To the problem of biologically active conformation of enkephalin.

A semi-rigid structural analog of [Leu5] enkephalin, possessing the azo-bridge between Tyr1 and Phe4 residues, was synthesized, along with two other linear enkephalin analogs: [4'-amino Phe4] enkephalin and [4'/hydroxyphenyl/-azo Phe4] enkephalin. The results of the determination of the analgesic activity of the synthesized compounds suggest that the biologically active conformation of the enkephalin molecule should be such that both aromatic rings, Tyr1 and Phe4, are situated in close proximity.