Accepted answer
At 37 °C 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. Met and Trp take up oxygen sixteen daltons at a time, and the oxidised species is more polar, so on a reversed-phase column it elutes ahead of the parent rather than behind it. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to 37 °C is weak evidence: the method may simply be integrating the degradant along with the parent and reporting the sum as one peak.
Start with the sequence, because which pathways are available depends on which residues are present.
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.
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.
Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.
Sequence decides which pathways are even available. Check the residues.
3Worth adding that residual moisture predicts this better than any printed date. – Dr_Yusuf_Adeyemi 9 months ago 2Is there a reason to prefer minus eighty here, or is minus twenty genuinely enough? – Dr_Aoife_Brennan 8 months ago add a comment