Synthesis and biological activity of ΔPhe4‐enkephalins*

The ΔPhe4‐enkephalins have been synthesized and examined in an in vitro receptor binding assay and an in vivo tail flick analgesia test. The ΔPhe4 residue was derived from Boc‐Gly‐Phe(β‐OH)‐OH by spontaneous dehydration and azlactonization. The dipeptide azlactone was coupled directly with H‐Leu‐OBzl to yield a tripeptide which was converted into the pentapeptides after stepwise coupling with two amino acids using the water soluble EDC‐HOBt method. Dehydroenkephalins were liberated with hydrogen fluoride in the presence of anisole. In the radioligand binding assay which did not contain an enzyme inhibitor [d‐Ala2, ΔPhe4, Leu5] enkephalin was almost twice as active as saturated [d‐Ala2, d‐Leu5] ‐enkephalin. The ΔPhe4‐enkephalins exhibited a considerably diminished activity as compared with the saturated peptide in the in vivo analgesic assay. These results are discussed with regard to the enzyme stability and receptor preference of dehydroenkephalins.