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

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.