Solid-Phase Synthesis and Opioid Activities of [D-Ala²]Deltorphin II Analogs

Abstract

[D-Ala²]Deltorphin II (DL-II) analogs having various aliphatic amino acids at positions 5 and 6 were synthesized by a solid-phase method and their opioid activites on electrically inducewd guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations were determined. During the synthesis of an analog, [tert-leucine(Tle)^{5, 6}]DL-II, we encountered difficulty in the coupling reaction between Tle⁵ and Tle⁶ with the usual diisopropylcarbodiimide (DIPCDI)-mediated tert-butoxycarbonyl (Boc) strategy, though the other analogs could be successfully synthesized. We found that the fluorenylmethoxycarbonyl (Fmoc)-Tle/DIPCDI/1-hydroxybenztriazole method was very useful for the synthesis of such a peptide having a sterically hindered sequence. Acid hydrolysis studies of the synthetic analogs suggested that the steric hindrance of consecutive aliphatic amino acid sequences depend upon the degree of branching at the β-carbon atom of the amino acids. In the MVD assay, two analogs, [Ala^{5, 6}] and [Tle^{5, 6}]DL-II showed remarkably low potencies while other analogs with Nva^{5, 6}, Nle^{5, 6}, Ile^{5, 6}, Leu^{5, 6} and Mle^{5, 6} substituted for Val^{5, 6}(DL-II) showed comparable or slightly lower potencies than DL-II. In the GPI assay, no remarkable changes in potency were observed between DL-II and this series of analogs. Conformational aspects of synthetic analogs were examined by comparing the circular dichroism spectra.