Proton NMR studies of angiotensin II and its analogs in aqueous solution

The 1H nuclear magnetic resonance (NMR) spectra of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) and five of its octapeptide analogs as well as angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) and angiotensin III (Arg-Val-Tyr-Ile-His-Pro-Phe) in aqueous solutions (90% H2O/10% D2O) were completely assigned by two-dimensional COSY and ROESY experiments. All of the peptides give rise to two distinct sets of signals. The minor set accounts for about 5% of the total population below pH 5.5 and increases to 12-20% around pH 7.0. The two sets of signals result from a cis-trans isomerization of the His-Pro peptide bond with the major resonances arising from the trans isomer. One analog in which the Pro is replaced with a D-Pro displays a very different isomerization behavior. The measured coupling constants JNH-alpha CH, the temperature dependence of the amide proton shifts and the relative intensities of the intraresidue and sequential NH-alpha CH ROEs, are all indicative of an extended backbone conformation for ANGII. However, some evidence for the existence of conformers with local structure involving preferred sidechain positions for the Tyr, His, Phe, and the carboxyl group of the Phe was found, particularly in the ROESY and pH-titration experiments. Moreover, pH effects and the unusual amide exchange behavior of the Arg epsilon NH suggests the presence of interactions between the Asp and Arg sidechains of ANGII. At low temperatures the Arg guanidinium NH2 protons were detected as two broad peaks which are related by sizeable exchange peaks in ROESY experiments. This behavior could be useful as a general probe for the study of Arg sidechain mobility and accessibility in other peptides and proteins.