Lyophilised powder, water, and the cold chain

Peptides are shipped as lyophilised powder for one reason: water is what lets them come apart. Freeze-drying removes it, and the resulting dry cake is far more stable than the same material in solution.

What degrades a peptide, and how

Several routes, and most of them need water. Hydrolysis cuts the backbone. Oxidation attacks particular residues — methionine, cysteine and tryptophan are the usual ones — and is driven by oxygen, light and trace metals. Deamidation converts asparagine and glutamine into other residues, changing the molecule while barely changing its mass. Aggregation does not break anything but takes material out of solution.

This is why dry, cold and dark are the three conditions worth protecting, and why repeated warming and cooling is worse than a single stable temperature: each cycle moves water around inside the vial.

Which numbers are real

The storage figures quoted around this market — minus twenty for powder, four degrees for solution, twenty-eight days once opened — are a mix. The temperature guidance follows straightforwardly from the chemistry. The twenty-eight day figure does not: it comes from the labelling convention for bacteriostatic water, and it describes how long a preserved diluent is considered good for repeated access, not how long a particular peptide stays intact in it.

Why bacteriostatic water rather than sterile water

Bacteriostatic water contains roughly 0.9% benzyl alcohol as a preservative. Bacteriostatic means it prevents bacteria from multiplying — it is not sporicidal and it does not sterilise. Sterile water has no preservative, which is why it is treated as single-use.

Benzyl alcohol is not inert towards every peptide, and it is not the right solvent for all of them. Some need an acidic diluent to dissolve properly, which is why acetic acid water exists alongside it.

Handling the material

• Let a vial reach room temperature before opening it. Cold glass pulls moisture out of the air and into the powder.
• Direct the stream of diluent down the side of the vial rather than onto the cake.
• Swirl rather than shake. Agitation drives aggregation and foaming at the air-liquid interface.
• Keep it out of the light, and keep the temperature steady rather than optimal-but-fluctuating.
• Cloudiness, stringiness or visible particles after reconstitution mean the material did not go cleanly into solution.

These are handling notes for a laboratory reference material. They are not a preparation protocol, and we do not publish one.