Light, Oxygen and Temperature: How Peptides Degrade
Storage advice is easier to follow when you know what it is protecting against. Peptide degradation is not one process but several, each affecting particular residues and each leaving a recognisable signature.
Oxidation
Three residues are meaningfully oxidation-prone. Methionine oxidises to the sulfoxide, adding 16 mass units. Cysteine oxidises to form disulfides, either intramolecular or between molecules. Tryptophan oxidises through several routes and is also the most light-sensitive residue.
Oxidation products are usually more polar than the parent and therefore elute earlier on reversed-phase HPLC. An early-eluting peak that grows over a vial’s life is the classic signature. Minimising headspace oxygen and keeping vials sealed until use are the practical mitigations.
Hydrolysis
Water attacks the peptide backbone, cleaving it. Aspartate-proline bonds are particularly labile. This is the route that lyophilisation is designed to suppress: with the water largely removed, the chemistry has no medium. It is also why reconstituted solutions have a working life measured in days while lyophilised powder keeps for far longer — see water content in lyophilised peptides.
Deamidation
Asparagine and glutamine side chains lose their amide group, converting to aspartate and glutamate. The mass change is small — one unit — but the charge change is not, so deamidated species often separate cleanly on ion-exchange chromatography while being hard to resolve by mass alone. Asparagine followed by glycine is the most susceptible motif. Rate rises sharply with pH and temperature.
Aggregation
Not a covalent change, but a practical one. Peptides can associate into oligomers and eventually visible particulate. Aggregation is promoted by concentration, agitation, freeze–thaw and by shear from vortexing or sonication — which is the reason reconstitution advice says to swirl rather than shake. See choosing a reconstitution solvent.
Light
Photodegradation matters mainly for tryptophan-containing sequences, and to a lesser extent tyrosine and phenylalanine. DSIP, with an N-terminal tryptophan, is a clear case — see our note on DSIP. Amber vials or foil wrapping and minimal bench time under ambient light are cheap precautions.
What this means for storage
Every one of these routes slows at lower temperature, which is why refrigeration at 2–8 °C is the single most effective measure. Equilibrate a sealed vial to room temperature before opening it so atmospheric moisture does not condense onto the powder, keep solutions briefly and refrigerated, and prepare fresh where the experiment allows. Full guidance: storage and stability.
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