To reconstitute a lyophilized peptide, let the sealed vial reach room temperature, choose a solvent that suits the peptide's charge and hydrophobicity, add a measured volume by pipette to reach a known concentration in mg/mL, and swirl gently until the solution is clear. Then sterile-filter if the assay needs it, split the stock into single-use aliquots, and store them cold. Everything below is about preparing in-vitro stock solutions at the bench; none of it is guidance for any other purpose.
A freeze-dried peptide is at its most stable as a dry cake. The moment you add liquid, the clock on hydrolysis, oxidation and deamidation starts, so reconstitution is less a single step than a set of decisions that decide how long the stock stays usable. If you are new to the chemistry, what peptides are covers the bonds and side chains that drive most of these choices.
Before you open the vial
Let it warm up sealed
Take the vial out of the freezer and leave it closed on the bench until it reaches room temperature. A cold vial opened in a humid room pulls moisture straight onto the powder, and lyophilized peptides are hygroscopic. Water on the cake lowers the effective peptide content of whatever you weigh or dissolve and speeds up degradation in the solid. The Anhydrolabs storage and handling page makes the same point: allow the vial to reach room temperature before breaking the vacuum seal, so atmospheric moisture does not condense onto cold material. A desiccator is the better place to do the warming if you have one.
Inspect the cake
Look at the cake through the clear window on the label before anything else. A dry, intact cake or loose powder is expected. A collapsed, shrunken, glassy or discolored cake suggests the material was exposed to heat or moisture, and it should not go into an experiment until you have checked it with the supplier.
Read the certificate first
The certificate of analysis tells you the identity by mass spectrometry, the purity by HPLC and, sometimes, the salt form. Two numbers matter for reconstitution:
- Net content, the mass of material in the vial (for example 5 mg).
- Peptide content, where reported. Synthetic peptides are usually isolated as salts, and the powder also carries some residual water, so the mass of peptide itself is somewhat lower than the net content.
For most screening work, calculating concentration from net content is fine as long as you record that you did. For quantitative work, such as a binding curve or a comparison between lots, calculate from peptide content or measure concentration directly (by UV absorbance for peptides containing Trp or Tyr, or by amino acid analysis).
Choosing a solvent
There is no universal solvent for peptides. Solubility depends on sequence: the number of charged residues, the share of hydrophobic residues, and whether the chain tends to form sheets and aggregate. A practical approach is to estimate the peptide's net charge at neutral pH and work from there, always testing a small portion before committing the whole vial.
Estimate the net charge
Count charges at about pH 7:
- +1 for each Lys (K) and Arg (R), and +1 for a free N-terminus.
- −1 for each Asp (D) and Glu (E), and −1 for a free C-terminal acid.
- His (H) is only partly charged at pH 7; count it as roughly neutral, or +1 in acidic solution.
- An amidated C-terminus or an acetylated N-terminus removes that terminal charge.
Then use the sum to pick a starting solvent.
| Peptide character | Typical first choice | If it does not dissolve |
|---|---|---|
| Net positive (basic) | Sterile water | Add dilute acetic acid (for example 10%) dropwise, then dilute |
| Net negative (acidic) | Sterile water or a neutral buffer | Add a small amount of dilute aqueous ammonia or ammonium bicarbonate, then dilute |
| Near neutral, under about 25% hydrophobic residues | Sterile water or buffer | Try dilute acid or base depending on the few charged residues present |
| Hydrophobic (roughly half or more of residues are L, I, V, F, W, M, A, Y) | A small volume of DMSO, then water or buffer added slowly | DMF or acetonitrile, if the assay tolerates them |
| Contains free Cys, prone to oxidation | Degassed, slightly acidic water or buffer | Avoid DMSO and basic pH; see below |
Water and dilute acid
Most short, charged peptides dissolve in sterile water. For basic peptides that resist water, a little dilute acetic acid protonates the side chains and the termini and usually brings them into solution. Add it a drop at a time and stop as soon as the solution clears, then bring the volume up with water or buffer. The pH of the final stock matters: deamidation of Asn and Gln and oxidation of Cys are both slower in mildly acidic conditions than at neutral or basic pH, so a slightly acidic stock tends to last longer than one at pH 8.
DMSO for hydrophobic peptides
Hydrophobic sequences often need an organic co-solvent. The usual approach is to wet the dry peptide with a small volume of neat DMSO until it dissolves completely, then add water or buffer slowly while mixing. If the solution turns cloudy, stop adding aqueous phase, add a little more DMSO until it clears, and accept a higher organic fraction or a lower final concentration.
Two cautions come with DMSO. First, it is itself biologically active in many cell systems, so keep the final concentration in the assay low and include a vehicle control at exactly the same DMSO concentration. Second, DMSO is an oxidant toward thiols: peptide chemists use DMSO-containing solutions deliberately to close disulfide bonds between cysteine residues (Tamamura and colleagues describe one such method). If your peptide has a free cysteine that must stay reduced, choose another co-solvent such as DMF or acetonitrile.
Peptides with free cysteine
A free thiol oxidizes in air to form disulfide-linked dimers, and it does so faster as the pH rises. For a peptide like glutathione, which carries one free cysteine, dissolve in degassed water or buffer on the acidic side of neutral, work quickly, and keep headspace small. Where the assay allows, a reducing agent in the buffer keeps the thiol in its reduced form.
Salt and buffer last
Dissolve the peptide in water or the minimal acid, base or organic phase first, and add concentrated buffer or salt afterwards. High ionic strength at the start can salt out a peptide that would otherwise have dissolved, and it is harder to rescue a precipitate than to prevent one.
Working out concentration
Concentration is simply mass divided by volume. Decide the stock concentration you want, then calculate the volume to add:
volume to add (mL) = net content (mg) ÷ target concentration (mg/mL)
| Net content in vial | Target stock | Volume of solvent |
|---|---|---|
| 2 mg | 1 mg/mL | 2.0 mL |
| 5 mg | 2 mg/mL | 2.5 mL |
| 5 mg | 5 mg/mL | 1.0 mL |
| 10 mg | 5 mg/mL | 2.0 mL |
| 10 mg | 10 mg/mL | 1.0 mL |
A few practical notes:
- Make the stock concentrated. A stock of 1 to 10 mg/mL is easier to keep stable and loses proportionally less material to container walls than a dilute one. Dilute into assay buffer on the day of the experiment.
- Mind the vial volume. The Anhydrolabs vial is a 3 mL format, so plan a reconstitution volume that leaves headspace for mixing, or dissolve in a smaller volume and transfer to a larger tube.
- Convert to molar units for the assay. Divide mg/mL by molecular weight (g/mol) to get mol/L, then scale. A 1 mg/mL solution of a 1,000 g/mol peptide is 1 mM.
- Use calibrated pipettes. Transfer solvent with a calibrated pipette and a fresh sterile tip. Positive-displacement pipettes handle DMSO and other viscous or volatile solvents more accurately than air-displacement pipettes.
Mixing without damaging the peptide
Add the solvent slowly down the inside wall of the vial rather than jetting it onto the cake. Then swirl or roll the vial gently until the solid is gone. Vigorous shaking and vortexing whip air into the solution, create a large air-water interface, and can drive aggregation, especially for longer and more hydrophobic sequences.
If the peptide dissolves slowly:
- Let it stand at room temperature for 10 to 20 minutes and swirl again.
- Try brief bath sonication in short bursts, keeping the vial cool between them.
- Check the pH; a basic peptide in an unbuffered neutral solution may just need a little acid.
- As a last resort, spin the tube down and use the clear supernatant, recording that the true concentration is now unknown until measured.
A correctly dissolved stock is clear and colorless. Haze, floating particles or a gel mean the peptide is not fully in solution, and any concentration you calculate from mass will be wrong.
Sterile filtration
If the stock will go into cell culture or be kept for weeks, pass it through a sterile 0.22 µm filter unit or a spin filter into a sterile tube. Choose a low-protein-binding membrane such as PVDF or PES, and rinse the dead volume through with a little extra solvent if you need every microgram. Filtration removes bacteria and particulates; it does not remove dissolved degradation products, so it is no substitute for a fresh stock.
Some losses come from the plastic and glass, not the filter. A PLOS ONE study measured by HPLC how fast three cationic membrane-active peptides adsorb to common sample containers and found that at typical experimental concentrations 90% or more could be lost from solution. The authors kept their own stocks at 100 µM or higher in low-binding polypropylene tubes to limit that loss. Strongly cationic peptides such as LL-37 are the obvious candidates for low-binding tubes and tips.
Aliquoting and storage after reconstitution
Divide the stock into single-use aliquots as soon as it is made. Each aliquot should hold what one experiment needs, so you never thaw and refreeze the same tube. Freezing concentrates solutes, can shift the pH of some buffers, and exposes the peptide to ice-water interfaces, all of which promote aggregation and chemical change. Label every tube with the compound, lot, concentration, solvent and date.
For storage, the Anhydrolabs storage and handling page sets out the figures this site works to:
| State | Condition | Guidance |
|---|---|---|
| Lyophilized, sealed | Frozen at −4 °F (−20 °C), protected from light | Move to the freezer on the day it arrives |
| Opening a vial | Let it reach room temperature first | Prevents condensation on cold material |
| Reconstituted | Refrigerated at 39 °F (4 °C) | Stability holds for about four weeks |
| Repeated use | Aliquot | Avoid freeze-thawing the same vial |
Sequences with Cys, Met, Trp, Asn or Gln are the least forgiving in solution, so for those, make smaller batches more often. The companion guide on how to store peptides goes into temperature, light and moisture in more detail.
A bench checklist
- Freezer to bench, vial closed, until it reaches room temperature.
- Inspect the cake; read the certificate for net content and salt form.
- Estimate net charge; choose water, dilute acetic acid or a DMSO co-solvent.
- Test solubility on a small portion if the material is scarce.
- Calculate volume from net content and target mg/mL.
- Add solvent by pipette down the vial wall; swirl, do not vortex.
- Confirm a clear solution; adjust pH or co-solvent if needed.
- Filter at 0.22 µm into a sterile low-binding tube if required.
- Aliquot, label and store cold.
Anhydrolabs supplies each compound as a lyophilized powder in vacuum-sealed vials, and as 10-vial kits for groups that run the same assay across many plates. Solvents, filters and consumables come from your usual laboratory supplier. For more on what separates a research-grade peptide from other material, see research peptides.