Why does fibrillation accelerate at minus 80 °C rather than proceeding linearly?

The particulars: fibrillation · minus 80 °C.

I understand the observation; what I do not understand is the mechanism behind it.

I have read the two review articles that come up first and both assert this without a citation to a primary source.

Can someone derive this rather than assert 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 minus 80 °C the same rule gives no useful multiplier at all, because below freezing the reaction is no longer happening in bulk solution. Ordered beta-sheet assembly, effectively irreversible, and its endpoint is opalescence you can see rather than a peak you can integrate. 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.

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.

On the detail: 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.

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

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

edited 17 Dec 2024 by sinead_gaffney — added a caveat about sampling

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.

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.

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

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

edited 19 Nov 2024 by forty_two_c — added the placebo-arm figures

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

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

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.

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

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

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

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.

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

The caveat is that none of these pathways can be seen by looking at a vial, and a clear solution can be substantially degraded.

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

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.

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.

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

At dilute concentrations, suspect adsorption before you suspect chemistry.