Catalytic oxidation processes can be applied for protein modification through activation of amino acid side chains and labeling reagents (Scheme 10A, 10B). As oxidation of proteins is one of the fundamental processes in living systems, there is a range of oxidative reactions used for protein modification in noncatalytic manners. The two major approaches for oxidative catalysis for protein modification occurs through activation of either amino acid side chains (e.g., thiyl radical generation) or labeling reagents (e.g., diazene generation). An example includes alkenylation of a cysteine residue by a gold-and silver-mediated system using allene-based labeling reagents. The gold catalyst was proposed to be useful for not only the single electron oxidation of cysteine, but also activation of allene labeling reagent as Lewis acid (Scheme 10C). The necessity of silver triflate additive may limit the utility of this catalysis, as a silver salt is known to induce precipitation of proteins (e.g., common staining protocols for protein gel known as silver staining). An oxidation process can be utilized for catalytic activation of labeling reagents, as the hemin-catalyzed tyrosine modification takes advantage of such a mechanism. Inspired by biological oxidation processes of luminol in firefly chemiluminescence, this approach proceeds by formation of tyrosine-reactive diazene reagent through oxidation of N-N bond to N=N bond. A previous approach of tyrosine-selective modification by Barbas and coworkers necessitated preparation of an unstable diazene reagent prior to protein modification processes, and this catalytic oxidation strategy omits the technical challenges through in situ generation of the active species. It is interesting that the most effective reagent proved to possess N-methyl group that should not be able to form neutral N=N bond species as proposed (Scheme 10C). The high activity of the N-methyl reagent may be indicative of potential involvement of single-electron oxidation by the iron complex.