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Bacteriostatic water is sterile water for injection with a preservative added, usually benzyl alcohol at 0.9% (9 mg/mL), so that a multi-use vial can be entered repeatedly without supporting bacterial growth. Sterile water for injection contains no preservative and comes in single-use containers.1, 2 For reconstituting peptides in the laboratory, neither is automatically the right choice: the best diluent depends on the peptide's chemistry and on what the solution is for.
This guide covers what the product labelling says about each, when benzyl alcohol can be a problem, how to reconstitute a lyophilised peptide aseptically, and the arithmetic for stock solutions. It is written for bench work with research material.
The US prescribing information for Hospira's Bacteriostatic Water for Injection, USP, listed on the National Library of Medicine's DailyMed, describes it as "a sterile, nonpyrogenic preparation of water for injection containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol added as a bacteriostatic preservative", supplied in a multi-use container from which repeated withdrawals may be made.1 The same label gives a pH of 5.7 (range 4.5 to 7.0) and a storage temperature of 20 to 25 °C.1 So 0.9% is the common concentration, but not the only one: check the label of the vial you actually have.
"Bacteriostatic" means the preservative inhibits bacterial growth; it does not sterilise a solution that has already been contaminated. The label still instructs users to "use aseptic technique for single or multiple entry and withdrawal from all containers".1
| Property | Bacteriostatic Water for Injection, USP | Sterile Water for Injection, USP |
|---|---|---|
| Preservative | Benzyl alcohol, 0.9% (9 mg/mL) or 1.1% (11 mg/mL) | None: contains "no bacteriostat, antimicrobial agent or added buffer" |
| Container | Multi-use vial | Single-use container only |
| After first opening | Repeated withdrawals permitted, with aseptic technique | "Do not reuse ... Discard unused portion." |
| Sterile and nonpyrogenic | Yes | Yes |
| Other labelled cautions | Not for use in neonates; some solutes may be incompatible with benzyl alcohol | Labelled "not isotonic" |
Sources: Hospira labels for each product on DailyMed.1, 2 For a single reconstitution that will be aliquoted and frozen straight away, the preservative's main advantage, safe repeated entry, never comes into play. For a vial that will be entered several times, it is the reason bacteriostatic water exists. Neither diluent changes the storage advice for the peptide itself; see how to store peptides.
The bacteriostatic water label carries the warning "NOT FOR USE IN NEONATES", because benzyl alcohol "has been associated with toxicity in neonates".1 Reports published in 1982 linked benzyl alcohol to deaths in newborn infants, described at the time as the gasping syndrome.3, 4 The label also cautions that some products "may be incompatible ... in a vehicle containing benzyl alcohol".1 In the lab, treat it as you would any other reagent: read the safety data sheet and keep the vial labelled.
It can. Benzyl alcohol is not an inert passenger.
If a peptide stock made up in bacteriostatic water goes into a cell-based assay, include a vehicle control containing the same final concentration of benzyl alcohol. If the assay is sensitive to it, use sterile water or a sterile buffer instead.
A peptide's solubility is determined mainly by its polarity, so start with the sequence: count the acidic residues (plus the C-terminal carboxyl group) and the basic residues (plus the N-terminal amino group).8 Bachem's guidance, summarised:8
| Peptide type | Starting solvent | Then |
|---|---|---|
| Basic (more basic than acidic groups) | A small amount of an acidic solvent such as acetic acid | Dilute to the working concentration; PBS at pH 7.0–7.4 is often enough at 1 mg/mL or less |
| Acidic (more acidic than basic groups) | A small amount of 0.1% aqueous ammonia | Dilute with water; PBS at 1 mg/mL or less often works |
| Neutral or hydrophobic | A small amount of DMSO, DMF, acetonitrile, methanol or isopropanol | Dilute with water or buffer; high organic concentrations are incompatible with cells |
| Contains free cysteine | Carefully degassed acidic buffer | Thiols oxidise rapidly to disulfides above pH 7 |
Sigma-Aldrich suggests making the stock in sterile distilled water or sterile dilute (0.1%) acetic acid where applicable, at a higher concentration than the assay needs, and diluting it further in assay buffer.9 Selank is a simple worked example: its sequence, TKPRPGP, carries a lysine and an arginine and no acidic side chains, so it counts as a basic peptide.10 The supplier's certificate of analysis may also state solubility information; see how to read a peptide certificate of analysis.
Reconstituting peptides does not always go to plan. Work through the problem in order rather than adding solvents at random:
Record whatever finally worked on the vial label and in your notebook, including the final pH if you added acid or base. The next person to use that lot will need it.
Concentration equals mass divided by volume. Dissolving 10 mg of peptide in 2.0 mL gives 10 ÷ 2 = 5 mg/mL. The same 10 mg in other volumes:
| Peptide mass | Diluent volume | Stock concentration |
|---|---|---|
| 10 mg | 1.0 mL | 10 mg/mL |
| 10 mg | 2.0 mL | 5 mg/mL |
| 10 mg | 5.0 mL | 2 mg/mL |
| 10 mg | 10.0 mL | 1 mg/mL |
Lyophilised peptide is not 100% peptide: it also contains counter-ions (often trifluoroacetate or acetate) and water. Bachem notes that both net peptide content and purity "must be taken into consideration when preparing solutions of biologically active peptides for assays".11 If 10 mg is the gross powder weight and the certificate reports 80% net peptide content, the vial holds about 8 mg of peptide, and 2.0 mL gives about 4 mg/mL. Check whether a stated fill refers to the gross powder or to the peptide.
Divide the mass concentration by the molar mass. For a hypothetical peptide of 1,000 g/mol, 5 mg/mL is 5 g/L ÷ 1,000 g/mol = 0.005 mol/L, or 5 mM. Use the molar mass of the peptide in the form it is actually supplied, or correct the mass for net peptide content first.
Use C1 × V1 = C2 × V2. To make 1 mL of a 10 µM working solution from a 5 mM (5,000 µM) stock: V1 = (10 µM × 1,000 µL) ÷ 5,000 µM = 2 µL of stock, made up to 1,000 µL with assay buffer. A volume that small is hard to pipette accurately, so a two-step serial dilution (for example 1:50, then 1:10) gives a more reliable result.
Follow the label of the specific product. Sterile water for injection is single-use: discard the unused portion.2 The bacteriostatic water label permits repeated withdrawals with aseptic technique, calls for storage at 20 to 25 °C, and advises against storing reconstituted solutions unless the maker of the dissolved product directs otherwise.1 For research peptides, that means following the peptide's own storage guidance rather than the diluent's. The catalogue lists a 3 mL bacteriostatic water vial; check the label of any diluent for its preservative, concentration and expiry before use. Background on how lyophilised peptides are made is in what research peptides are.
Bacteriostatic water is sterile, nonpyrogenic water for injection containing benzyl alcohol as a preservative, usually 0.9% (9 mg/mL); Hospira's US label also lists a 1.1% version. The preservative inhibits bacterial growth, which allows a multi-use vial to be entered more than once with aseptic technique. It does not sterilise a solution that has already been contaminated.
Bacteriostatic water contains benzyl alcohol as a preservative and comes in multi-use vials. Sterile water for injection contains no bacteriostat, antimicrobial agent or added buffer, is supplied only in single-use containers, and the unused portion should be discarded after opening. Both are sterile and nonpyrogenic. Because benzyl alcohol can affect some proteins and cells, the right choice depends on the application.
Often, yes. Supplier guidance for research peptides suggests sterile distilled water or sterile dilute acetic acid for many stock solutions, with other solvents for acidic or hydrophobic sequences. Sterile water has no preservative, so reconstitute aseptically, aliquot straight away and freeze. Its advantage is that there is no benzyl alcohol to interfere with protein stability or cell-based assays.
Vigorous shaking creates foam and makes it harder to see whether the peptide has fully dissolved. Swirl or roll the vial gently and give it time: Bachem notes that some peptides occasionally take up to several hours to dissolve, that several minutes of sonication in a water bath can help, and that excessive warming should be avoided.
Divide the peptide mass by the diluent volume: 10 mg in 2 mL gives 5 mg/mL. For accuracy, correct the mass for net peptide content from the certificate of analysis, because lyophilised powder also contains counter-ions and water. To convert to molarity, divide by the molar mass; a 1,000 g/mol peptide at 5 mg/mL is 5 mM. Then use C1V1 = C2V2 for working dilutions.
A laboratory guide to storing lyophilised and reconstituted peptides: freezer temperatures, moisture, oxidation-prone residues, light, freeze–thaw cycles and adsorption to containers.
A section-by-section guide to reading a peptide certificate of analysis: identity, HPLC purity, net peptide content, counter-ions, water, endotoxin, and the red flags that mean a COA does not check out.
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.
