The molecular structure of Act-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-degrees, 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 studies, including H-1 NOE
experiments, the conformational equilibrium in CDCl3 solution is dominated by the
conformer which is present in the crystal. The chiroptical properties of 1 are also
discussed, in comparison to those of mono- and dithionated models of beta-turns.
