Phosphate Buffered Saline (PBS)

Phosphate-buffered saline is a solution of sodium chloride and phosphate salts, and it is probably the most widely used solution in biology. It does two things at once: it holds pH steady near 7.4, and it matches the salt concentration of body fluid so that cells placed in it neither swell nor shrink.

The buffering is the part worth understanding. A buffer is a mixture of a weak acid and its corresponding base, and the pair works as a chemical shock absorber: add acid and the base half absorbs it, add base and the acid half does the same. Phosphate is used here because the point at which it buffers most effectively happens to sit close to the pH of blood.

Why that matters is straightforward. Proteins hold their shape through a balance of charges, and pH determines those charges. Move the pH and a protein can unfold, lose activity or precipitate. Unbuffered water has no resistance to pH change at all — dissolving almost anything in it moves the pH somewhere — which is why an experiment sensitive to protein behaviour uses a buffer rather than water.

• pH held near 7.4: Phosphate buffers most strongly close to physiological pH, which is why this particular salt pair is the standard choice rather than one of the alternatives.

• Isotonic with body fluid: The salt concentration matches that inside cells, so water does not rush in or out across their membranes. Cells in plain water burst; cells in this do not.

• Used where cells are involved: It is the usual choice for washing, diluting and handling cells, which is a different job from dissolving a lyophilised peptide.

• Phosphate is not inert: It precipitates with calcium and interferes with some enzyme assays, which is a real limitation and the reason other buffers exist.

The three solvents answer different questions. Bacteriostatic water is for dissolving a lyophilised peptide and keeping the vial usable over time. Acetic acid water is for peptides that will not dissolve at neutral pH, where shifting the charge is the only way to get them into solution. This one is for work in which a biological system has to remain viable and the conditions around it controlled.

A practical consequence follows. A peptide dissolved in an acidic solvent and then diluted into a buffered, physiological solution may come back out of solution as the pH returns to neutral, because it is being returned to the conditions it would not dissolve in. That is a property of the molecule rather than a fault of either solution, and knowing it in advance saves a wasted preparation.

Bacteriostatic Water and Acetic Acid Water are the other two solvents, and between them they cover most laboratory requirements for getting a peptide into solution.