<p>Bioconjugation reactions are a fundamental synthetic method for generating artificial peptides and proteins. Despite the potentially superior properties of bioconjugates at hydrophobic amino acid residues compared with those at hydrophilic amino acids, methods to target hydrophobic amino acids with moderate reactivity under mild and metal-free conditions are limited. Here we report the first electrochemically-promoted tryptophan (Trp)-selective bioconjugation of peptides and proteins in neutral aqueous media. The unique electrochemical cooperation of two radicals, keto-ABNO and 4-oxo-TEMPO, was critical to suppress both anodic overoxidation of the products and cross reactivity. Systematic cyclic voltammetry analysis suggested that these two radicals, containing similar redox potentials but contrasting steric demands, had distinct electrochemical roles (reactant and electrochemical mediator). This new protocol will be an important advance toward clean and scalable syntheses of chemically modified biologics.</p>