Nuclear magnetic resonance conformational studies of the Cα–Cβ fragments of oxytocin, oxytocinoic acid, and tocinoic acid in aqueous solution

Abstract

We have recorded the ¹H n.m.r. spectra (300 and 270 MHz) of oxytocin, oxytocinoic acid, and tocinoic acid in aqueous solution in the presence and absence of lanthanide ions. From the analysis of the C_αHC_βH₂ regions of the spectra, the J_α,β coupling constants have been derived and found to be similar for corresponding residues in the different molecules indicating these fragments to have similar conformations in the three molecules. The coupling constants for the side-chains of Tyr, Ile, Gln, and Asn have been used to estimate the fractional populations of the three minimum-energy staggered rotamers. The Cys(1)–Cys(6) fragments also have similar conformations and, for tocinoic acid, this conformation has been partially defined by analysing the combined coupling constant and lanthanide induced shift data. The binding of lanthanide ions does not affect the conformation (no change in coupling constants when La³⁺ ions bind) and because tocinoic acid, oxytocin, and oxytocinoic acid have similar conformations for their Cys(1)–Cys(6) fragments, the conformational information derived for tocinoic acid will be valid also for oxytocin and oxytocinoic acid.