Storing lyophilised peptides: temperature, moisture, and light

Lyophilisation exists because water is the medium in which most degradation chemistry happens. Remove it and hydrolysis slows dramatically, deamidation slows, and the material becomes storable on a timescale of years rather than days.

That stability is contingent on staying dry. A lyophilised cake is hygroscopic and will pull moisture from room air given the opportunity, which is why the seal and the headspace matter as much as the freezer temperature. Vials backfilled with an inert gas and closed with an intact stopper preserve the dry state; a vial that has been opened repeatedly in a humid room has been partly undone.

Visible cake collapse — a powder that has slumped, become glassy, or gone from fluffy to dense — is a moisture signal worth taking seriously before use.

Colder is better, with diminishing returns. For long-term storage of lyophilised material, −20 °C is the common working standard and −80 °C is used where inventory will be held for years or where the sequence is known to be fragile. Refrigeration at 2–8 °C is appropriate for material in active use over shorter periods.

Short excursions to ambient temperature during shipping are generally tolerable for lyophilised powder, which is precisely why it is shipped in that form. The risk from a brief warm period is far lower than the risk from a solution experiencing the same excursion.

The more damaging pattern is cycling. Repeatedly moving vials between freezer and bench produces condensation on cold glass each time the vial meets room air, and that water ends up in the cake. Letting a vial equilibrate to room temperature before opening it is the single cheapest habit in peptide handling.

Sequences containing tryptophan, tyrosine, phenylalanine, methionine or cysteine are photosensitive to varying degrees. Amber vials, foil overwrap, or simply a closed freezer box address this adequately; the requirement is ordinary care rather than darkroom conditions.

Oxygen drives the other major pathway. Methionine and cysteine oxidise readily, and cysteine-containing peptides can additionally form disulfide-linked dimers. Inert headspace and an intact seal are the defence, which is another argument for aliquoting rather than repeatedly opening a single stock vial.

Once in solution, the stability calculus changes completely. Water is back, so hydrolysis and deamidation resume; the peptide is dilute, so adsorptive losses to container walls become proportionally significant; and if the solution is not sterile, microbial growth is possible.

Solutions are best treated as short-lived. Refrigerated, expect days. Frozen in aliquots, expect longer but not indefinitely, and remember that each freeze-thaw cycle is itself a stress. Aliquoting at the point of reconstitution — rather than repeatedly thawing one tube — is what makes frozen storage worthwhile.

Labelling matters more than it seems. A tube whose concentration, solvent, lot and date are not written on it is, in practice, a tube of unknown material, however carefully it was prepared.