4.2. Target-Detected In-Cell NMR
In the target-detected in-cell NMR only the target of interest is isotopically labeled (i.e., 15N labeled protein). For instance, target proteins can be isotopically labeled during cell growth in isotopically enriched (13C, 15N, or both 15N/13C) media [410]. The cell type and the labeling method may vary across experiments. Different cell types, including bacteria [411], oocytes [412], yeast cells [413], mammalian cells [414], HeLa cells [415] and even insect cells [416] have been reported in the literature. The fact that in-cell NMR applies to more than one cell type testifies of the versatility and potential application of this technique.
In terms of labeling, 15N is one of the most commonly used approaches [417] when the targets of interest are proteins. Recently, 19F labeling has been reported as a useful probe for protein-ligand interactions [418]. It was shown that 19F can reveal information about the dynamics of protein-ligand interactions [419]. Methyl groups [420] have also been used as probes for proteins and complexes in vivo [420], proving that labeling specific chemical groups instead of the entire biomolecule (i.e., protein) is feasible.