A theoretical conformational analysis (molecular mechanics study) of nine cyclic tetrapeptides, structurally related to the highly mu-receptor-selective dermorphin analogue H-Tyr-D-Orn-Phe-Asp-NH2, was performed. These compounds display considerable diversity in their mu-receptor affinity and selectivity. A systematic search and subsequent energy minimization in absence of the exocyclic Tyr1 residue and Phe3 side chain revealed the constrained nature of the 11-13-membered ring structures contained in these analogues. No more than four low-energy conformers (within 2 kcal/mol of the lowest energy conformation) were found in each case. After attachment of the Tyr1 moiety and Phe3 side chain to the "bare" low-energy ring structures, a systematic search and energy minimization of these exocyclic moieties resulted in a limited number of low-energy conformational families for all compounds. Five analogues with high mu-receptor affinity--H-Tyr-D-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-Phe-D-Asp-NH2, H-Tyr-D-Asp-Phe-Orn-NH2, H-Tyr-D-Asp-Phe-A2bu-NH2 (A2 bu: alpha, gamma-diaminobutyric acid) and H-Tyr-D-Cys-Phe-Cys-NH2--all showed a tilted stacking interaction between the Tyr1 and Phe3 aromatic rings in the lowest or second lowest energy conformation found. The same kind of stacking was not possible in low-energy conformers of the four analogues with poor affinity for the mu-receptor [H-Tyr-L-Orn-Phe-Asp-NH2, H-Tyr-D-Orn-D-Phe-Asp-NH2, H-Tyr-D-Orn-Phe(NMe)-Asp-NH2 [Phe(NMe): N alpha-methylphenylalanine], and H-Tyr-D-Orn-Phg-Asp-NH2 (Phg: phenylglycine)].(ABSTRACT TRUNCATED AT 250 WORDS)