Peptide oxidation: methionine, cysteine and tryptophan

Last updated: 2026-10-10 · Research use only. Not for human or veterinary use.

Short answer

Oxidation is one of the most common chemical changes in stored peptides. Methionine converts to methionine sulfoxide (+16 Da), free cysteines form disulfides or further oxidised species, and tryptophan can oxidise to several products. Limiting oxygen, light, metal ions and moisture reduces it. [1]

At a glance

Term	Definition / specification
Methionine	→ Met sulfoxide, +16 Da per site (L-Methionine, CID 6137)
Cysteine	→ disulfide (−2 Da per bond) or sulfinic/sulfonic acids (L-Cysteine, CID 5862)
Tryptophan	→ several oxidised products (+4, +16, +32 Da)
Detection	LC-MS mass shift; earlier HPLC elution for Met(O)
Accelerants	Oxygen, light, trace metals, peroxides, water
Factors that slow it	Lower temperature, less light, less oxygen and moisture, fewer metal ions

Methionine sulfoxide

Adding one oxygen makes the residue more polar, so the oxidised peptide usually elutes slightly earlier in RP-HPLC and shows +16 Da in MS.

Reversibility

Methionine sulfoxide can be reduced back enzymatically in biology, which is part of why it is studied as a regulated modification.

FAQ

How do I spot oxidation on a COA?

Look for a +16 Da species in the MS data or a new peak just before the main HPLC peak.

Does freezing stop oxidation?

It slows it substantially; oxygen and light exposure still matter.

Related reference pages

Sources

Manta B et al. Regulated methionine oxidation by monooxygenases. Free Radic Biol Med. 2017. PMID: 28229915 · PMC5540442
Lockhart C et al. Methionine Oxidation Changes the Mechanism of Aβ Peptide Binding to the DMPC Bilayer. Sci Rep. 2019. PMID: 30976055 · PMC6459879
Lai MC et al. Solid-state chemical stability of proteins and peptides. J Pharm Sci. 1999. PMID: 10229638
PubChem. L-Methionine (C5H11NO2S, 149.21 g/mol). CID 6137
PubChem. L-(+)-Cysteine (C3H7NO2S, 121.16 g/mol). CID 5862

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Plain

Sable Peptides. (2026). peptide-oxidation. UK Research Peptide Reference. Retrieved 11 October 2026, from https://www.sablepeptides.co.uk/ref/peptide-oxidation

APA

Sable Peptides. (2026, October 11). *peptide-oxidation*. UK Research Peptide Reference. https://www.sablepeptides.co.uk/ref/peptide-oxidation

BibTeX

@misc{sable_peptide-oxidation_2026,
  author = {{Sable Peptides}},
  title = {{peptide-oxidation}},
  year = {2026},
  howpublished = {\url{https://www.sablepeptides.co.uk/ref/peptide-oxidation}},
  note = {Accessed 11 October 2026. CC BY 4.0}
}

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Date modified: 2026-10-16