Oxidation

Oxidation is one of the most destructive chemical degradation pathways for research peptides. It primarily targets methionine (Met) and cysteine (Cys) residues, whose sulfur-containing side chains are highly reactive toward reactive oxygen species (ROS), dissolved oxygen, transition metals (Fe²⁺, Cu²⁺) and UV light.

In practice, methionine oxidizes to methionine sulfoxide (Met-SO), then to methionine sulfone (Met-SO₂) under harsh conditions. Cysteine forms unwanted intermolecular disulfide bridges or oxidizes to sulfenic/sulfonic acid. Tryptophan (Trp), histidine (His) and tyrosine (Tyr) may also undergo oxidation, producing kynurenine or dityrosine adducts.

Consequences on a research peptide are severe: HPLC purity collapse, extra peaks in LC-MS (the characteristic +16 Da mass shift of Met-SO), aggregation through crosslinked S-S bridges, loss of biological activity in in vitro assays, and unpredictable responses in experimental models.

Effective prevention levers include: storage under inert atmosphere (argon or nitrogen), addition of antioxidants (free methionine, EDTA to chelate metals, ascorbic acid), lyophilization rather than solution storage, vacuum-packed amber vials, temperature ≤ -20 °C for long-term stocks, and nitrogen-purging of reconstitution water before use.

The most sensitive peptides in the catalog (BPC-157 contains one Met, TB-500 several Met, semaglutide contains one Met, oxytocin contains Cys residues forming a native disulfide bridge to preserve) demand absolute vigilance. An opened, reconstituted vial left at room temperature loses several purity percentage points within hours of air exposure. Systematic glovebox handling, immediate aliquoting, controlled batch workflow: these habits separate reproducible research from datasets polluted by oxidation artifacts.