Novel approaches in selective tryptophan isotope labeling by using Escherichia coli overexpression media.

NMR-based investigations of large protein complexes require optimized isotopic labeling schemes. We report new methods to introduce stable isotopes into tryptophan residues; these are fine-tuned to the requirements of the particular protein NMR experiment. Selective backbone labeling was performed by using a new -ketoacid precursor as an additive in cell-based overexpression media. Additionally, we developed synthetic routes to certain isotopologues of indole with C-13-H-1 spin systems surrounded by C-12 and H-2. The corresponding proteins, overexpressed in the presence of these precursor compounds, can be effectively analyzed for conformational changes in tryptophan residues in response to external stimuli, such as interaction with other proteins or small molecules.