Almost every research peptide is supplied as a dry powder in a glass vial. That powder is lyophilized, or freeze-dried. The reason is simple: a peptide deteriorates in water, and hardly at all when dry.
What goes wrong in solution
Water makes a number of degradation reactions possible:
- Hydrolysis. The bond between two amino acids breaks, and the chain falls apart.
- Oxidation. Methionine, cysteine, and tryptophan in particular react with oxygen.
- Deamidation. Asparagine and glutamine lose an amide group, which changes the charge of the peptide.
- Aggregation. Chains clump together and precipitate.
How fast this happens depends on the sequence, the pH, and the temperature. In a dry powder, these reactions come to a near standstill.
Freeze-drying in three steps
- Freezing. The peptide solution is frozen. The water turns to ice.
- Primary drying. Under vacuum, the ice turns directly into vapor without melting first. This is called sublimation.
- Secondary drying. The temperature is raised slightly to remove the last of the bound water.
Because the material is never liquid and warm at the same time, the structure of the peptide stays intact. What remains is a light, porous cake or a thin layer at the bottom of the vial.
What the powder looks like
A few milligrams of peptide is very little material. The vial can look almost empty, or the powder may sit as a thin film against the glass. The cake can also come loose or crumble during transport. None of this says anything about quality. The quantity and the purity are stated on the certificate of analysis.
Storage
Lyophilized material has three enemies: moisture, heat, and light.
- Keep the vial closed, cool, and dark. Follow the storage temperature on the label.
- Let a cold vial reach room temperature before you open it. Otherwise, moisture from the air condenses on the powder.
- Avoid repeated warming and cooling.
The powder attracts water. The shorter a vial stays open, the better.
And after that?
Once a peptide is dissolved, the degradation reactions described above apply again. How long a solution remains usable depends on the compound and the solvent. Record in your lab notebook when a solution was prepared and how it was stored, so that measurements can be traced back later.
You will find more terms from this article in the glossary.
For a deeper look at storage, the degradation routes per amino acid and receiving a shipment, see Storing peptides in the lab.
This article is background information on laboratory research. It is not medical advice or instructions for use. All products are for research use only.