# Selective oxidative modification of tryptophan and cysteine residues using visible light responsive Rh doped SrTiO3 photocatalyst

## TL;DR

This study shows a new photocatalyst can selectively modify specific amino acids in proteins using visible light.

## Contribution

A visible light-responsive Rh-doped SrTiO3 photocatalyst enables selective oxidation of tryptophan and cysteine in amino acids and peptides.

## Key findings

- g-STO:Rh selectively reacts with tryptophan and cysteine among 20 amino acids.
- Tryptophan is oxidized to N-formylkynurenine, while cysteine forms cystine.
- Selective oxidation occurs in peptides containing tryptophan residues.

## Abstract

In recent years, amino acids and peptides have attracted significant attention in food and medical fields due to their functionality. These functionalities largely depend on the chemical properties of their side chains, and efficient methods for selective side chain modification are desired. In this study, we investigated the selective modification of amino acids and peptides using rhodium-doped SrTiO3 (g-STO:Rh), a visible light-responsive photocatalyst. HPLC and LCMS analyses revealed that g-STO:Rh exhibited selective reactivity toward tryptophan and cysteine among the 20 protein-constituent amino acids. While cysteine was oxidatively dimerized to cystine, tryptophan underwent selective oxidation of its indole ring side chain, forming N-formylkynurenine. Furthermore, studies on dipeptides and tripeptides containing tryptophan have demonstrated that selective oxidation proceeds similarly to tryptophan residues within peptides.

The online version contains supplementary material available at 10.1038/s41598-025-04870-z.