Does aspartimide formation dominate for liraglutide held at 25 °C?

The case in front of me: aspartimide formation · liraglutide · 25 °C.

I want to know whether this is a real physical effect or an artefact of how it is measured.

What prompted the question is an inconsistency between two sources I otherwise trust.

Why does this happen, and what would falsify the usual explanation?

At 25 °C the question is which route is fastest, not whether aspartimide formation happens — and the routes do not share an activation energy, so their ranking changes with temperature. 25 °C is 20 kelvin above the 5 °C middle of a 2–8 °C refrigerator. The ten-degree rule of thumb — degradation rate roughly doubling per 10 K — makes that about 4 times the refrigerated rate, which is an order-of-magnitude statement and not a shelf life. That multiplier is an average over every route at once, which is exactly why it cannot tell you which one wins. A cyclic imide at Asp, eighteen daltons lighter, which then reopens to a mixture including the iso-aspartyl form — same formula as the parent, different molecule, and invisible to a mass-only method. So the way to answer it for your vial is to pick the method that sees aspartimide formation specifically and run it against a control held cold, rather than to infer a mechanism from a purity number that averages all of them.

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

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

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.

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.

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.

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.

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.

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.

Specifically, 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.

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

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.

At dilute concentrations, suspect adsorption before you suspect chemistry.