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

Details up front: hydrolysis · 4 °C.

I can predict the outcome but I cannot explain it, which means I will get the next case wrong.

I would like to know how confident the field actually is about this.

What is actually going on here, physically?

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. Backbone amide bonds cleave, so every product is shorter than the parent and the mass ladder they leave behind is the evidence that it happened. 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 short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.

Light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.

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.

Aggregation at air-liquid interfaces is established from surface-tension and particle-count studies and is the basis for anti-agitation handling guidance.

At dilute concentrations, suspect adsorption before you suspect chemistry.

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.

The underlying point is that 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.

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

Answer first: the degradation pathways worth knowing are hydrolysis, deamidation, oxidation, aggregation and adsorption, and each has a different trigger and a different mitigation.

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.

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.

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

The honest answer is that most reported "degradation" is adsorption and dilution error rather than chemistry.

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.

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

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

This is answerable from the chemistry rather than from anecdote, which is unusual and welcome.

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.

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

edited 5 Aug 2024 by micron22 — clarified the distinction between purity and content