The conformation of angiotensin II in solution. III. An analysis of Gd³⁺-induced perturbations of the ¹H nmr spectrum

The interactions of human angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) with Eu³⁺, La³⁺, and Gd³⁺ are monitored in H₂O and D₂O by ¹H nmr. The peptide forms 1:1 complexes with the trivalent lanthanide ions in which the carboxyl groups of Asp-1 and Phe-8 form the metal binding site. Addition of La³⁺ induces no significant perturbations in the C^αH-NH coupling constants or chemical shifts of the low field resonances of the peptide, indicating that the conformation of the interior residues of the peptide backbone is unchanged upon metal complexation. The addition of Gd³⁺ induces differential line-broadening of the resonances of the angiotensin II spectrum. These perturbations are discussed qualitatively in terms of the relative distance of each residue from the metal-binding site, and calculations are performed to estimate the absolute metal-proton distances for six of the hydrogens in the hormone (the NH's of Val-2, Tyr-4, Ile-5, His-6, and Phe-8, and the C2-H of the His-6 imidazole). Various literature models for the conformation of the hormone is aqueous solution are discussed in terms of their consistency with these distances.