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How to store peptides: keep lyophilised peptide sealed, dry, dark and frozen, let the vial reach room temperature in a desiccator before you open it, and avoid keeping peptides in solution for longer than you need to. Supplier guidance ranges from −20 °C for short-term storage to −80 °C, or at least below −15 °C, for the long term, and solutions should be split into aliquots and frozen rather than kept for weeks in the fridge.1, 2
The rest of this guide explains why each rule exists, so you can judge a specific sequence rather than follow a checklist blindly. It is written for laboratory handling of research material.
| Form | Short term | Longer term | Main risks |
|---|---|---|---|
| Lyophilised powder, sealed | −20 °C (Sigma-Aldrich suggests this for 1–2 weeks); some peptides tolerate 4 °C briefly | −80 °C (Sigma-Aldrich), or below −15 °C with −50 °C or lower preferred (Bachem) | Moisture uptake, oxidation, light |
| Lyophilised powder, opened | Reseal quickly; purge with nitrogen or argon if the sequence contains C, M or W | Return to the freezer in a desiccated secondary container | Condensation on cold powder, air oxidation |
| Stock solution | Up to about a week at 4 °C for many peptides (Sigma-Aldrich) | Single-use aliquots frozen below −15 °C; "a few weeks" (Bachem) | Deamidation, oxidation, freeze–thaw damage, adsorption, microbial contamination |
Sources: Bachem and Sigma-Aldrich handling guidelines.1, 2
Peptides do not simply "go off". They undergo specific chemical reactions, and each storage rule targets one of them. A review of solid-state stability lists deamidation, peptide bond cleavage, oxidation, the Maillard reaction, β-elimination and dimerisation or aggregation as the major reactions affecting peptides and proteins in the dry state, and identifies temperature, moisture content, excipients and the physical state of the solid as the main factors that influence them.3
Asparagine (N) and glutamine (Q) side chains can lose their amide group. In a classic study by Geiger and Clarke, the synthetic hexapeptide VYPNGA deamidated with a half-life of only 1.4 days at 37 °C and pH 7.4, and replacing the glycine after the asparagine with a bulkier leucine or proline slowed degradation 33- to 50-fold.4 Sequence, temperature and pH all matter, which is one reason suppliers advise against storing peptides in solution.1
Peptides containing cysteine (C), methionine (M) or tryptophan (W) are prone to air oxidation, and cysteine-containing peptides oxidise slowly over time at a rate that depends on the sequence and storage conditions.2 Bachem notes that free cysteine thiols "will be rapidly oxidized to the disulfides at pH > 7".1 Two sequences in our catalogue illustrate the point: Semax (MEHFPGP) contains a methionine, and MOTS-c (MRWQEMGYIFYPRKLR) contains two methionines and a tryptophan, as their PubChem records show.5, 6 For sequences like these, keeping oxygen out of the vial matters more than it does for a sequence with none of those residues.
Tryptophan, tyrosine, phenylalanine and cysteine/cystine residues undergo photo-oxidation, and a review of protein biologics notes that light exposure can alter primary, secondary and tertiary structure.7 Sigma-Aldrich advises that it is "always advisable to store away from bright light".2 An amber vial or a closed box inside the freezer costs nothing.
Both suppliers treat the freezer as the default. Bachem notes that peptides may be shipped at room temperature and, for short-term use, kept in a refrigerator at 4 °C; Sigma-Aldrich says that while some peptides are stable at 4 °C, it recommends −20 °C for short-term storage and −80 °C for longer storage.1, 2 A few days at ambient temperature in transit is therefore not unusual, but a vial that will sit for months belongs in the freezer.
The short answer is: briefly. Bachem advises that peptides "should not be stored in solution (even sterile and oxygen-free solution) because they may slowly undergo chemical degradation", that frozen solutions "may be kept for a few weeks", and that long-term storage of solutions is not recommended, especially for sequences containing N, Q, C, M or W.1 Sigma-Aldrich gives a similar picture: peptide solutions are generally stable for up to a week at 4 °C, sterile buffers at pH 5–6 and frozen aliquots prolong storage life, and exposure to pH above 8 should be avoided.2
In practice that means:
Size aliquots to the experiment, not to the vial. Suppose 10 mg of peptide is dissolved in 2.0 mL to give a 5 mg/mL stock, and each assay needs about 40 µL of stock. Twenty aliquots of 50 µL use 1.0 mL, leaving a small margin in every tube for pipetting losses, and the remaining 1.0 mL can be split the same way as a second set. Label every tube before it goes into the freezer, keep the aliquots from one lot together, and note on the box how many remain, so that nobody thaws a tube just to count them. The arithmetic for stock and working concentrations is set out in our reconstitution guide, linked above.
Freezing is not gentle. In a study of model enzymes frozen without cryoprotectants, slow freezing with fast thawing preserved the most activity, while fast freezing with slow thawing caused the most damage. The authors attributed the damage to denaturation at the ice–liquid interface during freezing and to recrystallisation during thawing, and found that pH shifts in some buffers during freezing reduced recovery further.8 Those were proteins rather than short peptides, but the practical lesson carries over: freeze each aliquot once, and thaw it once.
There is no universal figure. Stability depends on the sequence (particularly the N, Q, C, M and W residues above), the residual moisture in the powder, the pH and composition of any solution, and the temperature history of the vial.1, 3 Use the storage conditions and any retest or expiry date on the certificate of analysis for your lot, and if a stock's age or history is uncertain, check it again by HPLC rather than assume. Our guide to HPLC and mass spectrometry purity testing explains what that involves.
Visual checks are crude, but they are free. Lyophilised powder that has turned sticky, glassy or shrunken may have taken up moisture, and a solution that is cloudy, discoloured or slow to redissolve after thawing may contain aggregated or degraded material. None of these observations tells you how much intact peptide is left, and a vial can look perfect while its contents have changed.
Analytical checks are more informative. On reverse-phase HPLC, degradation typically shows up as new or growing impurity peaks beside the main peak. Mass spectrometry can then point to the chemistry: oxidation of a methionine adds about 16 Da (one oxygen atom), deamidation of an asparagine or glutamine adds about 1 Da, and a dimer appears at roughly twice the expected mass. Deamidation also produces isomerised and racemised forms that share the same mass, which is why chromatographic separation matters as well as the mass reading.4 If a result matters, compare the stored material with the certificate of analysis for the same lot, or with a freshly opened vial.
Peptides stick to surfaces, and the loss can be large enough to ruin an experiment. When three cationic membrane-active peptides (mastoparan X, melittin and magainin 2) were measured by HPLC, 90% or more could be lost from solution at typical experimental concentrations through rapid adsorption to the walls of common glass and plastic containers.9 A separate study of radiolabelled endocrine peptides, including ghrelin, GLP-1, insulin and leptin, found important differences in binding between surfaces; siliconising the tubes decreased recovery, whereas adding 1% bovine serum albumin improved it.10
For what a certificate should tell you about storage conditions, water content and counter-ions, see how to read a peptide certificate of analysis. For how lyophilisation produces the powder in the first place, see what research peptides are and how they are made.
Keep the sealed vial dry, dark and frozen. Supplier guidance ranges from −20 °C for short-term storage to −80 °C, or at least below −15 °C, for longer periods. Store it with desiccant, let it reach room temperature in a desiccator before opening so that moisture does not condense on the powder, and avoid repeated freeze–thaw cycles.
Not long, and it depends on the sequence. Sigma-Aldrich says peptide solutions are generally stable for up to a week at 4 °C, and Bachem says frozen solutions may be kept for a few weeks but advises against long-term storage in solution, especially for sequences containing asparagine, glutamine, cysteine, methionine or tryptophan. Frozen single-use aliquots last longest.
Only briefly, and only as dry powder. Bachem notes that lyophilised peptides may be shipped at room temperature, but both Bachem and Sigma-Aldrich recommend freezer storage for anything beyond short-term use. Peptide solutions should be refrigerated for use within days or frozen in single-use aliquots, not left on the bench.
The freezer, for anything longer than short-term use. Sigma-Aldrich recommends −20 °C for short-term storage and −80 °C for longer storage, and Bachem recommends below −15 °C, with −50 °C or lower preferred for long-term storage. A refrigerator at 4 °C is acceptable for short-term use of some peptides and for solutions you will use within days.
Because cold powder draws moisture from the air. Peptides tend to be hygroscopic, and opening a cold vial lets water condense on the lyophilised cake. Bachem notes that absorbed moisture reduces the overall peptide content and may decrease stability, so it recommends letting the container reach ambient temperature in a desiccator before opening and weighing.
What bacteriostatic water actually contains, how it compares with sterile water, when benzyl alcohol can interfere, and how to reconstitute a lyophilised peptide and calculate stock concentrations.
How reverse-phase HPLC and mass spectrometry are used to test peptide purity and identity, how to read a chromatogram and a mass spectrum, and what each method cannot tell you.
A plain-English primer on research peptides: what a peptide is, how peptides are built one amino acid at a time on a resin bead, why they arrive as a freeze-dried powder, and what "research use only" does and does not mean.
