Short answer
Sterile water is water that has been sterilised and contains nothing else. Bacteriostatic water is sterile water with a preservative, normally 0.9 % benzyl alcohol, which prevents bacterial growth in a vial that is opened more than once. Dilute acetic acid is an acidic solvent that helps basic peptides dissolve.
All three are laboratory reagents; the choice depends on the peptide's charge and on whether the preservative or the pH interferes with the assay. They are sold by Vesta for laboratory use only.
Key facts
- Sterile water has no preservative and is intended for single use once opened.
- Bacteriostatic water usually contains 0.9 % benzyl alcohol, which inhibits bacterial growth but can interfere with cell cultures and some assays.
- Dilute acetic acid is used to dissolve basic, hydrophobic peptides that do not dissolve well in neutral water.
- A peptide's net charge at the working pH predicts its solubility; acidic peptides dissolve better in dilute base and basic peptides in dilute acid.
- These reagents are for laboratory use only and must not be used with humans or animals.
Three reagents, three purposes
Sterile water is the simplest: water that has been sterilised, with no added substances. Bacteriostatic water is the same water with a preservative that stops bacteria multiplying, so it can be used from a vial several times. Dilute acetic acid is a solvent with a low pH that is used when a peptide will not dissolve in neutral water.
Each choice carries consequences for the experiment, which is why the identity and the lot of the reagent should be recorded.
What bacteriostatic water contains and why it matters
The usual preservative is benzyl alcohol at 0.9 %. It prevents growth of many bacteria but does not kill all organisms and does not make the water sterile after contamination. In a laboratory, benzyl alcohol can affect cell viability, enzyme activity and some analytical methods, so a preserved diluent is not appropriate for every experiment. If a protocol does not call for a preservative, use sterile water.
Sterile water: what sterile does and does not mean
A sterile product has been treated so that viable microorganisms are absent when the container is closed. Once opened, it can be contaminated, and without a preservative it should be used promptly. Sterility is not the same as low endotoxin or purity; if an assay is sensitive to endotoxin, check that the lot is tested for it.
Dilute acetic acid and peptide solubility
Whether a peptide dissolves depends on its sequence and net charge. Peptides with many basic residues (lysine, arginine, histidine) carry a positive charge in acid and dissolve more easily in dilute acetic acid than in neutral water. Peptides with many acidic residues behave the other way. Hydrophobic peptides may need a small amount of organic solvent.
The concentration of the acetic acid solution supplied by Vesta is given in the batch documents and should be checked with support before it is used for a particular peptide.
Compatibility with the assay
A low pH may change the structure of a protein target, a preservative may inhibit a cell line, and either may change the retention of a peptide on an HPLC column. Before using a diluent in a new assay, run a vehicle control containing the diluent alone and compare it with the assay buffer. Record the lot number of the diluent together with the lot number of the peptide.
Record keeping and laboratory-only use
Record the product, the strength, the lot, the date opened and the date discarded. Discard diluents according to the laboratory's policy once the expiry or in-use period has passed. The products described here are laboratory reagents and are not intended for use in humans or animals; no dosing or administration guidance is given on this site.
Frequently asked questions
Can I use bacteriostatic water for cell culture?
Usually not, because benzyl alcohol can be toxic to cells. Use sterile water or a medium prepared for the cell line, and check the protocol.
How do I know which solvent a peptide needs?
Calculate the net charge of the sequence at neutral pH and test a small amount first. Basic peptides generally dissolve in dilute acid, acidic peptides in dilute base, and hydrophobic peptides may need an organic co-solvent.
Does a diluent have an expiry?
Yes. The batch documents give the information for each lot, and an opened multi-use vial has a shorter in-use period than an unopened one.
Primary references
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