How Keto-ABNO Bridges the Boundary Between Chemistry and Biology

Basic Information

common name: 9-Azabicyclo[3.3.1]nonan-3-one N-oxyl (Keto-ABNO)

English name: Norpseudopelleterine-N-oxyl

CAS No.: 7123-92-4

Molecular formula: C8H12NO2

Molecular weight: 154.19

EINECS No.: 205-516-1

9-Azabicyclo[3.3.1]nonan-3-one N-oxyl (Keto-ABNO, CAS No. 7123-92-4) is a stable nitroxide radical with a bicyclic structure. Its catalytic versatility arises from a unique combination of steric compactness, high oxidation potential, and reversible interconversion among three oxidation states: hydroxylamine, N-oxyl radical, and oxoammonium ion. The value of these molecular features is fully demonstrated in the way Keto-ABNO has expanded from organic small-molecule synthesis to protein modification.

1. Catalytic Foundations in Organic Synthesis

Keto-ABNO was originally developed by Dupeyre and Rassat as a radical that is stable at room temperature. In 2012, the Kanai group at the University of Tokyo reported the use of Keto-ABNO as a catalyst for the aerobic oxidation of amines. This work established an efficient method for oxidizing amines to imines under mild conditions. Furthermore, by using a chiral copper–bis(oxazoline) complex, catalytic asymmetric amino acid synthesis from glycine derivatives and nitroalkanes can also be achieved.

In the field of alcohol oxidation, Keto-ABNO, owing to its relatively low steric hindrance and high oxidation potential, can oxidize secondary alcohols and unactivated aliphatic alcohols that are difficult to handle with TEMPO-based systems. The Kanai group developed a three-step synthetic route to Keto-ABNO, enabling gram-scale preparation and providing a material basis for subsequent application studies.

2. Key Features That Cross the Chemistry–Biology Boundary

The ability of Keto-ABNO to extend from small-molecule catalysis to protein modification stems from several molecular features:

Its steric compactness allows Keto-ABNO to efficiently approach sterically constrained amino acid residues in proteins. Its high oxidation potential provides a thermodynamic driving force for oxidation reactions under mild conditions. Its reversible three-state interconversion enables it to act both as an oxidation catalyst and as a covalent modification reagent that binds to target residues. In addition, Keto-ABNO has a certain degree of compatibility with aqueous media, which is a fundamental prerequisite for chemical modification of proteins.

3. Serine-Selective Oxidative Cleavage

One application of Keto-ABNO in protein modification is serine-selective oxidative cleavage. The Kanai group used a Cu(I)/Keto-ABNO catalytic system to achieve selective oxidation of serine residues in peptides and proteins in aqueous media. The hydroxymethyl group of the serine side chain is oxidized to an aldehyde, which then triggers peptide bond hydrolysis, leading to selective cleavage of the peptide chain at serine sites.

This method provides a chemical tool for protein structural analysis and sequence analysis. Traditional protein cleavage mainly relies on proteases, whose specificity is limited by the substrate scope of the enzyme. The Cu/Keto-ABNO system offers an alternative chemical route.

4. Tryptophan-Selective Bioconjugation

While developing the serine cleavage method, the Kanai group found that when the substrate was a peptide containing tryptophan, Keto-ABNO selectively formed an adduct with tryptophan residues. This discovery opened up applications of Keto-ABNO in protein labeling.

The Keto-ABNO/NaNO2 system can achieve selective covalent modification of tryptophan residues under aqueous conditions. This method is also applicable to peptides containing oxidation-sensitive amino acids such as serine, lysine, tyrosine, and methionine; these residues remain unaffected during the reaction. In addition, this method can introduce fluorescent probes, functional molecules, and other payloads into target proteins.

5. Electrochemical Tryptophan-Selective Bioconjugation

In 2025–2026, the Kanai group further reported an electrochemically promoted method for tryptophan-selective bioconjugation. This method uses a dual N-oxyl radical system in which Keto-ABNO cooperates with 4-oxo-TEMPO. Keto-ABNO serves as the tryptophan-reactive reagent, 4-oxo-TEMPO acts as an electron mediator, and NaBr is used to suppress product overoxidation and radical degradation.

Cyclic voltammetry and UV-Vis spectroelectrochemical analysis indicate that a pre-association exists between tryptophan and Keto-ABNO, which lowers the oxidation barrier. This method enables protein-level tryptophan-selective labeling in neutral buffer, bringing the conditions closer to physiological ones.

Keto-ABNO has gradually developed from an organic oxidation catalyst into a tool molecule in the field of protein chemistry, with applications that cross the disciplinary boundary between chemistry and biology. Through its steric compactness, high oxidation potential, and reversible three-state interconversion, Keto-ABNO shows application value in serine-selective oxidative cleavage, tryptophan-selective bioconjugation, and electrochemical tryptophan labeling, providing alternative technical routes for protein engineering and chemical biology research.