Methionine oxidation is the conversion of the thioether sulfur in a methionine side chain to a sulfoxide, adding one oxygen atom and therefore 16 Da to the peptide mass. Further oxidation to the sulfone adds another 16 Da but is much slower. Tryptophan and cysteine oxidise by related routes, but methionine is the most reactive residue in typical storage conditions.
Why methionine oxidation matters in peptide research
It is the most frequently observed chemical change in stored peptides, and it needs no dramatic mistreatment: dissolved oxygen, trace metal ions such as iron or copper, light and elevated pH are enough. The consequences are conformational as well as chemical, because a sulfoxide is markedly more polar than a thioether, so an oxidised residue at or near a binding surface can reduce receptor potency substantially while leaving the sequence otherwise intact.
Detection is straightforward once looked for: a plus-16 satellite on mass spectrometry, and an earlier-eluting shoulder on reversed-phase HPLC because the oxidised form is less lipophilic. Mitigation is prosaic — keep material lyophilised and cold, exclude light, avoid metal contact, and minimise headspace air. See the storage guide and COA reading guide.