Peptides degrade by chemistry, and every one of those chemical routes needs water. That single fact explains almost everything about how to store them.

The degradation routes

Four things happen to a peptide over time in solution.

  • Hydrolysis cleaves the peptide backbone, accelerated at extremes of pH and by heat. Aspartic acid residues are particularly vulnerable, and Asp-Pro and Asp-Gly bonds are the classic weak links.
  • Oxidation hits methionine, cysteine, tryptophan and histidine. Dissolved oxygen, light and trace metal ions all drive it. This is the one that adds 16 daltons and shows up on a mass spec.
  • Deamidation converts asparagine and glutamine to acidic residues, with Asn-Gly sequences the fastest. Strongly pH and temperature dependent.
  • Aggregation, where molecules stick together into dimers and larger species that are not biologically active and can be immunogenic. Driven by concentration, agitation and freeze-thaw.

Take away the water and hydrolysis and deamidation almost stop. That is the whole point of lyophilisation.

Lyophilised powder

Freeze drying removes water by sublimation under vacuum, leaving a porous cake with typically 1 to 5 percent residual moisture. In that state most peptides are stable for years frozen, and for months to years refrigerated.

Practical figures, which vary by sequence:

  • At minus 20 degrees Celsius, sealed and protected from light, expect several years for a typical peptide
  • At 2 to 8 degrees Celsius, expect one to two years
  • At room temperature, expect weeks to a few months, with sequences containing methionine, cysteine or tryptophan at the shorter end

The last of those is the one people get wrong in the anxious direction. Lyophilised peptide surviving a few days in transit at ambient temperature is normal and expected. Reputable manufacturers ship lyophilised material without ice for exactly this reason. If a courier delay leaves a powder vial warm for a week, the material is very likely fine.

What matters more than temperature for powder is moisture ingress. Peptide cake is hygroscopic. If you take a cold vial out of the fridge and open it immediately, water condenses on the cold glass and gets absorbed into the powder. Let a vial come to room temperature fully, 20 to 30 minutes, before breaking the seal.

Reconstituted solution

Once you add water the clock speeds up dramatically. Refrigerated at 2 to 8 degrees, most reconstituted peptides hold for two to four weeks. At room temperature, days.

The diluent choice matters here. Bacteriostatic water contains 0.9 percent benzyl alcohol as a preservative, which suppresses bacterial growth and allows repeated needle entries into the same vial. Sterile water for injection has no preservative and should be treated as single use, or at most 24 hours refrigerated, because every needle entry is a contamination event. For anything you intend to draw from more than once, use bacteriostatic water.

Two caveats. Benzyl alcohol is not appropriate for neonates and is generally avoided in pregnancy. And a small number of peptides are physically incompatible with benzyl alcohol or with the pH it produces, so check the manufacturer's reconstitution guidance rather than assuming.

Technique when reconstituting: run the water down the inside wall of the vial slowly rather than firing it directly at the cake. Then swirl gently or let it dissolve on its own. Do not shake. Shaking creates an air-liquid interface with shear forces, and that is the classic driver of aggregation. If your solution goes cloudy, develops visible particles, or shows a film, discard it.

Freeze-thaw

Freezing a reconstituted solution is possible but each cycle costs you. As ice forms, the remaining liquid concentrates the peptide and the buffer salts and shifts local pH, and ice crystal surfaces denature protein at the interface. Repeated cycles compound this. Losses of a few percent per cycle are typical, more for aggregation-prone sequences.

If you must freeze solution, aliquot into single use volumes first so nothing gets thawed twice. Practically, for most people, reconstituting only what you will use in two to three weeks and keeping it refrigerated is simpler and better.

Light and container

Tryptophan, tyrosine, phenylalanine and cysteine are photosensitive. UV exposure drives oxidation and can break disulphide bonds. Amber vials help, and keeping the vial in its box in the fridge helps just as much. Peptides also adsorb to surfaces, particularly at low concentrations, so a very dilute solution in a plain plastic tube can lose a meaningful fraction of its content to the tube wall. Glass or low binding plastic is the right container.

Travelling

  1. Carry it on. Cargo holds cycle through wide temperature ranges and bags get lost. Medication is exempt from standard liquid limits at most security checkpoints, including OR Tambo and Cape Town International, but declare it at the scanner.
  2. Carry documentation. A prescription, a pharmacy label, or at minimum the product packaging. Peptides are not well understood by border officials and the paperwork resolves conversations quickly.
  3. Check the destination's law before you fly. This is the part that goes wrong. Several countries treat peptides that are unscheduled at home as controlled substances, and the UAE and Singapore in particular have severe penalties for possession of unapproved pharmaceuticals.
  4. Insulated pouch with a gel pack for reconstituted solution. Frozen gel packs are generally permitted through security if fully frozen at the checkpoint. Do not let the vial sit against the gel pack directly, since freezing a solution vial is worse than letting it warm slightly.
  5. For a trip of a week or less, lyophilised powder needs nothing. Ambient in your hand luggage is fine. Reconstitute at your destination.

Where the guidance runs thin

Most of the specific stability numbers in circulation come from manufacturer testing on particular sequences under particular buffer conditions, and they do not transfer cleanly between peptides. There is very little published peer-reviewed stability data on the specific compounds sold in this market at the specific concentrations people reconstitute them to. Treat the ranges above as sensible defaults rather than measured facts for your vial, and when a supplier gives you a stability figure, ask whether it came from a study or from a spreadsheet.