Synthetic enkephalins. Addicting properties and conformational studies in solution.

The addicting properties of [Leu5]enkephalin in mice are conserved in the LSer3 analogue and lost both in the L-Ser2 analogue and in all the L-Cha4 derivatives of the above peptides. Fluorescence measurements in water show the presence of hydrogen-bonded tyrosul OH groups in [Leu5]-enkephalin and in its LSer2 analogue. The Phe4/Cha replacements do not influence these equilibria, but they affect the near u.v. dichroism of the hydrogen bonded tyrosyl residues. In the peptide absorption region in water solution, only [Leu5]-enkephalin and its cyclohexylalnyl derivative show a positive dichroism towards high frequencies, which is maintained in 8 M ura. No clear relation is found between conformation(s) in solution and biological activity. A II' beta-turn, with residues in positions 2 and 3 at the corners, is suggested for the conformation of enkephalin bound to the receptors involved in the bioassay here used