Comparative conformational analysis of CCK-B agonist Boc-Trp-Phg-Asp-(1-Nal)-NH2 and CCK1-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 using 1HNMR spectroscopy and restrained molecular dynamics.

The tetrapeptide Boc-Trp-Phg-Asp-(1-Nal)-NH2 is a potent CCK-B agonist. Interestingly, bis-methylation of the C-terminal carboxamide group of this compound leads to Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 which behaves as a CCK-B antagonist in electrophysiological studies on hippocampal neurones (Corringer et al., 1993). In order to ascertain whether bismethylation of the terminal carboxamide group has an influence on the conformational preferences of the peptide, we have undertaken a comparative conformational analysis of the two tetrapeptides by the combined use of 2D NMR spectroscopy and restrained molecular dynamics. The solution conformation of the two peptides were examined by 1H NMR in a d6-DMSO/H2O (80:20) mixture. 1H-1H distance constraints, derived from 2D NOESY and ROESY experiments, were used as inputs for subsequent restrained molecular dynamics simulations. Comparison of the NMR and molecular modeling data indicates different conformational preferences for these two peptides. Interestingly, the aromatic side chains of the CCK-B antagonist Boc-Trp-Phg-Asp-(1-Nal)-N(Me)2 in its preferential conformation, overlap their corresponding moieties in the two non peptide CCK-B antagonists L-362,260 and LY-288,513. The differences in conformational behaviour of the studied tetrapeptides could, at least in part, account for their opposite agonist/antagonist profile, a findings which could serve for the design of new conformationally restricted CCK-B analogs.