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