Accepted answer
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 37 °C the same rule gives about 9.2 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.
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.
Light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Sequence determines which pathways apply, so general statements are general.
A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.
8Adding a vote because this deserves more of them. – t_oyelaran 2 months ago 7Two lots stored differently, reassayed at a year — the difference was smaller than I expected. – birk_nordahl 8 days ago add a comment