The particulars: tirzepatide · 6.67 mg/mL.
I would like the arithmetic checked rather than the conclusion asserted.
I have deliberately not used an online calculator because I want to be able to check the result.
What is the general form of this calculation?
6.67 mg/mL is 6670 µg/mL — roughly 667 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 6.67 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.
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.
| Pathway | Dominant when | Detected by |
|---|---|---|
| Deamidation | Solution, neutral to alkaline pH | RP-HPLC, +1 Da on MS |
| Oxidation | Light, trace metals, peroxides | RP-HPLC, +16 Da on MS |
| Hydrolysis | Solution, extremes of pH | RP-HPLC, fragment masses |
| Aggregation | Agitation, interfaces, high concentration | SEC, visual haze; often invisible on RP-HPLC |
| Freeze-concentration damage | Freeze-thaw of buffered solution | SEC, loss of recovered content |
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.
Cold, dry, dark, still. Those four words cover most of the mitigation.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsAsk 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.