Why does deamidation accelerate at room temperature rather than proceeding linearly?

The case in front of me: deamidation · room temperature.

I would like the mechanism, because I want to be able to reason about the cases nobody has written about.

I have tried to reason it out from first principles and got to two contradictory conclusions.

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 room temperature the same rule gives about 3.4 times the refrigerated rate, and another 10 K would roughly double it again. Asn and Gln lose the amide through a succinimide intermediate, so the product is one dalton heavier and usually resolves as a shoulder on the main peak rather than as a peak of its own. 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.

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.

Degradation pathway by condition

More usefully, 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.

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

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

edited 28 Dec 2024 by h_pergande — reworded for clarity after a comment

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.

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

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

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

edited 27 Apr 2025 by plate_count_9k — added the method parameters

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

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.

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.

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

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.

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.

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.

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.