Dehydro-enkephalins. III. Synthesis and biological activity of [delta Ala2, Leu5]-enkephalin

[delta Ala2, Leu5]-enkephalin has been prepared and shown to be more active than the parent saturated enkephalin in a binding assay using rat brain membranes and [3H]dihydromorphine as a tracer. In a comparison of potencies against [3H]dihydromorphine and [3H]-[D-Ala2, D-Leu5]-enkephalin as tracers, [delta Ala2, Leu5]-enkephalin showed preference for micro opiate receptors, possibly due to the hydrophobicity of the delta Ala2 residue. A synthetic tetrapeptide enkephalin [delta Ala2]-desLeu5-enkephalin had weak activity and high selectivity for the micro receptors. O-Acylation of a serine residue in the peptide was achieved by coupling between the peptide and a carboxylic acid using DCC and a catalytic amount of 4-dimethylaminopyridine.