Selective 'unlabeling' of amino acids in fractionally C-13 labeled proteins: An approach for stereospecific NMR assignments of CH3 groups in Val and Leu residues

A novel methodology for stereospecific NMR assignments of methyl (CH3) grou ps of Val and Leu residues in fractionally C-13-labeled proteins is present ed. The approach is based on selective 'unlabeling' of specific amino acids in proteins while fractionally C-13-labeling the rest. A 2D [C-13-H-1] HSQ C spectrum recorded on such a sample is devoid of peaks belonging to the 'u nlabeled' amino acid residues. Such spectral simplification aids in unambig uous stereospecific assignment of diastereotopic CH3 groups in Val and Leu residues in large proteins. This methodology has been demonstrated on a 15 kDa calcium binding protein from Entamoeba histolytica (Eh-CaBP).