Single residue modifications of the delta opioid receptor selective peptide, [d-Pen2,d-Pen5]-enkephalin (DPDPE)

Six analogs of the highly delta opioid receptor selective, conformationally restricted, cyclic peptide d -Pen²,d -Pen⁵]enkephalin, Tyr-d -Pen-Gly-Phe-d -PenOH (DPDPE), were synthesized and evaluated for opioid activity in rat brain receptor binding and mouse vas deferens (MVD) smooth muscle assays. All analogs were single amino acid modifications of DPDPE and employed amino acid substitutions of known effects in linear enkephalin analogs. The effect on binding affinity and MVD potency of each modification within the DPDPE structural framework was consistent with the previous reports on similarly substituted linear analogs. Conformational features of four of the modified DPDPE analogs were examined by ¹H NMR spectroscopy and compared with DPDPE. From these studies it was concluded that the observed pharmacological differences with DPDPE displayed by diallyltyrosine¹-DPDPE (DAT¹]DPDPE) and phenylglycine⁴-DPDPE (Pgl⁴]DPDPE) are due to structural and/or conformational differences localized near the substituted amino acid. The observed enhanced μ receptor binding affinity of the carboxamide terminal DPDPE-NH₂ appears to be founded solely upon electronic differences, the NMR data suggesting indistinguishable conformations. The observation that the α-aminoisobutyric acid substituted analog Aib³]DPDPE displays similar in vitro opioid behavior as DPDPE while apparently assuming a significantly different solution conformation suggests that further detailed conformational analysis of this analog will aid the elucidation of the key structural and conformational features required for action at the δ opioid receptor.