REVERSE TURN CONFORMATION OF N-THIOACETYL THIOPROLYL GLYCINE N'-METHYLAMIDE IN THE CRYSTAL AND IN SOLUTION

The molecular structure of Prot-Gly-NHCH3/AcPSI(CSNH)-ProPSI(CSNH)-Gly -NHCH3/ (1) is reported. In the crystal 1 adopts a conformation with a Z(trans) tertiary (thioacetyl pyrrolidine) thioamide bond and torsion angles (PHI2 = -73.2-degrees, PHI2 = 151.9-degrees, PHI3 = 83.5-degre es, PHI3 = 5.5-degrees) which are rather close to the values of a type II beta-turn. The sulfur atom of the thioacetyl group is the acceptor of a 1<--4 intramolecular H-bond. Based on 400 MHz H-1 and C-13 NMR s tudies, including H-1 NOE experiments, the conformational equilibrium in CDCl3 solution is dominated by the conformer which is present in th e crystal. The chiroptical properties of 1 are also discussed, in comp arison to those of mono- and dithionated models of beta-turns.