The Tyr-ene reaction shown in the previous subsection can also proceed under one-electron transfer conditions when 1methyl-4-phenylurazole or N-methyl phthalic hydrazide is used instead of 4-phenylurazole (Scheme 2B). In 2020, Nakamura and co-workers reported that nitrogen-centered radicals generated by electrode oxidation of N-methyl phthalic hydrazide can modify Tyr residues on variety of proteins. Similarly, Lei and co-workers reported that nitrogen-centered radicals generated by electrochemical oxidation of phenothiazine are also effective for modifying Tyr residues. In 2022, Weng and co-workers introduced a modification method targeting Trp residues using azidyl radicals (N3･, Scheme 2B). These radicals add to indole side chain, enabling the modification of peptides with 20 or more residues. Azidyl radicals are generated electrochemically from Mn II -N3, accompanied with C=N double bond formation. Weng and coworkers subsequently reported that trifluoromethyl radicals (CF3･) are also applicable to modify Trp residues. The methods described in the above two subsections are notable for specifically targeting Tyr or Trp residues for protein modification. These residues are appealing targets because of their low surface exposure, the controlled nature of their modification reactions, and the minimal impact on post-modified protein structure. However, the conversion of phenol and indole side chains typically requires stringent reaction conditions. Electrochemical methods are noteworthy as the strategies enable efficient modification of Tyr and Trp residues under mild conditions, serving as a powerful tool for protein modification.