Synthesis, biology, NMR and conformation studies of the topographically constrained δ-opioid selective peptide analogs of [β-iPrPhe³]deltorphin I

Replacement of Phe³ in the endogenous δ-opioid selective peptide deltorphin I with four optically pure stereoisomers of the topographically constrained, highly hydrophobic novel amino acid β-isopropylphenylalanine (β-iPrPhe) produced four pharmacologically different deltorphin I peptidomimetics. Radiolabeled ligand-binding assays and in vitro biological evaluation indicate that the stereoconfiguration of the iPrPhe residue plays a crucial role in determining the binding affinity, bioactivity and selectivity of [β-iPrPhe³]deltorphin I analogs: a (2S,3R) configuration of the iPrPhe³ residue in [β-iPrPhe³]deltorphin I provided the most desirable biological properties with binding affinity (IC₅₀=2 nM), bioassay potency (IC₅₀=1.23 nM in MVD assay) and exceptional selectivity for the δ-opioid receptor over the μ-opioid receptor (30 000). Further conformational studies based on two-dimensional NMR and computer-assisted molecular modeling suggested a model for the possible bioactive conformation in which the Tyr¹ and (2S, 3R)-β-iPrPhe³ residues adopt trans side-chain conformations, and the linear peptide backbone favors a distorted β-turn conformation.