Conformation of angiotensin II. Evidence for a specific hydrogen bonded conformation

The peptide amide hydrogen exchange rate of human angiotensin II in H2O have been measured at room temperature by the transfer of solvent saturation method. The data are consistent with the assumption of a highly motile dynamic equilibrium between folded and highly solvated conformations. The NH of His6 is observed to exchange more slowly than predicted, suggesting that it is a participant in an internal hydrogen bond. Several models previously suggested in the literature for the conformation of the peptide in aqueous solution are examined, and most are found to be inconsistent with the exchange data. Evidence in support of a structure for the Ile5-His6 fragment of the hormone involving a C7eq-C5 bend is presented.