Solvent-Dependent Conformations of Cyclic Tetrapeptide. II

The solvent-dependent conformations of cyclo(–δ-Ava–L-Pro–)2 were characterized by means of the 1H and 13H NMR, CD, and IR spectra. CD results in CH2Cl2, CH3CN, CH3OH, and H2O indicated that the change in polarity of the solvent induces a reversible change in the conformation of cyclo(–δ-Ava–L-Pro–)2, revealed by the decrease of the negative peak near to 230 nm and then by the gradual appearance of a negative trough near to 210 nm. The NMR data in CD3Cl, CD2Cl2, CD3CN, and CD3OH confirmed that cyclo(–δ-Ava–L-Pro–)2 has a C2 symmetric conformation consisting of all-trans peptide-bond backbones in all solvents, and that the change in polarity of the solvent induces a difference in the rotational states of the Pro αC–C=O single bond.