Accepted answer
two weeks is 14 days at a temperature where the chemistry all but stops and the physics does not. minus 20 °C is 25 kelvin below a refrigerator, and below the glass transition of a lyophilised cake the ten-degree rule of thumb stops applying at all — solid-state chemistry is not slow liquid chemistry, it is a different regime, and the failure modes that survive it are mechanical rather than chemical. In a frozen solution the solute is excluded from the growing ice, so the unfrozen fraction concentrates and the buffer's pH moves as one salt crystallises before the other. The damage is done at the transitions, and 14 days of stable hold between them contributes very little. Reconstituted material has no certificate; the one in the box describes the powder.
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.
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.
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.
edited 20 Feb 2026 by marta_okonkwo — added the method parameters