Anthranilic acid, the new player in the ensemble of aromatic residue labeling precursor compounds

The application of metabolic precursors for selective stable isotope labeling of aromatic residues in cell-based protein overexpression has already resulted in numerous NMR probes to study the structural and dynamic characteristics of proteins. With anthranilic acid, we present the structurally simplest precursor for exclusive tryptophan side chain labeling. A synthetic route to ¹³C, ²H isotopologues allows the installation of isolated ¹³C– ¹H spin systems in the indole ring of tryptophan, representing a versatile tool to investigate side chain motion using relaxation-based experiments without the loss of magnetization due to strong ¹J _CC and weaker ²J _CH scalar couplings, as well as dipolar interactions with remote hydrogens. In this article, we want to introduce this novel precursor in the context of hitherto existing techniques of in vivo aromatic residue labeling.