Electrochemical Tryptophan-Selective Bioconjugation

Bioconjugation reactions are a fundamental synthetic strategy for generating artificial peptides and proteins. Although bioconjugates at hydrophobic amino acid residues can offer superior product homogeneity compared to those at hydrophilic residues, methods to target moderately reactive hydrophobic amino acids under mild and metal-free conditions are limited. In this study, we present the first electrochemically promoted, tryptophan (Trp)-selective bioconjugation that is applicable at the protein scale in a neutral buffer. The unique electrochemical cooperation of two radicals, keto-ABNO and 4-oxo-TEMPO, along with NaBr, was key to accelerating the reaction while simultaneously suppressing both anodic overoxidation of the products and cross-reactivity. Systematic cyclic voltammetry (CV) and UV-visible absorption spectroelectrochemistry (SEC) analyses revealed that these two radicals, which have similar redox potentials but differing steric demands, serve distinct electrochemical roles (as reactant and electrochemical mediator, respectively). Additionally, NaBr was suggested not to function as a redox mediator, but instead likely prevents the decomposition of oxoammonium active species. This electrochemical protocol marks a significant advance toward novel processing techniques for chemically modified biologics.