Cyclic hexapeptides related to somatostatin. Conformational analysis employing 1H-NMR and molecular dynamics.

We report the conformational analysis of a series of cyclic hexapeptides related to the hormone somatostatin utilizing 1H NMR spectroscopy and NOE restrained molecular dynamics. The conformational preferences and results from biological analysis of these analogs (previous paper) allow for refinement of the current understanding of the structure-activity relationship of somatostatin. For most of the molecules examined, a beta II' turn about the D-tryptophan-lysine residues, postulated to be required for biological activity, was present. From the NOE restrained molecular dynamics, it can be seen that the turn structure is important for the maintenance of the proper orientation of the side chains of the adjacent phenylalanine, tryptophan and lysine. The biologically active analogs have the side chains of lysine and D-tryptophan extended away from the 18-membered ring in close proximity to each other for a significant portion of the dynamic simulations. Although other conformations are accessible and monitored during the simulations, we believe this is important for biological recognition. The absence of the beta II' turn at the D-tryptophan-lysine disrupts this side chain array producing inactive molecules. The role of the bridging region, the Phe-Pro dipeptide, is to stabilize the beta II' turn and help maintain the proper orientation of the biologically important side chains.