Assignments of ^lH Nuclear Magnetic Resonances of the Cystyl, Asparaginyl, and Aromatic Residues of Arginine Vasopressin in D₂O. A Comparison with Lysine Vasopressin and Oxytocin in Terms of Solution Conformation

The resonances of the C^αand C^βprotons of the cystyl. asparaginyl, and aromatic residues of [8-arginine]vasopressin (AVP) in D₂O at pD 3.8 and 20°C were assigned in a rigorous manner by the use of isotopic isomers of AVP that contain specific replacements of protons by deuterons and by comparison of¹H NMR characteristics of AVP to those of [8-lysine]vasopressin (l.VP) and oxytocin (OT). Although there is extensive overlap of resonances of C^βprotons even at 360 MHz, all of the chemical shifts of these protons and most of the couplings between them and their vicinal C protons could be determined, at least to a first approximation. It was concluded that the cyclic moieties (residues 1-6) of AVP, LVP, and OT possess essentially the same overall backbone conformation, and that the side-chain conformation-or rotamer populations-about the O^α-C^βbonds of the cystyl residue (positions 1 and 6). the tyrosyl residue (position 2), and the asparaginyl residue (position 5) are similar. This study indicates that selective replacements of C^βprotons by deuterons are necessary to improve the accuracy of coupling constants extracted from 360-MHz spectra of AVP for use in conformational analysis.