A reflection on ketoABNO: the crossing point between organic synthesis and protein modification

In 2012, we reported that the <i>N</i>-oxyl radical ketoABNO functions as an effective catalyst for the mild aerobic oxidation of amines to imines (T. Sonobe, K. Oisaki and M. Kanai, <i>Chem. Sci.</i>, 2012, <b>3</b>, 3249, https://doi.org/10.1039/C2SC20699D). Its catalytic versatility arises from a unique combination of steric compactness, high oxidation potential, and the ability to reversibly interconvert among three oxidation states-hydroxyamine, <i>N</i>-oxyl, and oxoammonium. Beyond amine oxidation, ketoABNO has also been applied to the oxidation of alcohols and aldehydes. More recently, its utility has extended beyond small-molecule transformations to include applications in protein modifications, such as serine-selective oxidative cleavage of proteins (in conjunction with a water-soluble copper-complex catalyst) and tryptophan-selective bioconjugation. In this Commentary, we highlight the development of ketoABNO as an oxidation catalyst and its emerging applications in biocompatible protein chemistry.