Lyophilized

Lyophilisation is freeze-drying. The solution is frozen, then held under vacuum so the ice converts straight from solid to vapour without passing through liquid. What is left is a dry cake of the compound and whatever else was in the vial before drying.

The reason it is done is stability. Peptides in solution degrade by routes that all need water: hydrolysis of the peptide bonds, oxidation, aggregation. Take the water out and most of that chemistry stops. A lyophilised vial stored properly can be stable for a long time, which is why it is the standard presentation for peptides that would not survive shipping as a liquid.

What is in the vial besides the peptide

A lyophilised cake is rarely pure compound. Formulations usually include bulking agents such as mannitol to give the cake structure, cryoprotectants to shield the molecule during freezing, and buffer salts. There will also be residual moisture, because drying is never absolute, and counter-ions left from synthesis, commonly acetate or trifluoroacetate.

This is the reason a vial labelled 5 mg may contain less than 5 mg of peptide. The label often describes total solids. Net peptide content is the figure that answers what fraction is actually the compound, and it is the number most often missing from a certificate of analysis. Where it is absent, the concentration you calculate after reconstitution is an upper bound rather than a measurement.

Reading the cake

The appearance of the cake carries some information, though less than is often claimed.

• A firm, intact cake is the expected result and tells you the drying cycle completed normally.
• Collapse, where the cake has slumped or looks glassy, means the temperature rose above the formulation's collapse point during drying. It is associated with higher residual moisture.
• A cake that has come loose and rattles is usually cosmetic, caused by shipping.
• Visible discolouration is not normal.

What appearance cannot tell you is identity, purity or potency. A cake can look textbook and be the wrong compound. Those questions need instruments, which is the entire argument for a certificate.

Two shelf lives, not one

This is the distinction that matters practically and it is regularly collapsed into a single number.

The dry vial is stable for a long period, commonly quoted in months to years when refrigerated, because the degradation routes need water. Storage conditions still matter: temperature, and protection from light for compounds sensitive to it.

The reconstituted vial is a different object with a different clock. Once water is added the chemistry restarts. The window usually quoted is around 28 days refrigerated, and as covered on the bacteriostatic water page, that figure is a property of the preservative rather than of the compound. For most research peptides there is no published stability study in solution at all.

So "how long does it keep" has two answers depending on which vial is being asked about, and conflating them is how a compound gets stored for a year in solution on the strength of a dry-powder figure.

Handling the transition

Reconstitution is the moment the dry vial becomes the wet one. The conventional guidance is to add the diluent slowly down the inside wall of the vial rather than directly onto the cake, and to swirl rather than shake. The reasoning is mechanical: peptides can be denatured by shear and by the air-liquid interface that foaming creates, and neither adds anything.

Repeated freeze-thaw cycles are the other avoidable insult. Each one is an opportunity for aggregation, which is why splitting a reconstituted vial into single-use aliquots is preferred over freezing and thawing the same vial repeatedly.

The reconstitution calculator handles the arithmetic once the water is in.

Related terms: bacteriostatic water, reconstitution, certificate of analysis.