The four routes of degradation
Nearly all loss of peptide integrity comes down to four mechanisms. Each has a corresponding control, and the controls are the whole of storage practice.
| Mechanism | What happens | Control |
|---|---|---|
| Hydrolysis | Water cleaves peptide bonds, breaking the chain | Keep the material dry. This is the entire reason it ships lyophilised. |
| Oxidation | Susceptible residues react with oxygen, altering the molecule | Limit air and light exposure; keep vials sealed and dark. |
| Aggregation | Molecules associate and come out of solution | Avoid shaking, foaming and repeated temperature swings. |
| Microbial growth | Contaminants multiply in solution | Sterile technique; a preserved solvent for multi-entry vials. |
Methionine, cysteine and tryptophan are the residues most prone to oxidation. Whether a given compound is vulnerable depends on whether its sequence contains them, which is one practical reason to know the sequence of the material you hold.
Lyophilised material
In its freeze-dried state a peptide is comparatively robust. Water has been removed, so hydrolysis is largely arrested, and the material tolerates long storage provided it stays dry, cold and dark.
- Keep the vial sealed until the moment of use.
- Store cold and desiccated. Freezer conditions are conventional for long-term storage; refrigeration is generally considered adequate for shorter periods.
- Protect from light.
- Always allow the vial to reach room temperature before opening, otherwise condensation forms inside it.
Reconstituted material
Once in solution the material is markedly less stable, because water is now present in quantity and hydrolysis proceeds. Solutions are working stock with a limited life, not a storage format.
- Refrigerate promptly after preparation.
- Keep it out of light.
- Discard if the solution becomes cloudy, develops particulates, or changes appearance in any way.
- Assume a short useful life, defined by your own protocol, rather than assuming it keeps indefinitely.
Freeze-thaw cycling
Every freeze-thaw cycle stresses the molecule. Ice crystal formation and the concentration changes that accompany freezing both promote aggregation, and the damage accumulates across cycles rather than resetting each time.
Aliquot once, immediately after reconstitution, into single-use volumes. Thawing one small aliquot leaves the rest untouched. Repeatedly thawing and refreezing a single container subjects the entire stock to every cycle.
Quick reference
| State | Temperature | Light | Practical life |
|---|---|---|---|
| Lyophilised, sealed | Frozen, desiccated | Dark | Long; the stable format |
| Lyophilised, short term | Refrigerated | Dark | Shorter, still stable |
| In solution | Refrigerated | Dark | Short; working stock only |
| In solution, aliquoted | Frozen | Dark | Extended, but thaw each aliquot once only |
A note on published stability figures. Specific durations quoted for peptides in solution vary widely between sources and depend on the sequence, the solvent, the pH and the storage temperature. Read any single number as sequence-specific rather than universal, and prefer the supplier's own stability data for the material in hand.