How to store peptides

Water is what degradation needs. Hydrolysis cleaves peptide bonds, oxidation proceeds through species carried in solution, and aggregation requires molecules mobile enough to find each other. Remove the water and all three slow dramatically.

That is why a lyophilised vial is a fundamentally different storage problem from a reconstituted one. Dry, cold and dark, a peptide can hold for a long time. In solution, the clock runs whatever you do, and your job is only to slow it.

This is the one rule with no exceptions in ordinary use. When a solution freezes, ice crystals form and grow, and the peptide is excluded into a shrinking volume of unfrozen liquid where it becomes far more concentrated than intended. The combination — mechanical damage from crystal edges, extreme local concentration, and the shift in pH as buffer components crystallise out at different rates — unfolds and aggregates peptide.

It also does not reverse on thawing. A vial that has been frozen and thawed may look completely normal and contain a meaningful fraction of aggregated, inactive material.

The practical implication is about fridge geography, not intent. The back wall of a domestic refrigerator and the shelf directly beneath the freezer compartment both dip below freezing regularly. Store vials toward the middle, and never in the door, where the temperature swings every time it opens.

Ultraviolet and short-wavelength visible light drive photo-oxidation, which hits certain amino acid residues particularly hard. Most vials are supplied in cartons for exactly this reason. Keep them in the carton, and if you decant to a storage box, use an opaque one.

Heat accelerates every degradation pathway at once, and the exposure that matters is usually cumulative rather than dramatic — a vial that sits out on a counter for an hour each day is being warmed repeatedly, and none of that is recovered by putting it back.

Repeated freeze-thaw cycling deserves its own mention even for dry vials. If you keep lyophilised stock frozen and remove it periodically, each cycle is a stress event. Where a vial will be entered repeatedly, keeping the working vial refrigerated and the reserve frozen avoids cycling the whole supply.

Reconstituted vials need to stay cold in transit, and the practical answer is an insulated cooler with gel packs rather than loose ice — loose ice melts into water that gets everywhere and, as it melts, provides no temperature floor. Keep vials from touching a frozen pack directly, since direct contact can drop the vial below freezing.

Air travel adds two constraints worth planning around: medication and the syringes that go with it belong in carry-on rather than checked baggage, because cargo holds are neither temperature-controlled nor recoverable if a bag goes astray; and keeping items in original labelled packaging, with any documentation you have, makes security screening straightforward. Rules vary by carrier and jurisdiction, so check rather than assume.