Probing opioid receptor–ligand interactions by employment of indolizidin‐9‐one amino acid as a constrained Gly2‐Gly3 surrogate in a leucine‐enkephalin mimic

Abstract: The relationship between the conformation and biological activity of Leu‐enkephalin was studied using (2S,6R,8S)‐9‐oxo‐8‐N‐(Boc)amino‐1‐azabicyclo[4.3.0]nonane‐2‐carboxylic acid [(2S,6R,8S)‐ 1 , I⁹AA] as a constrained Gly²‐Gly³ dipeptide surrogate. [I⁹AA]^2,3‐Leu‐enkephalin 12 was assembled using solid‐phase peptide synthesis on Merrifield resin with TBTU as the coupling reagent. The in vitro assays indicated that [I⁹AA]^2,3‐Leu‐enkephalin 12 exhibited affinities for the µ‐ and δ‐opioid receptors that were three orders of magnitude lower than that of Leu‐enkephalin, as well as partial agonist character for both receptors. In in vivo assays for spinal analgesia, the indolizidinone analog 12 showed significantly enhanced duration of action, indicating an increased metabolic stability. Conformational analysis was performed using NMR and CD spectroscopy. The amide temperature coefficients and ³J_NH‐CαH coupling constants for 12 could not support a hydrogen‐bonded β‐turn structure; however, its CD spectrum indicated a turn conformation. Incorporation of indolizidinone amino acid 1 into Leu‐enkephalin thus provided additional support for the importance of a turn conformation for the biological activity of the native peptide.