A model for the delta-receptor-bound conformation of enkephalin.

Sets of low-energy structures were determined by energy calculations for two cyclic analogues of enkephalin (Ek), [D-Pen2, D-Pen5]-Ek and [D-Pen2, L-Pen5]-Ek, possessing the highest specificity towards delta-opioid receptors. Comparison of mutual spatial orientations of the alpha-amino group and aromatic moieties of the Tyr and Phe residues permitted one to suggest a model for the delta-receptor-bound conformation of enkephalin-related peptides. The model involves a pronounced gamma-like turn of the peptide backbone centred on the Gly3 residue.