Unraveling the Reactive Oxygen and Nitrogen Species Responsible for Oxidation of Methionine, Cysteine, Tyrosine and Tryptophan in Atmospheric Pressure Nonthermal Plasma Treatment

Nonthermal plasma is a promising food preservation method, yet its effects on proteins remains underexplored. In this study, aqueous solutions of methionine, cysteine, tyrosine and tryptophan at pH 4.75 were exposed to a multihollow surface dielectric barrier discharge using dry air, humid air (75% relative humidity) or Ar/O₂ (99/1%). Native amino acids and their degradation products were quantified using HPLC-FLD/DAD and LC-MS/MS. To identify key reactive species, the same amino acids were treated with individual reactive oxygen and nitrogen species (RONS; ¹O₂, O₃, H₂O₂, ^•OH, O₂NOOH, ONOOH, ONOOCO₂^-, OCl^-). Ozone emerged as the dominant oxidant across all plasma conditions, with nitrogen species also contributing. Amino acid sensitivity to plasma varied, with methionine and tryptophan showing higher oxidation (8-27%) than cysteine and tyrosine. Major oxidation products included methionine sulfoxide, cysteic acid, levodopa and <i>N</i>'-formylkynurenine. These findings lay the groundwork for applying plasma technology to food protein treatment.