Natural-abundance ¹⁵N NMR studies of Turkey ovomucoid third domain. Assignment of peptide ¹⁵N resonances to the residues at the reactive site region via proton-detected multiple-quantum coherence

Heteronuclear two-dimensional ¹H{¹⁵N} multiple-quantum (MQ) spectroscopy has been applied to a protein sample at natural abundance: ovomucoid third domain from turkey (Meleagris gallopavo), a serine proteinase inhibitor of 56 amino acid residues. Peptide amide ¹H NMR assignments obtained by two-dimensional ¹H{¹H} NMR methods (R. Krishnamoorthi and J. L. Markley, unpublished data) led to identification of the corresponding ¹H{¹⁵N} MQ coherence cross peaks. From these, ¹⁵N NMR chemical shifts were determined for several specific backbone amide groups of amino acid residues located around the reactive site region of the inhibitor. The results suggest that amide ¹⁵N chemical shifts, which are readily obtained in this way, may serve as sensitive probes for conformational studies of proteins.