Assignment of polypeptide 1H and 13C signals according to the amino acid sequence by multifrequency heteronuclear decoupling

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By 13C{ 1H} heteronuclear selective multifrequency decoupling one can assign the NMR signals in polypeptide spectra to specific positions of amino acid residues in the primary structure. Use was made of decoupling of the interresidue spin-spin interaction of carbonyl carbon 13C′ and H α proton in the natural-abundance 13C NMR spectrum. Interactions of the carbonyl carbon with H α and H β protons of the same amino acid residue are suppressed by additional irradiation. The assignment technique is illustrated by lysine and alanine signal assignment in the NMR spectra of apamin, an 18-membered bee-venom polypeptide neurotoxin.