Peptide Storage and Handling

Peptides are supplied lyophilised because water is the reagent in most of the reactions that destroy them. Freeze-drying removes it: the solution is frozen, then dried under vacuum so ice sublimates directly to vapour without passing through a liquid phase, leaving an amorphous glassy solid.

Two things follow. Hydrolysis of the peptide backbone needs water and effectively stops without it. And in a glassy solid, molecular mobility is low enough that reactions requiring molecules to find each other — aggregation, disulfide scrambling — proceed orders of magnitude more slowly than in solution. A lyophilised peptide is not inert, but it is operating on a timescale of years where the same peptide in solution operates on days.

Everything below is a consequence: keep it dry, keep it cold, keep it dark, and once it is in solution accept that the clock has started.

Chemical degradation follows Arrhenius behaviour: as a working approximation, reaction rate roughly doubles for every 10 °C increase. That single relationship explains the whole temperature hierarchy.

• Ambient. Lyophilised peptide tolerates room temperature for the days a shipment is in transit. It is a transport condition, not a storage condition.
• 2–8 °C. Reasonable for material in active use over weeks.
• −20 °C. The standard long-term condition for lyophilised peptide, and adequate for most sequences for a year or more.
• −80 °C. For multi-year storage, and for sequences known to be fragile — those containing cysteine, methionine, tryptophan, or Asn-Gly and Asp-Gly motifs.

A frost-free freezer is a poor choice for either form. Its automatic defrost cycle works by warming the compartment periodically, which is precisely the thermal cycling you are trying to avoid.

Lyophilised peptide is hygroscopic. The amorphous solid has a large effective surface area and will pull water out of the air readily, and once it does, hydrolysis and deamidation resume in the solid state.

This produces the single most important handling step in this article, and it is routinely skipped: let a cold vial reach room temperature before you open it. A vial taken from −20 °C and unstoppered immediately is a cold surface in humid air, and water condenses onto the solid directly. Twenty to thirty minutes on the bench, still sealed, is enough for a small vial. Reseal promptly, and where a sequence is known to be moisture-sensitive, store the closed vial in a secondary container with desiccant.

Visible signs of moisture ingress are worth knowing: a cake that has collapsed, gone glassy at the edges, or become tacky rather than free-flowing has taken up water.

Photo-oxidation is residue-specific rather than general. Tryptophan is the most photolabile of the twenty common amino acids, followed by tyrosine, histidine, methionine and cysteine. Sequences containing none of those are comparatively indifferent to light; sequences containing tryptophan should be handled as genuinely light-sensitive.

Amber glass, foil overwrap, or simply keeping vials in their opaque outer packaging handles this. The precaution costs nothing and the failure mode — a +16 Da oxidation product that shows up as a new shoulder on a chromatogram — is irreversible.

Reconstitution restores the water that lyophilisation removed, and with it every reaction the dry state suppressed. The main routes, in rough order of how often they cause trouble:

• Deamidation. Asparagine and, more slowly, glutamine lose their side-chain amide to give aspartate or glutamate. The rate is strongly sequence-dependent — Asn-Gly is the notorious motif, because the small glycine offers no steric hindrance to the cyclic succinimide intermediate. Deamidation is fastest at alkaline pH.
• Aspartimide formation and isomerisation. Aspartate residues can cyclise to a succinimide that reopens to a mixture of the original aspartate and an iso-aspartate with an altered backbone. Mass is unchanged, so this defect is invisible to a simple mass measurement and shows only as a new chromatographic peak.
• Oxidation. Methionine to the sulfoxide, cysteine to disulfides or higher oxidation states, tryptophan to a range of products. Dissolved oxygen and trace metal ions drive it.
• Hydrolysis. Backbone cleavage, fastest at Asp-Pro bonds and under acidic conditions.
• Aggregation. Sequences with a propensity to form beta-sheets can associate irreversibly, especially at higher concentration.

As a working expectation, a peptide in aqueous solution is best used within days at 2–8  °C, or held frozen for longer. Solutions near neutral to mildly acidic pH are generally more stable than alkaline ones, which is one of several reasons solvent choice is not arbitrary — see the peptide solubility guide for how to select one, and how to reconstitute peptides for the procedure.

Repeatedly freezing and thawing a peptide solution is more damaging than the time spent frozen, for reasons that are physical rather than thermal.

As a solution freezes, pure water crystallises first and everything dissolved in it is excluded into a shrinking unfrozen fraction. Solute concentration in that fraction climbs by orders of magnitude, which accelerates every concentration-dependent process, aggregation above all. At the same time, buffer components can crystallise at different rates and shift the pH of the remaining liquid substantially — sodium phosphate buffers are the classic example, capable of dropping several pH units on freezing as the dibasic salt crystallises out preferentially. And the growing ice-water interface itself is a surface at which peptides adsorb and unfold.

The remedy is to freeze once. Divide the reconstituted solution into single-use aliquots immediately after preparation, in volumes matched to how the material will actually be consumed, and thaw each aliquot only once.

Two details make aliquoting work properly. Use low-binding polypropylene tubes: peptides adsorb to plastic and glass surfaces, and at low concentrations the loss is a measurable fraction of the total rather than a rounding error. And thaw on ice or at 2–8 °C rather than in a warm hand or a water bath, so the sample spends as little time as possible in the partially frozen state where the concentration and pH effects above are at their worst.

Label every aliquot with the compound, the lot number from the original vial, the concentration, the solvent, and the date of preparation. Lot number matters more than it looks: it is the only thing connecting a tube in a freezer box back to a certificate of analysis, and without it an anomalous result six months later cannot be traced to a batch, a storage condition, or a preparation error.

• Peptide Solubility Guide
• How to Reconstitute Peptides
• What Are Peptides?
• Peptide Glossary