Support of¹H NMR assignments in proteins by biosynthetically directed fractional¹³C-labeling

Biosynthetically directed fractional incorporation of¹³C into proteins results in nonrandom¹³C-labeling patterns that can be investigated by analysis of the¹³C-¹³C scalar coupling fine structures in heteronuclear¹³C-¹H or homonuclear¹³C-¹³C correlation experiments. Previously this approach was used for obtaining stereospecific¹H and¹³C assignments of the diastereotopic methyl groups of valine and leucine. In the present paper we investigate to what extent the labeling patterns are characteristic for other individual amino acids or groups of amino acids, and can thus be used to support the¹H spin-system identifications. Studies of the hydrolysates of fractionally¹³C-labeled proteins showed that the 59 aliphatic carbon positions in the 20 proteinogenic amino acids exhibit 16 different types of¹³C-¹³C coupling fine structures. These provide support for the assignment of the resonances of all methyl groups in a protein, which are otherwise often poorly resolved in homonuclear¹H NMR spectra. In particular, besides the individual methyl assignments in Val and Leu, unambiguous distinctions are obtained between the methyl groups of Ala and Thr, and between the γ- and δ-methyl groups of Ile. In addition to the methyl resonances, the γCH₂ groups of Glu and Gln can be uniquely assigned because of the large coupling constant with the δ-carbon, and the identification of most of the other spin systems can be supported on the basis of coupling patterns that are common to small groups of amino acid residues.