What is Peptide oxidation?

Peptide oxidation is the reaction of susceptible side chains with oxygen-derived species, leaving the backbone intact but changing the chemistry of individual residues. It can occur at any stage, from production and purification through storage and handling of both dry and dissolved material, and its products can differ in structure, aggregate more readily and lose biological activity compared with the native molecule ^[1].

Which residues are at risk

Among the amino acids, cysteine, methionine, tyrosine, phenylalanine and tryptophan react fastest with common biological oxidants ^[2]. In practice, a sequence is checked for these residues first:

Tryptophan is especially reactive: it reacts with hydroxyl radical at a diffusion-limited rate and quickly with singlet oxygen ^[2]. When bovine α-crystallin was exposed to a Fenton (iron and peroxide) system, tandem MS located hydroxytryptophan, N-formylkynurenine and kynurenine on specific Trp residues, plus oxidised Met ^[3].

What drives it at the bench

• Dissolved oxygen and the air in a vial's headspace.
• Trace transition metals with peroxide, which generate hydroxyl radicals (the chemistry used in ^[3]).
• Light, which in the presence of photosensitisers generates singlet oxygen, an oxidant that tryptophan reacts with rapidly ^[2].
• Neutral to alkaline pH for free thiols, which form disulfides readily.

Practical counters follow directly: degassed buffers, minimal headspace, light-protected tubes, frozen single-use aliquots rather than a stock repeatedly exposed to air, and dry storage of the lyophilised solid for as long as possible.

Worked example: Met oxidation in Semax

Semax (Met-Glu-His-Phe-Pro-Gly-Pro) has an N-terminal methionine; its monoisotopic mass is 813.348 Da.

On reversed-phase HPLC, oxidising one Met to the sulfoxide lowered retention by 2.37% acetonitrile on average, though the shift varied from −9.1% to +0.4% with sequence, and the two sulfoxide diastereomers can partly resolve into two peaks ^[4]. A genuine sulfoxide therefore usually appears as a separate, earlier peak with its own retention time. A +16 Da signal sitting exactly under the parent peak suggests oxidation during electrospray rather than in the sample.

Sequence design can remove the problem. Melanotan II carries norleucine where α-MSH has methionine; norleucine has the same side-chain length without the sulfur, so it cannot form a sulfoxide. Its His and Trp remain oxidisable.

Common misunderstandings

• "A dry powder doesn't oxidise." Oxidation affects solid as well as liquid preparations ^[1]; dryness slows it.
• "+16 Da always means methionine." Trp, Tyr and Phe hydroxylation give the same nominal shift; MS/MS fragmentation localises the site ^[3].
• "It only costs a percent of purity." The oxidised form is a different molecule that may be less active or more aggregation-prone ^[1], so in a sensitive assay it acts as an impurity with its own biology.

The COA guide explains how mass spectrometry results are presented, and the storage guide covers conditions that limit oxidation alongside deamidation.