Covalent bond formation between the catalyst and protein has also been reported, with the Ru(TAP)₂phen²⁺ complex (TAP = 1,4,5,8-tetraazaphenanthrene; phen = 1,10-phenanthroline) enabling tryptophan (Trp) labeling via a SET reaction. For photocatalysts such as flavin, acriflavine, and 2,4,6triphenylpyrylium (TPT), it is postulated that the excited photocatalyst abstracts an electron from the substrate. Flavin, in particular, can accept two electrons and two protons to achieve its reduced H₂-Fl state, after which it regenerates its ground state by donating electrons to oxygen. Lumiflavin-based photocatalysts have been applied in selective Tyr modification of proteins using a phenoxazine dialdehyde tag. Flavin-catalyzed Tyr modifications with tyramide have also found applications in analyzing cell-cell interactions by enabling controlled reactivity on cell membrane surfaces. Additionally, acriflavine, with its higher cell membrane permeability compared to organometallic complexes, has been adapted for controlling reactions within cells. TPT, known for its high oxidation potential (+2.55 V vs saturated calomel electrode (SCE)), has even been reported to facilitate modifications of Phe residues, which are typically challenging to activate.