Measuring 13Cβ chemical shifts of invisible excited states in proteins by relaxation dispersion NMR spectroscopy

A labeling scheme is introduced that facilitates the measurement of accurate 13Cβ chemical shifts of invisible, excited states of proteins by relaxation dispersion NMR spectroscopy. The approach makes use of protein over-expression in a strain of E. coli in which the TCA cycle enzyme succinate dehydrogenase is knocked out, leading to the production of samples with high levels of 13C enrichment (30–40%) at Cβ side-chain carbon positions for 15 of the amino acids with little 13C label at positions one bond removed (≈5%). A pair of samples are produced using [1-13C]-glucose/NaH12CO3 or [2-13C]-glucose as carbon sources with isolated and enriched (>30%) 13Cβ positions for 11 and 4 residues, respectively. The efficacy of the labeling procedure is established by NMR spectroscopy. The utility of such samples for measurement of 13Cβ chemical shifts of invisible, excited states in exchange with visible, ground conformations is confirmed by relaxation dispersion studies of a protein–ligand binding exchange reaction in which the extracted chemical shift differences from dispersion profiles compare favorably with those obtained directly from measurements on ligand free and fully bound protein samples.