NMR Evidence against γ Turn and β Turn Models of Angiotensin II in Aqueous Solution

Abstract

WE wish to report NMR data refuting both the γ turn and β turn models¹ proposed for the conformation of [Asn¹, Val⁵-antiotensin II in aqueous solution. The assignment of all the resolved resonances in the 220 MHz PMR spectrum in D₂O to specific hydrogens has recently been published². By studying the spectra in H₂O as a function of pH, the peptide ΝΗ-α-CH coupling constants were determined at acid pH and partially assigned³: (1) 6.5 ± 0.3, Arg-2; (2) 6.0±0.5; (3) 7.2 ± 0.5; (4) 7.3±0.3, Phe-8; (5) 7.9 ± 0.3; and (6) 8.0±0.4. Bleich et al.⁴ have reported exchange experiments which allow the tentative assignment of the coupling constants (5) and (6) to the two valine residues at positions (3) and (5). Decoupling experiments (J. D. G., J. Dadok, and G. R. M., unpublished) of the α protons in the presence of the water resonance have made possible unambiguous assignments of the amide resonances (Table 1).