What I have: a GLP-1 receptor agonist · 10 mg/mL.
I can do the algebra. I am not confident about the conversion factors.
If there is a standard way to lay this out, I would rather learn that than invent one.
What is the general form of this calculation?
10 mg/mL is 10000 µg/mL — roughly 1000 times the concentration at which surface adsorption is measurable. Losses to glass and plastic matter in the low microgram-per-millilitre range, where a monolayer on the wall is a real fraction of what is in solution. At 10 mg/mL that same monolayer is a rounding error. If you see an apparent loss at this concentration, suspect the dilution step or the assay before you suspect the wall.
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.
On the detail: light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.
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.
edited 8 Aug 2024 by kwn_analytical — corrected a unit error in the worked example
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe 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.
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.
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.
Swirl, never shake. Aggregation is a handling problem more than a time problem.
The honest answer is that most reported "degradation" is adsorption and dilution error rather than chemistry.
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.
Sequence decides which pathways are even available. Check the residues.
Answering this needs the physical state, since a dry powder is protected from most of these and a solution is protected from none.
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.
A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.
Concretely, this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
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.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Cold, dry, dark, still. Those four words cover most of the mitigation.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.