Storing lyophilised peptides
Temperature, light, moisture and the one handling mistake that undoes careful storage.
Lyophilised peptide is stable, but stability is conditional. The conditions are not complicated, and most degradation in practice comes from a small number of avoidable errors.
Temperature
Cold and dry is the rule. For short periods, refrigeration is generally adequate. For longer-term storage, a freezer is the normal choice, and deep-freeze conditions are used where material needs to be held for extended periods.
What matters as much as the absolute temperature is stability of temperature. Repeated freeze-thaw cycling is harder on peptide material than simply being held slightly warmer throughout. A vial that is taken out, warmed, returned and re-frozen several times is being treated worse than one kept steadily in a fridge.
Moisture, and the condensation trap
This is the error that catches people. A vial taken straight from cold storage and opened immediately will draw moisture from the room air onto the cold glass and the cold powder. The material is hygroscopic, and the water it absorbs is exactly what lyophilisation was meant to remove.
Allow a sealed vial to reach room temperature fully before breaking the seal. Depending on vial size this takes a reasonable number of minutes, and it is worth the wait.
Light
Several amino acid residues — tryptophan, tyrosine, methionine and cysteine among them — are susceptible to photodegradation and oxidation. Keep vials in their packaging or in a dark container rather than on an open bench under laboratory lighting.
Oxygen
Vials are typically sealed under an inert atmosphere. Once a seal is broken that protection is gone. Peptides containing methionine or cysteine are the most oxidation-prone and benefit most from minimising the time a vial spends open.
In solution
Once reconstituted, the clock speeds up considerably. Solutions are far less stable than dry powder, and should be treated as short-lived working material rather than storage. Aliquoting into single-use portions avoids repeatedly freezing and thawing the same stock.
Records
Note the date a vial was opened and the conditions it has been held under. When a result looks anomalous, storage history is one of the first things worth being able to check, and memory is not a record.