Conformational analysis of cyclic partially modified retro-inverso enkephalin analogues by proton NMR

The conformations of five cyclic retro-inverso enkephalin analogues have been probed by proton NMR. After assignment of peaks, intramolecularly hydrogen-bonded amide protons were detected by temperature perturbation. Carbonyl hydrogen-bond acceptors were surmised from the computer simulations of minimum energy conformations of Hassan and Goodman [Hassan, M., & Goodman, M. (1986) Biochemistry (preceding paper in this issue)]. Hydrogen bonds were identified in dimethyl-d6 sulfoxide solutions and monitored as H2O was added. One hydrogen bond was observed in each of the retro-inverso-modified enkephalin analogues although in the parent analogue H-Tyr-c-(D-A2bu-Gly-Phe-Leu) two were detected. The change in solvent altered the conformations of two of the analogues.