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Complex formation between bovine neurophysin II and oxytocin molecules containing 85% 13C enrichment in specific amino acid residues was studied using 13C nuclear magnetic resonance spectroscopy. Chemical shift and relaxation time values of the analogue [13C-Leu3]oxytocin, [13C-Gly9]oxytocin, and the doubly labeled [13C-Ile3 Gly9]oxytocin were obtained for the hormones in the absence and presence of neurophysin. The results showed that certain 13C nuclear magnetic resonance parameters of residue 3 but not of residue 9 of oxytocin are altered upon binding to neurophysin. These observations suggest that residue 3 but not residue 9 is involved in the protein-hormone interaction and they demonstrate the general applicability of selective 13C enrichment for the study of peptide-protein interactions.

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