Research peptides ship as a lyophilized (freeze-dried) powder because the dry solid survives storage and transport far better than a solution. Reconstitution is the step that turns that powder back into a stock of known concentration for laboratory work. The method below is general; your own protocol overrides it. The numbers specific to a lot (net content, water content, counter-ion, storage temperature) come from that lot’s Certificate of Analysis.
Choosing a diluent
Sterile water dissolves most research peptides and is the default. Bacteriostatic water is sterile water containing benzyl alcohol at 0.9 % (w/v) as a preservative, and it is the choice when one stock will be sampled repeatedly over days rather than used up at once.
Bacteriostatic does not mean sterilizing. Benzyl alcohol inhibits the growth of organisms introduced into the solution; it does not reliably kill an established population and it will not make a contaminated stock clean. Benzyl alcohol at that concentration is also an organic co-solvent some assays will not tolerate, so check compatibility before the stock goes into cell culture.
Not every sequence goes into neutral water. Solubility follows charge, so the practical rule is to push the peptide away from its isoelectric point, the pH at which it carries no net charge and dissolves least readily:
- Basic sequences (net positive, rich in Arg, Lys, His) dissolve in water or in dilute acetic acid.
- Acidic sequences (net negative, rich in Asp and Glu) dissolve in dilute ammonium bicarbonate or ammonium hydroxide.
- Strongly hydrophobic sequences may need a small volume of DMSO or acetonitrile first, diluted into aqueous buffer afterwards, with the final organic fraction kept low enough for the assay.
Working out net charge from the sequence itself, rather than by category, and which co-solvent suits a sequence that will not go into either dilute acid or dilute base, is covered in peptide solubility.
The salt form matters here, and the COA records it. Vitro’s certificates carry a counter-ion row measured by HPLC; on the specimen certificate that row reads Counter-ion (acetate), meaning the lot is supplied as an acetate salt rather than as a free base or a TFA salt. The counter-ion shifts the pH of an unbuffered solution, and acetate is the preferred salt over TFA for cell-based work.
Calculating a working concentration
Concentration is mass divided by volume: milligrams of peptide in the vial, divided by milliliters of diluent added, gives mg/mL. Reconstituting a 10 mg vial in 2 mL gives 5 mg/mL. Lyophilized powder displaces some volume of its own, under a few percent at these scales, so the volume you add is close enough to the final volume.
| Vial | + 1 mL | + 2 mL | + 3 mL |
|---|---|---|---|
| 5 mg | 5.0 mg/mL | 2.5 mg/mL | 1.67 mg/mL |
| 10 mg | 10.0 mg/mL | 5.0 mg/mL | 3.33 mg/mL |
| 20 mg | 20.0 mg/mL | 10.0 mg/mL | 6.67 mg/mL |
| 50 mg | 50.0 mg/mL | 25.0 mg/mL | 16.67 mg/mL |
Vitro’s COA carries the three rows that resolve the difference. Net content (Gravimetric) states how much material the vial holds against its labeled Net content / vial: on the specimen certificate, 50.3 mg against a 50 mg ± 5 % specification. Water content by Karl Fischer reports 5.1 %, and Counter-ion (acetate) reports 6.8 %. Around 12 % of that weighed solid is therefore not peptide. Reconstituted in 2 mL it gives about 22 mg/mL of peptide, not the 25 mg/mL the vial label implies. For most bench work the label figure is close enough; where the concentration is itself the result, use the per-lot values. Which fields a certificate carries, and how to read each one, is covered in reading a Certificate of Analysis. Vial sizes per compound are listed in the catalog.
Technique
Whatever the diluent, add it slowly and aim it at the inside wall of the vial so it runs down onto the powder rather than jetting into it. Then swirl or roll the vial gently and set it down until the solid is gone.
Do not shake or vortex. Peptides adsorb to the air–liquid interface, unfold there, and aggregate; shaking multiplies that interface with every bubble it makes, and shear at a collapsing bubble surface adds to the damage. Foam is the visible symptom, but the loss happens whether or not foam persists, so the rule holds even when a vial looks fine afterwards.
Dissolution usually takes a few minutes with intermittent swirling. Some sequences take considerably longer; leaving the vial to stand beats agitating it. Inspect the finished solution against both a light and a dark background before anything else happens to it. A clear solution is colorless and free of visible particles, and haze shows up far better against dark than against white.
When the powder does not dissolve
Incomplete dissolution shows up as persistent haze or opalescence, visible particulate or flecks, an undissolved film clinging to the glass, or a gel that will not disperse. Material still in the solid phase is not in the stock, so the concentration is lower than the arithmetic says, by an amount you cannot see well enough to estimate. Four causes account for most of it, and they are usually distinguishable by eye; what drives each one is covered in peptide solubility. One of the four needs defining: the isoelectric point is the pH at which the peptide carries no net charge, so nothing keeps molecules apart and solubility is at its minimum.
| Observation | Likely cause | Check |
|---|---|---|
| Clears on adding more diluent | Solubility limit exceeded | Target concentration vs. what the sequence supports |
| Will not wet at all | Wrong solvent for the charge state | Acidic vs. basic sequence; organic co-solvent |
| Cloudy, clears on adding acid or base | pH near the isoelectric point | pH of the diluent; counter-ion on the COA |
| Gel or haze that never clears | Aggregation | Whether the vial was shaken; hydrophobic sequence |
Moving the pH a unit or so away from the isoelectric point, in either direction, is usually enough to recover solubility. Aggregation is the failure that frequently does not reverse on its own. Once molecules have associated into β-sheet, warming and sonication often recover only part of the material.
A lot that meets its purity specification can still be difficult to dissolve. Purity is an area percent from a chromatogram, not a solubility measurement. See peptide purity by HPLC and MS for what that figure does and does not cover.
Filtration
Passing a reconstituted stock through a 0.22 µm syringe filter removes particulate and is sterilizing-grade against bacteria and fungi, which is why it is the standard step before a solution goes near cell culture. It is not sterilization in general. Mycoplasma are 0.2–0.3 µm, have no cell wall, and deform through a 0.22 µm membrane routinely; viruses pass freely. Nothing dissolved is removed either: degradation products, counter-ions, residual solvent and endotoxin all pass straight through, so filtration cleans up a solution without improving the material in it.
Membrane chemistry decides how much peptide survives the step. Nylon and cellulose esters bind peptides appreciably; PES and PVDF membranes sold as low protein binding lose less and are the sensible default. Losses are worst at low concentration, where the fixed amount adsorbed by the membrane is a large fraction of what you started with.
Small volumes lose twice over. A syringe filter holds back a fixed hold-up volume, tens of microliters for a 4 mm device and 75–100 µL for a 25 mm one, and the first material through is further depleted by adsorption. Filtering 1 mL through a 25 mm filter can therefore strand close to a tenth of the preparation in the housing alone. Match the device to the volume, and where the concentration has to be exact, pre-wet the membrane with diluent or confirm the stock afterwards.
A filtered stock is only as clean as what it lands in, so filter into sterile labware and handle the syringe, the filter and that vessel so that none of them recontaminates it. That handling is covered in aseptic technique.
Vial and septum handling
Let a vial taken from the freezer or refrigerator reach room temperature while it is still sealed, which takes roughly 20–30 minutes for a small vial. Opening it cold pulls humid air onto a surface below the dew point, and lyophilized powder is hygroscopic: water picked up in that moment stays in the vial and feeds hydrolysis for the rest of its life. That is the same reason the powder is stored desiccated, covered in storing research peptides.
Before piercing the septum, swab it with 70 % isopropanol or ethanol and let it dry. The alcohol needs contact and evaporation time to do anything; wiping and immediately puncturing mostly carries wet alcohol into the vial.
- Coring. A needle punched straight through a rubber septum can cut out a plug that drops into the solution. Entering at about 45° with the bevel up and rotating to vertical as it passes reduces it, as does not re-piercing the same spot, which also preserves the seal.
- Pressure. Withdrawing liquid without replacing the volume pulls a vacuum that resists withdrawal and encourages bubbles. Introduce an equal volume of filtered air first, slowly.
- One direction only. Withdrawn liquid goes into a sterile tube, never back into the vial. Anything that has been through a needle and barrel has left the sterile path, and returning it puts whatever it collected into the whole stock.
Storing the reconstituted solution
Aliquot into single-use volumes as soon as the solution is clear and inspected. A stock in solution is far less stable than the dry powder it came from, and every extra entry into the parent vial is another chance to contaminate it.
Refrigerate at 2–8 °C for work over days and freeze at −20 °C or below for longer, following the storage conditions stated on the lot’s COA rather than a general rule. Repeated freeze–thaw is the main avoidable loss: each cycle concentrates solutes in the shrinking liquid fraction and can shift pH as buffer components crystallize out at different rates. Single-use aliquots avoid it.
Label every aliquot at the moment it is filled, with:
- compound and lot number
- concentration in mg/mL, and the diluent used
- the date of reconstitution
The lot number is what ties a tube at the back of a freezer to a certificate, and it is what you enter at verify to confirm that certificate against our records. Do not top up an old aliquot with fresh solution: the mixture has no single reconstitution date, and the older material sets the stability of the whole tube.
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure, or prevent any disease.