Conditions: semaglutide · 5 mg/mL.
Please show the division. I want to check my own against yours.
I would like the general form as well as the specific number, so I can apply it again.
Where is my error, and what is the correct working?
5 mg/mL is 5000 µg/mL — roughly 500 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 5 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.
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.
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.
Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.
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.
edited 9 Mar 2025 by loss_on_drying — added the method parameters
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Browse resultsAnswering 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.
In practice, 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.
Deamidation via the succinimide intermediate is well characterised, with sequence-dependent rates highest for asparagine-glycine motifs.
Cold, dry, dark, still. Those four words cover most of the mitigation.
edited 8 Apr 2025 by lyoph_cake — reworded for clarity after a comment
In practice, asparagine and glutamine are the deamidation risk, and methionine is the oxidation risk.
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.
Light exposure matters for tryptophan-containing sequences and for anything with a chromophore. Amber vials and a closed box are free mitigations.
Apparent loss in a dilute preparation is usually adsorption rather than degradation and is worth ruling out first.
Swirl, never shake. Aggregation is a handling problem more than a time problem.
The short version: water enables most of it, oxygen enables oxidation, surfaces enable adsorption, and agitation enables aggregation.
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.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Sequence determines which pathways apply, so general statements are general.
At dilute concentrations, suspect adsorption before you suspect chemistry.
Stated carefully, this is answerable from the chemistry rather than from anecdote, which is unusual and welcome.
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.
The caveat is that none of these pathways can be seen by looking at a vial, and a clear solution can be substantially degraded.
Sequence decides which pathways are even available. Check the residues.
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.