A series of phenylalanine homologs with elongated phenylalkyl side chains (R=–(CH 2) nC 6 H 5, n= 1–4) have been incorporated into opioid peptide morphiceptin at position 4 in order to elucidate a role of the amino acid residue in the molecular mechanism of receptor activation. The receptor specificity and selectivity of peptides synthesized were examined in the radio-ligand receptor binding assays using tritiated DAGO-and DADLE-enkephalins.[ ...