Conformational studies of cyclo-gly-pro(S) by X-ray diffraction and proton magnetic resonance

The X-ray crystal structure and the PMR spectrum in ²H₂O and DMSO-d₆ of cyclo-glycyl-4-thiaprolyl has been determined. In the crystal, the diketopiperazine ring of the molecule adopts a boat conformation and the thiazolidine ring an envelope conformation, very similar to the analogous compound cyclo-glycyl-prolyl. Comparison of proton-proton dihedral angles derived from the crystal structure and from vicinal coupling constants in solution indicates that the conformation of cyclo-glycyl-4-thiaprolyl is nearly the same in crystal and in solution in the limits of applied mehods.