The dermorphin-derived cyclic tetrapeptide analogues H-Tyr-C[D-Cys-Phe-Cys]NH2 and H-Tyr-C[D-Cys-Phe-D-Cys]NH2 are opioid agonists at the µ and δ receptor. To enhance the metabolic stability of these peptides, we replaced the disulfide bridge with a bis-methylene moiety. This was achieved by solid-phase synthesis of the linear precursor peptide containing allylglycine residues in place of the Cys residues, followed by ring-closing metathesis. In the case of the peptide with L-configuration in the 4-position both the cis and the trans isomer of the resulting olefinic peptides were formed, whereas the cis isomer only was obtained with the peptide having the D-configuration in position 4. Catalytic hydrogenation yielded the saturated -CH2-CH2- bridged peptides. In comparison with the cystine-containing parent peptides, all olefinic peptides showed significantly reduced µ and δ agonist potencies in the guinea pig ileum and mouse vas deferens assays. The -CH2-CH2-bridged peptide with L-configuration in the 4-position was equipotent with its cystine-containing parent in both assays, whereas the bis-methylene analogue with D-configuration in position 4 was 10-27-fold less potent compared to its parent. The effect of the disulfide replacements with the -CH=CH- and ‑CH2-CH2- moieties on the conformational behavior of these peptides was examined by theoretical conformational analysis which provided plausible explanations in terms of structural parameters for the observed changes in opioid activity.
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