Comparative Study of Tryptophan Oxidation with 1O2, O3,•OH, O2NOOH, ONOOH, and OCl–: Key and Novel Oxidation Products Identified with High-Resolution Mass Spectrometry

Tryptophan, one of the most oxidation-prone amino acids, forms distinct products with different reactive oxygen and nitrogen species, yet oxidant-specific fingerprints have not been compared. Untargeted LC-QTOF-MS profiled tryptophan oxidation in ammonium acetate buffer (pH 4.75) by 1O2, O3,•OH, O2NOOH, ONOOH, and OCl-. Feature intensities were hierarchically clustered and structurally assigned. Established markers (N'-formylkynurenine, oxindolylalanine, kynurenine, hydroxypyrroloindole carboxylic acid, and dioxindolylalanine) and a tryptophan dimer occurred across most conditions, while many oxidants also produced specific oxidized dimers. ONOOH and O2NOOH showed strongly overlapping nitration/nitrosation-dominated fingerprints, while ONOOH also prominently yielded pyridazinoindole, a previously unreported feature. O3 oxidation was characterized by quinoline-4-ol, kynurenine-related products, and multiple Trp+3O signals, whereas 1O2 favored hydroperoxypyrroloindole carboxylic acid, quinoline derivatives, and dimers involving fragmented tryptophan. •OH and OCl- yielded fewer unique features. Comparison with nonthermal plasma-treated tryptophan indicated ozone-dominated chemistry, with additional reactive nitrogen species contributions under humid multihollow surface dielectric barrier discharge conditions.