Bacteriostatic water vs sterile water vs acetic acid water for peptides
Freeze-dried peptides have to be dissolved before they can be used in an experiment, and the liquid you choose matters more than it seems. It decides how long the solution stays free of bacteria, whether the peptide dissolves properly and, for some peptides, how stable it stays. This guide covers the three solvents used most often in peptide work.
At a glance
| Bacteriostatic water | Sterile water | Acetic acid water | |
|---|---|---|---|
| What it is | Sterile water with 0.9% benzyl alcohol | Sterile water, nothing added | Sterile water with a small amount of acetic acid |
| Stops bacteria growing | Yes | No | Partly, through its low pH; not a validated preservative |
| Vial use | Multiple punctures | Single use | Depends on the product |
| Best for | Most peptides, solutions used over days or weeks | Solutions used at once, peptides sensitive to benzyl alcohol | Peptides that dissolve poorly or are unstable at neutral pH |
Bacteriostatic water
Bacteriostatic water is sterile water containing 0.9% (9 mg per mL) benzyl alcohol. "Bacteriostatic" means it stops bacteria from multiplying; it does not kill them or sterilise a contaminated solution. Its job is to keep a vial usable after the stopper has been pierced several times, when a little contamination can get in each time.
That is why it is the default solvent for research peptides: a reconstituted vial is usually sampled many times over days or weeks. For the same reason, an opened vial of bacteriostatic water itself does not last indefinitely. Pharmacy practice in the US (USP <797>) gives opened multi-dose vials 28 days after the first puncture unless the manufacturer states otherwise. Unopened vials are stored at controlled room temperature, 20–25 °C.
When benzyl alcohol is a problem
Benzyl alcohol can interact with some proteins and larger peptides. In a published study on a freeze-dried protein, reconstituting with 0.9% benzyl alcohol caused more aggregation (clumping) than reconstituting with plain water, and later work showed it can favour partly unfolded forms that clump more easily. Most small research peptides tolerate it well, but if the supplier or the literature for a specific compound recommends a different solvent, follow that.
Sterile water
Sterile water (water for injection) is sterile and pyrogen-free, with nothing added. Without a preservative, once the vial is opened nothing stops bacteria from growing, so it is meant for single use: prepare what you need and discard the rest. Use it when a solution will be used immediately, or when a peptide should not be exposed to benzyl alcohol.
Acetic acid water
Acetic acid water is sterile water with a small amount of acetic acid, which makes it slightly acidic. A common strength is 0.6%, about 0.1 mol/L. Some peptides dissolve poorly in neutral water, especially basic peptides that carry a positive net charge, and a slightly acidic solution helps them dissolve and can keep them more stable. A well-known example is LR3 IGF-1, which the original manufacturer supplies freeze-dried from 0.1 M acetic acid.
One approach used in labs is to dissolve the peptide in a small volume of dilute acetic acid first and then dilute it to the final volume with bacteriostatic water, so the solution keeps a preservative.
Which one to use
- Most peptides, used over several days: bacteriostatic water.
- Solution used straight away, or a compound sensitive to benzyl alcohol: sterile water.
- Peptide that will not dissolve, or that the supplier says needs an acidic solvent: acetic acid water.
If a peptide does not dissolve fully, do not shake it hard or heat it. Let it stand, swirl gently, and check the solubility information for that compound before trying another solvent.
Working out the concentration
The volume of solvent only changes the concentration, not the amount of peptide in the vial. Divide the amount in the vial by the volume added:
- 5 mg + 2 mL of solvent = 2.5 mg/mL
- 10 mg + 2 mL of solvent = 5 mg/mL
- 10 mg + 1 mL of solvent = 10 mg/mL
Write the concentration and the date on the vial. A more dilute solution is easier to measure accurately in small volumes; a more concentrated one takes less space in the refrigerator.
Good practice
- Wipe the stopper with an alcohol swab before every puncture and use a new sterile needle each time.
- Let the solvent run down the inside wall of the peptide vial; swirl, do not shake.
- Keep reconstituted peptides in the refrigerator, whichever solvent you used.
- Do not use a solvent vial that looks cloudy or has particles in it.
For how long reconstituted peptides keep, see our storage guide.
References
- United States Pharmacopeia. General Chapter <797> Pharmaceutical Compounding — Sterile Preparations.
- Bacteriostatic Water for Injection, USP (0.9% benzyl alcohol). Manufacturer prescribing information.
- Roy S, Jung R, Kerwin BA, Randolph TW, Carpenter JF. Effects of benzyl alcohol on aggregation of recombinant human interleukin-1-receptor antagonist in reconstituted lyophilized formulations. J Pharm Sci 2005;94(2):382–396.
- Zhang Y et al. Mechanism for benzyl alcohol-induced aggregation of recombinant human interleukin-1 receptor antagonist in aqueous solution. J Pharm Sci 2004.
- Merck / Sigma-Aldrich. Solubility guidelines for peptides. Technical article.