Trp residues can also be modified by aromatic electrophilic substitution reactions under appropriate conditions (Scheme 1B). In this approach, Trp residues are selectively modified by oxoammonium anchors generated by the polar oxidation of the stable organoradical (keto-ABNO). The redox potential of keto-ABNO is lowered when it complexes with Trp residues, enabling functional group tolerance and selective modification of Trp residues.