Crystal structures of two cyclic pseudopentapeptides containing ψ[CH 2 S] and ψ[CH 2 SO] backbone surrogates

The solid state conformations of cyclo[Gly–Proψ[CH 2 S]Gly– D –Phe–Pro] and cyclo[Gly–Proψ[CH 2 –(S)–SO]Gly– D –Phe–Pro] have been characterized by X‐ray diffraction analysis. Crystals of the sulfide trihydrate are orthorhombic, P2 1 2 1 2 1 , with a = 10.156(3) Å, b = 11.704(3) Å, c = 21.913(4) Å, and Z = 4. Crystals of the sulfoxide are monoclinic, P2 1 , with a = 10.662(1) Å, b = 8.552(3) Å, c = 12.947(2) Å, β = 94.28(2), and Z = 2. Unlike their all‐amide parent, which adopts an all‐ trans backbone conformation and a type II β‐turn encompassing Gly‐Pro‐Gly‐ D ‐Phe, both of these peptides contain a cis Gly 1 ‐Pro 2 bond and form a novel turn structure, i.e., a type II′ β‐turn consisting of Gly– D –Phe–Pro–Gly. The turn structure in each of these peptides is stabilized by an intramolecular H bond between the carbonyl oxygen of Gly 1 and the amide proton of D ‐Phe 4 . In the cyclic sulfoxide, the sulfinyl group is not involved in H bonding despite its strong potential as a hydrogen‐bond acceptor. The crystal structure made it possible to establish the absolute configuration of the sulfinyl group in this peptide. The two crystal structures also helped identify a type II′ β‐turn in the DMSO‐d 6 solution conformers of these peptides. © 1993 John Wiley & Sons, Inc.