Methionine can be oxidised by naturally-occurring reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) to form methionine sulfoxide (MetO), and by further oxidation to a methyl sulfone derivative (MetOO). Oxidation of methionine results in a mixture of the diastereomers methionine-(S)-S-oxide and methionine-(R)-S-oxide (Lavine 1947, Kim et al. 2014).
