Accepted answer
At room temperature 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. Molecules associate without any covalent change, so the mass is unchanged and a reversed-phase run — which is performed in organic solvent — mostly dissolves the evidence before it can be measured. That is why the two measurements are not interchangeable and why an unchanged purity figure after an excursion to room temperature 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.
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.
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.
3Two lots stored differently, reassayed at a year — the difference was smaller than I expected. – Dr_Colm_Fitzhenry 8 days ago 2Does the same reasoning apply to material already in solution, or is that a different curve? – bac_or_bust 9 months ago add a comment