Eiweiss-und Lederforschung, Munich, Germanq synopsis
Trifluoroet.hatio1, benxyl alcohol, and n-butanol bind to the peptide and acetyl CO groups of po1y-O-acetyl-Lhydroxyproline in dichloromethane via hydrogen bonds. The binding affinity decreases from trifluoroethanol to n-butanol. For t.he acetyl 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 times when it is in the helix I1 conformation (all peptide bonds trans) than when it assumes the helix I conformation (all peptide bonds cis). This preference is explained by the higher ac-cessibi1it.y of t,he pept.ide CO groups in the I1 helix. The small additional energy which results from the preferential binding is sufficient to induce a complete I -* I1 transition because of the very high cooperativity of the system. The quantitative dependence of the equilibrium constant. s for the propagation step of the transition on solvent composihn (ratrio of triflrion)et.hanol or benzyl alcohol to n-but,anol) is derived from the binding data. It agrees satisfact,orily with the empirical relat,ion obtained from a best tit to transition curves of Gamer et. al.
I1 conversion of poly-Lproline is therefore an example of a conformat.iona1 transition whose solvent dependence can be explained by a binding mechanism.