Short answer
Lyophilization removes water from a frozen peptide solution under vacuum, leaving a dry, porous powder called a cake. Dry peptides are more stable than peptides in solution because water drives hydrolysis, deamidation and other degradation.
But freeze-drying does not purify the material and does not guarantee a particular amount of peptide, so identity, purity, content and moisture still have to be measured and recorded for each batch.
Key facts
- Lyophilization has three stages: freezing, primary drying by sublimation and secondary drying to remove bound water.
- The result is a porous cake whose appearance can vary from batch to batch without a change in quality.
- Residual moisture is measured by Karl Fischer titration and is typically a few percent for peptides.
- Peptide powders are hygroscopic, so exposure to humid air raises the moisture content.
- A lyophilized appearance says nothing about purity, content or identity.
Why peptides are freeze-dried
Most degradation reactions of peptides need water: hydrolysis of peptide bonds, deamidation of asparagine and glutamine, oxidation reactions that are driven by dissolved oxygen, and aggregation. Removing water slows these reactions substantially. After synthesis and purification, the peptide is in a mixture of water and acetonitrile, and lyophilization is the gentle way of turning it into a solid without heating it.
The three stages
In the first stage the solution is frozen so that water becomes ice. In primary drying the pressure is lowered and the ice sublimes directly to vapour, which removes most of the water while the product stays frozen. In secondary drying the temperature is raised a little under vacuum to desorb water that is bound to the solid. The process is controlled so that the cake does not melt or collapse.
What the cake looks like and what it means
A lyophilized peptide is often a white, fluffy or compact cake. Its volume and texture depend on the concentration, the vial, the excipients and the drying cycle. A small cake or a powder that has broken into pieces is not by itself a sign of poor quality.
A collapsed, shrunken or yellowed cake can indicate that moisture entered or that the peptide has degraded, so the appearance on arrival should be recorded together with the batch information.
Residual moisture and hygroscopicity
Even after drying, peptide powders contain some water, and many take up moisture from the air quickly, especially salts with acidic or basic side chains. Residual moisture is measured by Karl Fischer titration and is part of the net peptide content calculation.
A vial that is opened in humid air before it has reached room temperature can pick up water, which is one reason why the handling conditions should be written into the laboratory record.
What lyophilization does not tell you
A freeze-dried powder can be impure, can contain the wrong peptide or can contain a lower amount of peptide than the label states. It does not remove impurities, counter-ions or salts. Identity (by mass spectrometry), purity (by HPLC) and content (by amino acid analysis or NMR) must be established separately, and a certificate of analysis for that batch should state them.
Documenting a lyophilized batch in the laboratory
On receipt, record the batch number, the appearance of the cake, the condition of the seal and the date. Follow the product-specific storage information supplied with the batch, because a generic temperature rule cannot replace compound-specific stability data. Record any excursions from the intended conditions and keep the vial closed until it has equilibrated to room temperature.
The guide on documenting storage conditions on this site gives a template.
Frequently asked questions
Does lyophilization sterilise a peptide?
No. Freeze-drying removes water but does not kill or remove microorganisms or endotoxin. Sterility is a separate property that has to be tested.
Is a lyophilized peptide stable indefinitely?
No. Dry peptides are much more stable than solutions, but they still degrade slowly, and the rate depends on the sequence, the moisture, the temperature and the counter-ion.
Why is there so little powder in a vial?
A few milligrams of peptide form a very small cake. The vial size is chosen for handling, not to match the volume of the powder.
Primary references
Related guides
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ChecklistLaboratory Receiving Inspection for Lyophilized Research PeptidesA receiving workflow for package condition, label agreement, batch documentation, seals, photographs, quarantine, and inventory release.
GuidePeptide Analytical Methods Compared: HPLC, LC-MS, Amino Acid Analysis, NMR and Karl FischerWhich test answers which question: identity, purity, content, water, counter-ion, stereochemistry and sterility, with the strengths and…
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Compound references · Research areas · Glossary · Methodology · All resources