Practice question

A mass increase of 16 daltons corresponds to addition of a single oxygen atom, atomic mass 15.995 Da. Methionine contains a sulfur thioether that is highly susceptible to reactive oxygen species, forming methionine sulfoxide with +16 Da shift and further oxidizing to sulfone with +32 Da. This artifactual or biologically relevant oxidation occurs during sample preparation, storage and oxidative stress, altering protein hydrophobicity and function. Acetylation adds C2H2O (+42 Da), carboxylation adds CO2 (+44 Da), and dehydration removes water (-18 Da), so oxidation uniquely explains the +16 signature commonly tracked in proteomics quality control.