α-Ketoacids as precursors for phenylalanine and tyrosine labelling in cell-based protein overexpression

C-13-alpha-ketoacid metabolic precursors of phenylalanine and tyrosine effectively enter the metabolism of a protein overexpressing E. coli strain to label Phe- and Tyr-residues devoid of any cross-labelling. The methodology gives access to highly selective labelling patterns as valuable tools in protein NMR spectroscopy without the need of N-15-chiral amino acid synthesis using organic chemistry.