Does fibrillation of semaglutide at room temperature show up as a loss of content or of purity?

Setup, so nobody has to ask: fibrillation · semaglutide · 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?

At room temperature 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. Ordered beta-sheet assembly, effectively irreversible, and its endpoint is opalescence you can see rather than a peak you can integrate. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to room temperature is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.

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

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.

Degradation pathway by condition

Stated carefully, 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.

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

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.

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.

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.

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

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.

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.

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

edited 1 Jan 2026 by siobhan_deasy — clarified the distinction between purity and content

The part that matters: asparagine and glutamine are the deamidation risk, and methionine is the oxidation risk.

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.

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

The underlying point is that aggregation is a physical process and is the one most often caused by handling rather than by time.

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.