How to store peptides

Peptides are stable as dry powders, but several things wear them down:

• Moisture. Peptide powders are hygroscopic: they pull water from the air. Absorbed water lowers the peptide content of what you weigh out and can reduce stability. Sequences rich in aspartic acid, glutamic acid, lysine, arginine or histidine take up water most readily.
• Oxygen. Cysteine, methionine and tryptophan oxidise easily. Free cysteine thiols oxidise to disulfides quickly above pH 7.
• Water over time. In solution, peptide bonds can slowly break and some residues change chemically. Sequences containing asparagine and glutamine are known for shorter shelf lives.
• Heat and light. Warmth speeds every one of these reactions; bright light is best avoided.
• Repeated freezing and thawing, which exposes the material to condensation and air each time.
• Microbes, once a peptide is dissolved in a non-sterile liquid.

Every storage rule below is a way of keeping one of these out.

Temperature. For long-term storage, keep the peptide in a tightly closed container in a freezer. Bachem's guidance is below −15 °C, with lower temperatures preferred (for example −50 °C or lower for long-term storage). GenScript recommends −20 °C and notes that most lyophilised peptides are stable for several years under those conditions.

Short-term. Bachem notes that peptides containing asparagine, glutamine, methionine, cysteine or tryptophan, which have limited shelf lives, may be kept in a refrigerator at 4 °C for short-term use. Short spells at room temperature, such as during shipping, are normal for many lyophilised peptides; Bachem ships most of its peptides at ambient temperature. What matters is where the peptide spends the long term.

Dryness. Keep vials tightly capped and, ideally, in a desiccator. Peptides with many charged residues are the most moisture-sensitive.

Light. Store away from bright light.

Oxygen-sensitive sequences. Peptides containing cysteine, methionine or tryptophan are best stored under an inert atmosphere (argon or nitrogen), and dissolved in oxygen-free water or buffer when the time comes.

This is the step most often done wrong:

1. Take the vial out of the freezer and let it reach room temperature before opening it, ideally inside a desiccator. Opening a cold vial draws in moist air, and the water condenses on the powder.
2. Weigh out what you need quickly.
3. Close the vial tightly and return it to the freezer.

If you will use a peptide several times, divide it into single-use portions on the first opening. Each portion then goes through one thaw, not many.

Both manufacturers say the same thing: don't store peptides in solution for long, even sterile and oxygen-free solution. A peptide's shelf life in solution is far shorter than as a powder, especially for sequences containing cysteine, methionine, tryptophan, aspartic acid, glutamine or an N-terminal glutamic acid.

When solution storage can't be avoided:

• dissolve in a buffer at pH 5 to 6 (GenScript);
• divide the solution into aliquots and freeze them at −20 °C or below;
• don't refreeze a thawed aliquot;
• consider passing the solution through a 0.2 µm filter to remove possible bacterial contamination (GenScript);
• expect frozen solutions to keep for a few weeks, not months (Bachem).

If you have dissolved more than you need, freeze-drying the excess again preserves it better than keeping it in solution.

The first hour matters more than people think. Our delivery checklist, which works for any supplier:

1. Check the outer packaging for damage, and look at any temperature indicator before you open the box.
2. Match the paperwork to the product. The batch number and expiry on each vial or box should match the delivery note, the packing list and the certificate of analysis. If anything doesn't match, set the product aside and contact the supplier before using it.
3. Move each product to its storage condition straight away. The packing list states it for every line.
4. Record the receipt date next to the batch number. It makes any later question about stability answerable.

If an experiment needs, say, 1 mg/mL of peptide, weighing out 1 mg of powder per millilitre will usually give you less than that. A peptide powder also contains bound water and counter-ions (such as acetate or trifluoroacetate) and a small amount of related peptide impurities.

GenScript puts the rule simply: "The amount of the target peptide is calculated by the multiplication of gross weight x net peptide content x HPLC purity." Bachem's worked example: 5 mg of bradykinin acetate with an HPLC purity of 98.7% and a net peptide content of 85.0% holds 4.195 mg of bradykinin. Bachem also notes that peptides with a large proportion of basic amino acids can be expected to have a relatively low peptide content, because more counter-ions are bound to them. Always use the figures on your own batch certificate; how to read a certificate of analysis shows where to find them.

Small tubes get mixed up. Label every aliquot with the peptide name, batch number, concentration, solvent and the date it was made, and keep a simple log of which aliquots have been thawed. When a result looks wrong months later, that log is how you tell a stability problem from a pipetting one.

This section is about preparing solutions for experiments in the laboratory. It is not guidance on preparing anything for use in people or animals.

There is no universal solvent for peptides. Solubility depends on the sequence, above all on its overall charge and how many water-repelling (hydrophobic) residues it has. Bachem's approach, by type of peptide:

Practical points from the same guidance:

• Test solubility on a small portion before committing the whole batch.
• Dissolution can take time, occasionally several hours.
• A few minutes in a sonicating water bath helps break up larger particles. Avoid warming the solution much.
• Peptides that clump together (aggregate) may dissolve with denaturants such as urea or guanidinium hydrochloride, but these interfere with most biological systems.
• For cysteine-containing peptides, use carefully degassed, slightly acidic buffers: free thiols oxidise quickly above pH 7.

We ship each product at the storage temperature on its label, with temperature indicators where needed, and the packing list states the storage condition for every line. Read more in storage and shipping, and how we supply laboratories in research laboratories. If a certificate of analysis lists water content or net peptide content, those figures are another reason to keep a powder dry; see peptide purity and net peptide content.

• Bachem, Handling and Storage Guidelines for Peptides (Knowledge Center)
• Bachem, FAQ: analysis of peptides
• GenScript, Peptide Storage and Handling
• GenScript, AccuPep QC testing