The sulfur of methionine is readily oxidised by air, peroxides or light to methionine sulfoxide, adding 16 Da. Cysteine (to disulfides and sulfonic acids), tryptophan and histidine can also oxidise.
The oxidised form is more polar and usually elutes slightly earlier in reversed-phase HPLC. Oxidation is slowed by protecting the material from air and light and by keeping it dry and cold.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.