Practice question

Methionine contains a thioether side chain highly susceptible to oxidation during sample handling, electrophoresis, or in vivo oxidative stress conditions. Addition of a single oxygen atom to sulfur forms methionine sulfoxide, increasing monoisotopic mass by 15.9949 Da, rounded to +16 Da in low-resolution spectra. This mass increment is a common variable modification included in proteomics database searches. Detection of +16 Da shift at methionine residues helps monitor oxidative damage, aging of proteins, and artifactual modifications, distinguishing true biological regulation from sample preparation induced oxidation in mass spectrometry analysis.