Proton Magnetic Double-Resonance Study of Angiotensin II (Asn¹ Val⁵) in Aqueous Solution Employing Correlation Spectroscopy. Assignment of Peptide NH Resonances and Transfer of Saturation from Water

Complete assignment of the peptide NH resonances of angiotensin II (Asn¹Val⁵) (AII′) in aqueous solution has been accomplished by double irradiation of α-CH absorptions. The Karplus equation was employed to calculate the dihedral angles ϕ of specific HNCH fragments from corresponding peptide NH-α-CH coupling constants. These data exclude the α helix, conventional β turn, γ turn, random coil, and the structure proposed by Weinkam and Jorgensen (J. Amer. Chem. Soc. 93, 7038 (1972)) as the preferred solution conformation of AII′ in water. Although the data are consistent with an order-disorder equilibrium, additional information is required for a definitive conformational analysis. The data suggest that the solution conformation of AII′ differs from its conformation at the receptor site. Transfer of saturation from the H₂O hydrogens indicates that the Arg² peptide NH hydrogen exchanges with the solvent at a significantly more rapid rate than do the other peptide NH hydrogens.