Does aspartimide formation of liraglutide at minus 20 °C show up as a loss of content or of purity?

The case in front of me: aspartimide formation · liraglutide · minus 20 °C.

This is one of those things that everyone repeats and nobody derives.

This matters practically, not just academically, because it changes what I would do next.

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

At minus 20 °C it can show up as either, and which one depends entirely on whether the product still elutes under the main peak. Purity is a ratio of areas, so a degradant only costs purity if the method resolves it. Content is a mass against a standard, so a degradant costs content whenever the parent is consumed — resolved or not. 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. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to minus 20 °C is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.

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

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.

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

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

Concretely, asparagine and glutamine are the deamidation risk, and methionine is the oxidation risk.

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.

On the detail: 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.

Apparent loss in a dilute preparation is usually adsorption rather than degradation and is worth ruling out first.

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

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

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.

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.

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 28 Apr 2025 by pierce_count — expanded the table to cover the lower concentration

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

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 determines which pathways apply, so general statements are general.

At dilute concentrations, suspect adsorption before you suspect chemistry.

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

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.

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.