Keto-ABNO: Organic Synthesis Meets Protein Modification

Basic Information

English name: Norpseudopelleterine-N-oxyl  

Common abbreviation: Keto-ABNO; 3-oxo ABNO  

CAS No.: 7123-92-4  

Molecular formula: C8H12NO2  

Molecular weight: 154.19  

EINECS No.: 205-516-1  

Appearance: Yellow crystalline solid

9-Azabicyclo[3.3.1]nonan-3-one N-oxyl—better known as Keto-ABNO or 3-oxo ABNO—is a stable nitroxide radical with a bicyclic skeleton. It didn’t start out as a protein chemistry tool. It was originally developed by Dupeyre and Rassat as a radical that remains stable at room temperature. In 2012, the Kanai group at the University of Tokyo reported its use as a catalyst for the aerobic oxidation of amines. From there, its applications gradually moved beyond small-molecule oxidation into protein modification and chemical biology. That’s what makes Keto-ABNO interesting: it sits right at the intersection of traditional organic synthesis and modern protein engineering.

1. What Lets It Cross from Organic Synthesis into Protein Modification

Keto-ABNO can extend from small-molecule catalysis to protein modification because of a few molecular features that work together.

It is compact and sterically unhindered. Unlike TEMPO, where four methyl groups create considerable steric bulk, Keto-ABNO has a compact bicyclic scaffold with a well-exposed active site. That makes it easier for the radical to reach sterically constrained amino acid residues in large substrates such as proteins.

It has a high oxidation potential and strong reactivity. Keto-ABNO’s oxidation potential (+521 mV vs. Ag/Ag+) is significantly higher than that of TEMPO (+320 mV), giving it more thermodynamic driving force for oxidation under mild conditions.

It can reversibly switch between three oxidation states. Keto-ABNO interconverts between hydroxylamine, N-oxyl radical, and oxoammonium forms. That flexibility allows it to act both as an oxidation catalyst and as a covalent modification reagent that binds to target residues.

And it is mild and water-compatible. Protein chemistry is sensitive—reactions usually need to be close to physiological conditions (neutral pH, aqueous media, room temperature), and heavy-metal catalysts are best avoided because they can denature proteins. Keto-ABNO fits those requirements surprisingly well.

2. Serine-Selective Oxidative Cleavage: An Artificial Peptidase Reaction

The first real breakthrough for Keto-ABNO in protein modification came from serine-selective oxidative cleavage.

A serine side chain carries a hydroxymethyl group (–CH2OH), which can be viewed as a special kind of primary alcohol. The Kanai group used a Cu(I)/Keto-ABNO catalytic system to selectively oxidize serine residues in peptides and proteins in aqueous media at room temperature and neutral pH.

The mechanism is essentially an artificial peptidase reaction. Keto-ABNO works together with Cu(I) to oxidize the serine hydroxymethyl group to an aldehyde. Formation of the aldehyde then triggers hydrolysis of the peptide bond, leading to selective cleavage of the peptide chain at the serine site. The method has been successfully applied to site-selective cleavage of peptide chains around ten residues long.

Why does this matter? Traditional protein cleavage relies on proteases such as trypsin, and their specificity is limited by the enzyme’s substrate scope. The Cu/Keto-ABNO system offers a chemical route to serine-site-selective cleavage, giving researchers another tool for protein structure analysis, sequence analysis, and protein engineering.

3. Tryptophan-Selective Bioconjugation: Metal-Free Chemical Labeling

While developing the serine cleavage method, the Kanai group stumbled on something unexpected: when the substrate was a peptide containing tryptophan, Keto-ABNO selectively formed an adduct with the tryptophan residue. That accidental finding opened up a new direction for Keto-ABNO in protein labeling.

It led to a metal-free, tryptophan-selective protein modification method. Before this, selective chemical modification of tryptophan was rare, and the few available methods often depended on heavy-metal catalysts or organic solvents—conditions that are not exactly friendly to intact proteins. The Keto-ABNO/NaNO2 system enables selective covalent modification of tryptophan residues under mild aqueous conditions, which was a real step forward. Keto-ABNO forms a C–O bond adduct at the C(3) position of the indole ring.

The functional-group compatibility is also notable. The method works on peptides containing oxidation-sensitive amino acids such as serine, lysine, tyrosine, and methionine; those residues remain unaffected, which is important for selectivity. It can also introduce fluorescent probes, anticancer drugs, and other functional molecules into target proteins.

More recently, between 2025 and 2026, the Kanai group developed an electrochemically promoted version of tryptophan-selective bioconjugation. This approach uses a dual N-oxyl radical system with Keto-ABNO  and 4-oxo-TEMPO: Keto-ABNO acts as the tryptophan-reactive reagent, while 4-oxo-TEMPO serves as the electron mediator. Cyclic voltammetry and UV-Vis spectroelectrochemical analysis indicate a pre-association between tryptophan and Keto-ABNO that lowers the oxidation barrier. The method allows protein-scale tryptophan-selective labeling in neutral buffer.

There is another interesting extension, too. A Keto-ABNO/H/TEMPO+ system has been used for tryptophan-selective bioconjugation in the synthesis of DNA-encoded libraries, where it can help build bioactive hydroxypyrroloindole carboxylic acid scaffolds.

Keto-ABNO s journey from an organic oxidation catalyst to a protein chemistry tool is a good example of how molecular properties can open doors across disciplines. Its compact scaffold, high oxidation potential, and reversible three-state chemistry give it a useful combination of reactivity and control—one that is now being explored in serine-selective cleavage, tryptophan bioconjugation, and electrochemical protein labeling.