Helix-inducing alpha-aminoisobutyric acid in opioid mimetic deltorphin C analogues

The achiral symmetric alpha-aminoisobutyric acid (Aib) replaced the critical N-terminal residues of the amphibian skin opioid deltorphin C (H-Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) without detriment to the physicochemical requirements for delta opioid receptor recognition. Substitutions by the alpha,alpha-dialkyl amino acid in place of D-Ala(2) or Phe(3), or both, exhibited high delta receptor affinity (K-i delta = 0.12-3.6 nM) and 5-9-fold greater selectivity (K-i mu/K-i delta = 5000-8500) than the parent compound. This is the first definitive demonstration that the D-chirality of alanine and the aromaticity of phenylalanine are replaceable by an achiral alpha,alpha-dialkylated residue without detrimental effects on ligand binding. Incorporation of the mono-alpha-alkyl amino acid L- or D-Ala at the third position also produced highly selective delta ligands (K-i mu/K-i delta = 2000-3500), albeit with reduced delta affinities (K-i delta = 6-15 nM). Replacement of the anionic residue Asp(4) by Aib yielded an opioid peptide that fit two-site binding models for the delta receptor (eta = 0.763; P < 0.0001) and displayed dual high affinity for both delta and mu receptors, emphasizing the repulsive effect by a negative charge at mu receptor sites and the insignificance of Asp for delta affinity. Molecular dynamics conformational analyses suggested that Aib residues caused distinct changes in deltorphin C secondary structure when substituted for D-Ala(2), Asp(4) and simultaneously D-Ala(2) and Phe(3) but not when substituted for Phe(3). These conformational changes might be critical factors for the proper orientation of reactive constituents of residues in the N-terminal region of deltorphin C. Disparities between binding data and functional bioassays of [Aib(3)] indicated that Phe(3) was required for bioactivity in mouse vas deferens but not for interaction with delta opioid receptors in rat brain membranes.