SYNTHESES AND CONFORMATIONS OF SOMATOSTATIN-RELATED CYCLIC HEXAPEPTIDES INCORPORATING SPECIFIC ALPHA-METHYLATED AND BETA-METHYLATED RESIDUES

The cyclic hexapeptide c[-Pro6-Phe7-D-Trp8-Lys9-Thr10-Phe11-] displays high biological activities in inhibiting the release of many bioactive molecules, including growth hormone, insulin, and glucagon. The superscript numbers refer to the location of the residues in native somatostatin. Conformational studies of this cyclic hexapeptide indicated considerable conformational flexibility in various regions of backbone and side chains. The flexibility prevents the elucidation of the ''bioactive conformation'' of this molecule when bound to a receptor. To reduce the conformational flexibility, we have synthesized one main chain methylated analog containing alpha-MeVal at position 10, c[-Pro6-Phe7-D-Trp8-Lys9-(S)-alpha-MeVal10-Phe11-], and two side chain methylated analogs containing beta-MeTrp and beta-MePhe at positions 8 and 11, respectively, c[-Pro6-Phe7-(2R,3S)-beta-MeTrp8-Lys9-Thr10-(2S,3S)-beta-MePhe11-] and c[-Pro6-Phe7-(2S,3R)-beta-MeTrp8-Lys9-Thr10-(2S,3S)-beta-MePhe11-]. The effect of main chain and side chain methylations has been studied using 500-MHz two-dimensional H-1-NMR and computer simulations. These main chain and side chain methylated analogs display constrained conformational preferences at the modified backbone and side chains. One of the side chain methylated analogs shows high potency in receptor binding. Conformational studies of these analogs provide valuable information about the main chain and side chain conformation required for receptor interaction. This study clearly demonstrated a novel approach using main chain and side chain methylations in the elucidation of the bioactive conformation of somatostatin analogs. This approach may have important implications in the study of other peptide hormones and neurotransmitters.