Glossary
Oxidation (Methionine)
Methionine oxidation adds 16 Da and is the most frequent storage-related change in peptides. How it arises, how you detect it and how to keep it to a minimum.
In methionine oxidation, the thioether sulfur of the methionine side chain becomes a sulfoxide; one oxygen atom is added, so the peptide gains 16 Da. A second step to the sulfone adds a further 16 Da but proceeds far more slowly. Tryptophan and cysteine oxidise through similar pathways, yet under normal storage conditions methionine is the most easily oxidised residue.
Why it deserves attention
No chemical change is seen more often in stored peptides, and it does not require any serious mishandling: dissolved oxygen, traces of metal ions like iron or copper, light and raised pH are sufficient. The effects go beyond chemistry to conformation, since a sulfoxide is considerably more polar than a thioether; an oxidised residue on or close to a binding surface can therefore cut receptor potency markedly although the rest of the sequence is unchanged.
It is easy to spot if you look for it: a +16 satellite peak in mass spectrometry, and a shoulder eluting earlier on reversed-phase HPLC, because the oxidised form is less lipophilic. Prevention is unglamorous — store the material freeze-dried and cold, keep it out of the light, avoid contact with metal and reduce the air in the headspace. See the storage guide and COA reading guide.
Related terms
degradation · stability · disulfide bridge