Storing lyophilised peptides: temperature, light and moisture

Almost everything in this catalog ships as a lyophilised powder. Lyophilisation, or freeze-drying, removes water by freezing a solution and then dropping the pressure so the ice sublimes straight to vapour without ever passing through a liquid phase. What is left is a dry cake, usually white and often barely visible at the bottom of the vial, because a few milligrams of peptide occupy very little space.

Dryness is the whole point. Peptide bonds hydrolyse in water, and side chains react with each other and with dissolved oxygen. Take the water away and most of that chemistry slows to a crawl.

What the storage line means

Each product page carries a storage line in its specification table, and there are only two in this catalog. Lyophilised peptides are specified at −20 °C, desiccated, protected from light. Sterile solutions, including the consumables, are specified at 2–8 °C, protected from light.

That line describes the conditions the purity specification is written against. It is a statement about how the material was qualified, not advice about a particular laboratory's arrangements. If a vial is held somewhere else, its stated purity no longer necessarily describes what is in it.

Each of the three conditions is aimed at a different failure mode. Temperature is the simplest: degradation reactions slow down as it drops, and while freezing does not stop them, a dry peptide held at −20 °C is stable over long periods. Desiccation matters because a freeze-dried cake is hygroscopic and will pull moisture out of whatever air it meets, undoing the reason for drying it. Light is the one people forget. Tryptophan, tyrosine, phenylalanine, methionine and cysteine are all photosensitive to some degree, and amber glass or an opaque box removes the variable for nothing.

Condensation and cold vials

The most common way moisture gets into a dry vial has nothing to do with the storage temperature. A vial taken from a freezer is far colder than the room, and opening it immediately lets humid air condense on the cold glass and the powder inside. Letting a vial reach room temperature before breaking the seal avoids that, and it costs nothing but a few minutes.

Repeated freezing and thawing causes trouble for the same underlying reason. Each excursion is another chance for condensation and another period spent at temperatures where reactions run faster. Manufacturers that supply peptides in ten-vial kits are partly solving this problem by packaging: a kit of ten smaller vials means a single vial is opened once rather than one large vial being opened ten times.

Shipping at ambient temperature

Kits leave the manufacturer at ambient temperature and travel by tracked international post. This is normal for lyophilised peptides — the dry state is what confers stability, and a few weeks in transit at room temperature is a much smaller insult than the same material would suffer in solution. The storage specification applies from the point the kit arrives.

Appearance on arrival

A lyophilised cake can look like a compact white plug, a thin film on the glass, or a scatter of powder that has come loose in transit. All three are ordinary. Movement during shipping breaks up the cake without changing its chemistry. A cake that has collapsed into a sticky or discoloured residue is a different matter. That appearance suggests the vial has taken on moisture or heat, and any kit arriving in that condition should be reported with the order reference before it is opened.

The batch report should stay with the vials it describes. Once a certificate and a lot are separated, there is no way to reunite them with certainty, and an untraceable vial is an unspecified one no matter how good the original report was.