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

The achiral symmetric R-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 ä opioid receptor recognition. Substitutions by the R,R-dialkyl amino acid in place of D-Ala2 or Phe3, or both, exhibited high ä receptor affinity (Kiä ) 0.12-3.6 nM) and 5-9-fold greater selectivity (Kií/Kiä ) 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 R,R-dialkylated residue without detrimental effects on ligand binding. Incorporation of the mono-R-alkyl amino acid L- or D-Ala at the third position also produced highly selective ä ligands (Kií/Kiä ) 2000- 3500), albeit with reduced ä affinities (Kiä ) 6-15 nM). Replacement of the anionic residue Asp4 by Aib yielded an opioid peptide that fit two-site binding models for the ä receptor (è ) 0.763; P < 0.0001) and displayed dual high affinity for both ä and í receptors, emphasizing the repulsive effect by a negative charge at í receptor sites and the insignificance of Asp for ä affinity. Molecular dynamics conformational analyses suggested that Aib residues caused distinct changes in deltorphin C secondary structure when substituted for D-Ala2, Asp4, and simultaneously D-Ala2 and Phe3 but not when substituted for Phe3. 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 [Aib3] indicated that Phe3 was required for bioactivity in mouse vas deferens but not for interaction with ä opioid receptors in rat brain membrane