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.