Why does oxidation accelerate at 4 °C rather than proceeding linearly?

Concretely: oxidation · 4 °C.

This is one of those things that everyone repeats and nobody derives.

This matters practically, not just academically, because it changes what I would do next.

Is the standard explanation correct, and if so, what is the evidence for it?

Because temperature enters the rate constant through an exponential, so equal steps in temperature multiply the rate instead of adding to it. Arrhenius puts the rate proportional to exp(−Ea/RT); the working approximation is a doubling per 10 K, which takes 5, 15, 25 and 35 °C to multipliers of 1, 2, 4 and 8. The steps in temperature are equal and the steps in rate are not, and that is the whole of the observation. At 4 °C the same rule gives about 0.9 times the refrigerated rate, and another 10 K would roughly double it again. Met and Trp take up oxygen sixteen daltons at a time, and the oxidised species is more polar, so on a reversed-phase column it elutes ahead of the parent rather than behind it. Ea differs by route, so the ranking of routes changes with temperature too — which is why accelerated data extrapolates badly and why nobody should read a 40 °C study as a fast version of a 5 °C one.

The relevant point is that a mass shift of plus one dalton is deamidation and plus sixteen is oxidation, so degradation is often visible in a mass spectrum if anyone looks.

Hydrolysis cleaves the backbone, most readily at aspartate-proline and aspartate-glycine sequences, and is acid-catalysed. In a dry solid it barely proceeds at all.

Reported and extrapolated stability by condition

Windows for the solid state are chemical; windows for solution are microbiological and usually shorter than the chemical limit.

Mechanically, freeze-thaw cycling drives aggregation through concentration at the ice interface and pH shifts as buffer components crystallise out at different rates. Each cycle costs something.

Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.

Nothing here is medical advice, and research-use compounds are not approved for human use.

Sequence decides which pathways are even available. Check the residues.

edited 9 May 2025 by thabo_maseko — added a caveat about sampling

Put another way, asparagine and glutamine are the deamidation risk, and methionine is the oxidation risk.

Oxidation targets methionine, cysteine and tryptophan, adding sixteen daltons per oxygen. It is catalysed by trace metals and promoted by dissolved oxygen and by light.

To be exact about it, light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.

Deamidation via the succinimide intermediate is well characterised, with sequence-dependent rates highest for asparagine-glycine motifs.

Sequence determines which pathways apply, so general statements are general.

Cold, dry, dark, still. Those four words cover most of the mitigation.

edited 9 May 2025 by marta_okonkwo — corrected a unit error in the worked example

Start with the sequence, because which pathways are available depends on which residues are present.

Aggregation is physical: peptides unfold at air-liquid interfaces and associate. Shaking maximises that interface, which is why swirling and shaking produce visibly different outcomes on the same vial.

Adsorption onto glass and plastic is significant at low concentrations — micrograms per millilitre — and negligible at milligrams per millilitre. It is the usual explanation for an apparent loss in a dilute preparation.

Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.

Apparent loss in a dilute preparation is usually adsorption rather than degradation and is worth ruling out first.

A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.

Aggregation is a physical process and is the one most often caused by handling rather than by time.

A mass spectrum resolves most of this: minus eighteen is dehydration or succinimide, plus one is deamidation, plus sixteen is oxidation, and an unchanged mass with a shifted retention time is an isomer.

At dilute concentrations, suspect adsorption before you suspect chemistry.

Answering this needs the physical state, since a dry powder is protected from most of these and a solution is protected from none.

Deamidation converts asparagine or glutamine to the corresponding acid via a succinimide intermediate, adding one dalton. It is base-catalysed, accelerates above neutral pH and is the dominant aqueous pathway for many peptides.

Swirl, never shake. Aggregation is a handling problem more than a time problem.