(4-Carboxamido)phenylalanine is a surrogate for tyrosine in opioid receptor peptide ligands

(S)-4-(Carboxamido)phenylalanine (Cpa) is examined as a bioisosteric replacement for the terminal tyrosine (Tyr) residue in a variety of known peptide ligands for the μ, δ, and κ opioid receptors. The Cpa-containing peptides, assayed against cloned human opioid receptors, display comparable binding affinity (K_i), and agonist potency (EC ₅₀) to the parent ligands at the three receptors. Cpa analogs of δ selective peptides show an increase in δ selectivity relative to the μ receptor. Cpa is the first example of an amino acid that acts as a surrogate for Tyr in opioid peptide ligands, challenging the long-standing belief that a phenolic residue is required for high affinity binding.