Bacteriostatic Water

This is purified water carrying benzyl alcohol at 0.9 percent by weight, supplied sterile in a sealed multi-use vial. Benzyl alcohol is a small aromatic alcohol that partitions into microbial membranes and inhibits growth at this concentration.

The distinction between bacteriostatic and sterile is worth being exact about, because the two are routinely confused. Sterile means the contents contain no viable organisms at the point of filling. Bacteriostatic means that organisms introduced afterward are inhibited from multiplying. The preservative does not sterilise the vial and does not undo contamination; it makes repeated withdrawal from one vial practicable where plain sterile water would support growth after the first entry.

Nor is the agent bactericidal at this concentration. It suppresses proliferation rather than killing, and its efficacy declines as the vial is depleted and repeatedly entered, which is the reason laboratory practice conventionally dates a vial at first entry and limits how long it stays in use.

For short synthetic peptides, which make up most of this catalogue, benzyl alcohol at this concentration is generally well tolerated and the preservative is the reason a lyophilised vial can be reconstituted once and sampled repeatedly.

Larger folded proteins are a different case. Benzyl alcohol is an amphiphile that interacts with hydrophobic protein surfaces, and its association with aggregation of folded proteins is documented in the formulation literature. For material whose activity depends on a maintained three-dimensional fold, such as a recombinant protein or a glycoprotein held together by non-covalent subunit association, a preservative-free diluent is the more conservative choice and a compatibility check is worth performing before committing a lot.

There is a related consideration for copper-complexed peptides and thiol-containing compounds, which are sensitive to their solution environment in ways that a simple peptide is not. The diluent should be chosen against the specific chemistry being dissolved rather than by default.

Benzyl alcohol carries an aromatic ring and therefore absorbs strongly in the low ultraviolet, with significant absorbance in the 250 to 260 nanometre region and a tail extending across the range where peptide bonds and aromatic side chains are measured. At 0.9 percent it is present in vast molar excess over any dissolved peptide.

The practical consequence is that concentration cannot be determined spectrophotometrically for material reconstituted in this diluent. An absorbance reading at 280 nanometres, or at 214 nanometres, is dominated by the preservative rather than by the peptide, and the resulting figure is meaningless. Concentration measurement requires either a preservative-free diluent or a method that separates the components first, such as chromatography with the benzyl alcohol peak identified and excluded.

The same excess interferes with reversed-phase analysis of a reconstituted sample, where benzyl alcohol produces a large peak of its own, and with any assay using a low-ultraviolet detection wavelength.