A Study of the Peptide Hormone Oxytocin and of Prolylleucylglycinamide by ¹⁵N NMR

We have performed an extensive ¹⁵N NMR study in water of the peptide hormone oxytocin and of L-prolyl-L-leucyi-glycinamide, the peptide with the sequence of the C-terminal tripeptide of oxytocin. The chemical shifts of all nitrogen nuclei in the two peptides have been determined. These shifts are shown to be very similar to those of N’-acetylamino acids in dimethyl sulfoxide. Deviation from this correlation may be related to conformational restrictions or lack of exposure to solvent. Spinlattice relaxation times (T₁i's), nuclear Overhauser enhancements (NOEs), and one-bond N-H coupling constants are reported. Dipolar relaxation dominates the observed T₁'s, and the ’ ¹⁵N relaxation measurements are consistent with isotropic motion of the tocin ring with a correlation time similar to that determined previously from ¹³C NMR measurements. Paramagnetic metal ions can be particularly troublesome in such ¹⁵N studies, and a technique for their removal is described.