Does hydrolysis dominate for retatrutide held at 25 °C?

Numbers first: hydrolysis · retatrutide · 25 °C.

I can predict the outcome but I cannot explain it, which means I will get the next case wrong.

I would like to know how confident the field actually is about this.

What is actually going on here, physically?

At 25 °C the question is which route is fastest, not whether hydrolysis 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. Backbone amide bonds cleave, so every product is shorter than the parent and the mass ladder they leave behind is the evidence that it happened. So the way to answer it for your vial is to pick the method that sees hydrolysis specifically and run it against a control held cold, rather than to infer a mechanism from a purity number that averages all of them.

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

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.

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

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

The short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.

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.

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

At dilute concentrations, suspect adsorption before you suspect chemistry.

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

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.

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

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

edited 8 Aug 2024 by nils_karlberg — corrected a unit error in the worked example

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.

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

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

The underlying point is that 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.

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

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

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