Evidence for the presence of hydrogen-bonded secondary structure in angiotensin II in aqueous solution.

Automated tritium-hydrogen exchange measurements have been made on the linear octapeptide Val(5)-angiotensin II amide. All six amide hydrogens of the peptide backbone are observable, and are resolved into three classes according to their exchange rates. The rate of exchange of the slowest class, t(1/2) of 300 min at 0 degrees C (pH 2.5), is compared with that of hydrogens that exchange abnormally slowly in other peptides. It is concluded that these slow hydrogens in angiotensin II are involved in secondary structure with either one or both forming stable, intramolecular hydrogen bonds. This finding demonstrates that linear peptides may have hydrogen-bonded conformations in aqueous solutions. Analysis of the pH dependence of the rate of exchange indicates that one peptide amide hydrogen, namely that of the Asn(1)-Arg(2) peptide bond, is not involved in hydrogen bonding and is freely accessible to the solvent. Thus, the finding of internal hydrogen bonding, together with the assignment of the environment of one peptide bond, places major constraints on the number of allowable conformations of this linear polypeptide hormone.