A series of diastereoisomers of endomorphin-1 (EM1, Tyr(1)-Pro(2)-Trp(3)-Phe(4)-NH(2)) have been synthesized and their potency measured using the guinea pig ileum assay. [D-Phe(4)]EM1 possessed 1/10 the potency of EM1, while potencies of [D-Tyr(1)]EM1 and [D-Trp(3)]EM1 were 50- and 100-fold lower, respectively. Drastic loss of activity occurred in the [D-Pro(2)]EM1 peptide. The structural determinants for the inactivity and reduced potency of the diastereoisomers were investigated using NMR spectroscopy and conformational analysis. Simulations of trans-[D-Pro(2)]EM1 using NOE-derived distance constraints afforded well-defined structures in which Tyr and Trp side chains stack against the proline ring. The inactivity of [D-Pro(2)]EM1 was explained by structural comparison with EM1 (, FEBS Lett. 439:13-20). The two peptides showed an opposite orientation of the Trp(3) residue with respect to Tyr(1), thus suggesting a role of Pro(2) as a stereochemical spacer in orienting Trp(3) and Phe(4) toward regions suitable for mu-receptor interaction. The agonist activity of [D-Tyr(1)]EM1 and [D-Trp(3)]EM1 was attributed to their ability to adopt low-energy conformations that mimic those of EM1. The requirements for mu-receptor activation were examined further by comparing EM1 with the mu-peptide [D-Ala(2), MePhe(4), Gly-ol]-enkephalin (DAMGO). Conformations of DAMGO with a Tyr(1)-MePhe(4) phenyl ring separation of approximately 12 A were found to mimic Tyr(1)-Phe(4) of EM1, thus suggesting overlapping binding modes between these two peptides.