Peptides do not have a single shelf life. They have several failure routes running in parallel, and storage conditions act on them unequally.

Hydrolysis

Water attacks the peptide bond, preferentially next to aspartate. This one is temperature-driven and water-driven, which is why the material is lyophilized, shipped with desiccant and kept cold. Dry and frozen, it is close to stopped.

Oxidation

Methionine, cysteine and tryptophan oxidise on exposure to air and light. Cold slows it. A sealed vial with the headspace displaced slows it more. A vial that has been opened, reconstituted and put back in the fridge with air above the solution is the worst case, and it is the common one.

Aggregation

The one cold does not fix, and can make worse. Freeze-thaw cycling drives aggregation directly, so a vial taken out and put back six times is in a worse state than one taken out once, even though every hour of that was spent at minus twenty.

• Aliquot on first reconstitution, so the stock is thawed once rather than once per experiment.
• Let a vial reach room temperature before opening it, or moisture condenses onto cold solid the moment the seal is broken.
• Keep it dark. Tryptophan and tyrosine do not need much light.
• Write the reconstitution date on the vial. The valid-until date on the certificate is for the dry material and stops applying the moment you add solvent.

Storage conditions for each product are on its own page — the specification block carries the temperature and the form as supplied.