Conformational analysis of potent and very selective delta opioid dipeptide antagonists

The 6 selectivity and antagonism of peptides containing L-tetrahydro-3-isoquinoline carboxylic acid (Tic) in second position can be attributed mainly to the Tyr-Tic unit. These properties can be further enhanced by substituting Tyr t with 2,6-dimethyi-L-tyrosyl (Dmt). Dmt-Tic-NH2, Dmt-Tic-OH, Dmt-Tic-Ala-NH2 and Dmt-Tic-Ala-OH are all more active and/ or selective than the corresponding [Tyrt]-parent peptides. In fact the selectivities of Dmt-Tic-OH and Dmt-Tic-Ala-OH are the highest ever recorded for opioid molecules, tH NMR spectra in a DMSOIwater mixture at 278 K reveal the presence of two similar conformers, characterised by a cis or trans Dmt-Tic bond, in all four peptides. A detailed conformational analysis in solution of Dmt-Tic-NH 2 shows that these conformers have a shape very similar to that of the bioactive conformation of Tyr-Tic-NH~ and to that of naltrindole.