be explained by a binding mechanism. |
