PROTON NMR CONFORMATIONAL-ANALYSIS OF CYCLIC BETA-CASOMORPHIN ANALOGSOF THE TYPE TYR-CYCLO[-N-OMEGA-D-ORN-XAA-YAA-GLY-]

A conformational study of the cyclic beta-casomorphin-5 analogues H-Ty r-cyclo[-D-Orn-2-Nal-Pro-Gly-] (1) (mu-selective agonist; 2-Nal = 2-na phthylalanine), H-Tyr-cyclo[-D-Orn-2-Nal-D-Pro-Gly-] (2) (mixed mu ago nist/delta antagonist) and H-Tyr-cyclo[-D-Orn-Phe-D-Pro-Gly-] (3) (hig hly potent mu and delta agonist) has been carried out using H-1 NMR sp ectroscopy. A complete assignment of the proton resonances of the thre e pentapeptides has been achieved. Compound 1 was shown to exist in tw o conformations, a major one (90%) characterized by a cis amide bond b etween 2-Nal(3) and Pro(4), and a minor one (10%) showing cis amide bo nds both between D-Orn(2) and 2-Nal(3) and between 2-Nal(3) and Pro(4) Peptides 2 and 3 each showed only one conformer with all-trans peptid e bonds in both cases. Temperature dependence studies of the amide pro ton chemical shifts indicated the existence of several intramolecular hydrogen bonds in the case of compounds 2 and 3 but not in the case of peptide 1. The backbone conformations of 2 and 3 were found to be sim ilar, both being characterized by two consecutive gamma turns around t he D-Pro(4) and D-Orn(2) residues, respectively, and by a D-Orn(2)-CO< --HNdelta-D-Orn(2) hydrogen bond. Altogether, the overall backbone con formation and the preferred side chain conformation were found to be r oughly similar for the three title peptides. For all three compounds a close proximity between the aromatic moiety of the 3-position residue (2-Nal or Phe) and the D(or L)-Pro(4) residue was established on the basis of ROESY experiments. The examination of low energy conformation s obtained in molecular modelling studies by taking into account the v arious experimentally found NMR parameters (NOEs, vicinal H,H coupling constants, torsion angles, H-bonds) led to proposals of the solution conformation for each peptide. These conformations are in close agreem ent with a pharmacophore model for mu opioid receptor binding compound s.