C-13 NMR-SPECTROSCOPY OF [20-PERCENT-1,2-C-13-2-GLY6]-BRADYKININ - ROLE OF SERINE IN REDUCING STRUCTURAL HETEROGENEITY

Substitution of a glycine residue for the serine residue at position six in the nonapeptide bradvkinin leads to a dramaic increase in the cis/trans ratio about the sixth peptide bond (~38%cis). Decreases in the cis/trans ratio determined by NMRin 1 M NaClO4and in 60%12CH3OH have been correlated with decreases in the 219-nm CD band. Chemical shifts of the 20%13C-labeled Gly6residue exhibit a very different pattern from that observed in the dipeptide Gly-L-Pro; the cis-trans shift dif-ference isM)~0ppm for the glycine carbonyl resonances and ~1 ppm for the glycine methylene resonances. In contrast, the cis-and trans-glycine carbonyl resonances are resolved in 60% methanol and in I M NaClo4. A series of model peptides has beenexamined to determine the structural factors leading to the observed shift pattern. These studies indicate the importance ofPhe8both in determining the cis/trans ratio and in producing the observed chemical shifts. Glycine resonances in the tripeptideGly-Pro-Phe are qualitatively similar to those observed in (Gly6]-bradykinin. The most probable explanation of these data is a favorable hydrophobic/solvent-excluding Pro-Phe association in the cis peptide, which leads to a rotation about the Pro Cα-carbonyl bond, placing the carbonyl oxygen in close proximity to the Gly6methylene carbon. In the trans peptide, electrostaticrepulsion between the Gly and Pro carbonyl oxygens opposes thi>effect. Significant differences in the cis/trans ratios betweenbradykinin and [Gly6]-bradykinin have prompted careful bioassay studies, which show the potency of [Gly6]-bradykinin tobe only 50-70% that of bradykinin, although at saturating levels of peptide the responses are within experimental error. Thesedata provide clucs about the functional role of serine in bradykinin. © 1979, American Chemical Society. All rights reserved.