CRYSTAL-STRUCTURE AND NMR CONFORMATION OF A CYCLIC PSEUDOTETRAPEPTIDECONTAINING URETHANE BACKBONE LINKAGES

Urethane bonds, derived from the hydroxyl group of the tyrosine side c hain, have been investigated as a new type of amide bond mimetic in th e design of pseudopeptides. The structure of a representative cyclic p seudotetrapeptide that consists of an - Ala - Tyr(urethane) Ala - Tyr( urethane) sequence fused into a rigid ring has been studied in the sol id state by x-ray crystallography and in solution by two-dimensional n mr techniques. The cyclic pseudotetrapeptide has an oblong shape. The backbone urethane bonds assume a trans-trans conformation. The carbony l groups in the ring have an alternating pattern of down, up, down, up with respect to the average ring plane. Solution nmr studies give obs erved nuclear Overhauser effects and coupling constants largely in agr eement with the crystal structure. However, in solution the observed s tructure is likely to be conformationally averaged, and in the average d structure, the urethane bond is perpendicular to the plane of the ar omatic ring of the tyrosine, while in the crystal it is close to this plane. These differences may be explained by intermolecular hydrogen-b onding interactions. Four aspects of the conformation of the cyclic ps eudotetrapeptide were investigated in detail: the tyrosine residue wit h the attached side-chain urethane bond (the tyrosine-urethane unit), the conformation of the two urethane backbone linkages, the conformati on of the two conventional peptide bonds within this unusual ring stru cture, and the tight turns within the cyclic pseudotetrapeptide. The c onformation of the tight turns present in the cyclic pseudotetrapeptid e is very similar to that of a beta-bend of type II. Intermolecular hy drogen bonding, joining adjacent layers of the cyclic pseudotetrapepti de in the solid state, resemble a parallel beta-pleated sheet. The pre sence of these structural motifs in the cyclic pseudotetrapeptide indi cates that the tyrosine urethane unit may find applications in peptide and protein engineering. (C) 1994 John Wiley & Sons, Inc.