Electrochemical Tryptophan-Selective Bioconjugation

Abstract

<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>

Single electron transfer-based peptide/protein bioconjugations driven by biocompatible energy input

“…c Electrochemical bioconjugation for labeling tryptophan by keto-ABNO, reported by Kanai and Oisaki in 2019 (ref. 78 ).…”

“…Kanai, Oisaki, and coworkers recently developed an electrochemical bioconjugation method for labeling tryptophan residues in peptides and proteins 78 . In comparison with the above electrochemical bioconjugation cases, this report showed that the electrochemical modification method could be applied to tryptophan, thus confirming the potential of electrochemical bioconjugation for use in modifying aromatic amino acids.…”

Toward Intracellular Bioconjugation Using Transition-Metal-Free Techniques

“…However, the reaction is incompatible with live systems, since the radical generatorsodium nitritealso generates ROS, which cause oxidative stress and membrane lipid peroxidation, , thus preventing the observation of the normal physiology of the cell. Keto-ABNO has been used in conjunction with 4-oxo-TEMPO in another work by the same group, to produce tryptophan conjugates, taking a unique, electrochemical approach, depicted in Scheme . The novel use of hybrid radical species for labeling resulted in the two N-oxyl-radicals, with similar redox potentials, working in tandem, at neutral pH to enhance specificity and suppress the formation of conjugates arising from the overoxidation or dehydration, unlike the previous work.…”

Protein Chemical Labeling Using Biomimetic Radical Chemistry

“…Kanai, Oisaki, and co-workers reported that 9-azabicyclo [3.3.1]nonane-3-one- N -oxyl (keto-ABNO, Figure 12) selectively labels tryptophan residues in the presence of 0.1% acetic acid and NaNO 2 [18]. Although keto-ABNO is oxidized by NOx in this method, they recently reported a method for activating the reaction by electrochemical oxidation [81]. They added 4-oxo-TEMPO as the electrochemical mediator to suppress both the anodic overoxidation of proteins and the cross reactivity to other amino acid residues (Figure 12).…”