Proton Magnetic Resonance Study of Angiotensin II (Asn1Val5) in Aqueous Solution

All the resolved resonances in the 220-MHz proton magnetic resonance (pmr) spectrum of angiotensin II (Asn1Val5) (All′) in D₂O have been assigned to specific hydrogens. In H₂O additional assignments were made of resonances originating from peptide NH hydrogens of Phe and Arg, primary amide NH hydrogens of Asn, and the four equivalent guanidino NH hydrogens of Arg. Conformational transitions associated with titration of the α-amino and/or the imidazole group(s) (pK_a = 6.6 ± 0.2) and with titration of the phenol group (pK_a = 10.2 ± 0.2) have been confirmed. Pmr determined pK_a values in H₂O at 23d and in D₂O at 4° (shown in parentheses) are all normal: carboxyl 3.07 ± 0.03, imidazole 6.26 ± 0.04 (6.82 = 0.02), α-amino (6.98 ± 0.04), and phenol 10.2 ± 0.2 (10.5 ± 0.5). The peptide NH-αCH coupling constants in acidic solution are: 6.5 = 0.3 (Arg), 6.0 = 0.5, 7.2 ± 0.5, 7.3 ± 0.3 (Phe), 7.0 ± 0.3, and 8.0 ± 0.4. Various classes of labile hydrogens were defined on the basis of their exchange rates as determined from broadening of their respective resonances. The pmr data are consistent either with a rapid equilibrium between various conformations or with a unique conformation of the hormone, but additional evidence is required to definitively determine the structure(s) significantly contributing to the equilibrium. Previously proposed structures excluded by these data include the α helix, the conventional β turn, the γ turn, and a structure stabilized by a salt bridge.