Accepted answer
25 °C is 20 kelvin above the 5 °C middle of a 2–8 °C refrigerator, which the ten-degree rule of thumb makes about 4 times the refrigerated rate — but the rule averages every route at once, and it is the ranking that changes with temperature, not just the speed. In aqueous solution at 25 °C the chemical routes that lead are deamidation at Asn, which proceeds through a succinimide and leaves a product one dalton heavier, and hydrolysis of the backbone, which leaves a mass ladder of shorter fragments. Both are strongly pH-dependent, and deamidation accelerates sharply above pH 7. Running alongside them is a physical route with no covalent change at all: association and aggregation, driven by the air-liquid interface rather than by temperature, and largely invisible to a reversed-phase method because the run is performed in organic solvent. So the honest answer is that at 25 °C deamidation usually leads on a chemical assay while aggregation leads on a plate that has been shaken, and which one you find is partly a statement about which method you chose. 4 times a refrigerated rate is an order-of-magnitude statement about a rate, not a shelf life. 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.
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.
It helps to be literal here: 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.