CD‐n.m.r. study of the solution conformation of bradykinin analogs containing α‐aminoisobutyric acid

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Abstract
The conformation in aqueous solution of several α‐aminoisobutyric acid (AIB)‐containing analogs of bradykinin (BK) has been probed by complementary CD and 1H n.m.r. measurements. The conclusion reached is that substitution of AIB for Pro2 and/or Pro3 in BK stabilizes a degree of β‐turn conformation in the N‐terminal tetrapeptide moiety of the resulting analogs. Changing the solvent from water to DMSO or TFE further enhances the contribution of particular hydrogen bonded structures to the time‐averaged conformation of these peptides. Bradykinin and [AIB7]‐BK adopt similar hydrogen bonded conformations in TFE, apparently with a contribution from a β‐turn involving their common Arg1‐Pro2‐Pro3‐Gly4 moiety. The contrasting biological activities of BK and its AIB‐analogs are considered in terms of the conformational analogy between the AIB‐residue and cis1 Pro and the propensity for a β‐turn at the N‐terminus of the peptide.