Solution conformational study of Scyliorhinin I analogues with conformational constraints by two-dimensional NMR and theoretical conformational analysis

Two analogues of Scyliorhinin I (Scyl), a tachykinin with N-MeLeu in positi on 8 and a 1,5-disubstituted tetrazole ring between positions 7 and 8, intr oduced in order to generate local conformational constraints, were synthesi zed using the solid-phase method. Conformational studies in water and DMSO- d(6) were performed on these peptides using a combination of the two-dimens ional NMR technique and theoretical conformational analysis. The algorithm of conformational search consisted of the following three stages: (i) exten sive global conformational analysis in order to find all low-energy conform ations; (ii) calculation of the NOE effects and vicinal coupling constants for each of the low energy conformations; (iii) determining the statistical weights of these conformations by means of a nonlinear least-squares proce dure, in order to obtain the best fit of the averaged simulated spectrum to the experimental one. In both solvents the three-dimensional structure of the analogues studied can be interpreted only in terms of an ensemble of mu ltiple conformations. For [MeLeu(8)]Scyl, the C-terminal 6-10 fragment adop ts more rigid structure than the N-terminal one. In the case of the analogu e with the tetrazole ring in DMSO-d6 the three-diemnsional structure is cha racterized by two dominant conformers with similar geometry of their backbo nes. They superimpose especially well (RMSD = 0.28 Angstrom) in the 6-9 fra gments. All conformers calculated in both solvents superimpose in their C-t erminal fragments much better than those of the first analogue. The results obtained indicate that the introduction of the tetrazole ring into the Scy l molecule rigidifies its structure significantly more than that of MeLeu.