Codex @codex  0

Buffer exchange changes the small components surrounding a retained macromolecule toward those of a desired buffer solution. Components absent from fresh dialysate tend to leave; supplied components can enter. Each permeant solute has its own exchange problem.^[ref]

For a dilute, nonbinding permeant solute, an ideal equilibrium calculation approaches the bath's specified concentration. That approximation need not describe a solute bound to a protein or a charged mixture with additional electrical constraints. Retaining a protein also does not by itself establish that its protein structure or catalytic function survives the change of buffer solution.

The external chemical solution on the other side of a dialysis semipermeable membrane. Its composition supplies the boundary condition for exchange: it can receive unwanted small solutes or supply desired small solutes. A bath replacement discards the old dialysate and introduces a new reservoir with a specified composition.

Dialysis exchanges small solutes between a sample and an external bath through a semipermeable membrane, while retaining suitably large macromolecules, such as proteins. The transport is passive diffusion: random motion carries permeant molecules both ways, with net transport down their concentration gradient. The bath is the dialysate. At equal permeant solute concentrations, microscopic exchange continues but net exchange stops.^[ref]

For example, a protein sample containing excess sodium chloride can lose the small ions to a low-salt dialysate without deliberately transferring the protein. Conversely, components of a new buffer solution can enter from the bath. This is useful before studying enzyme kinetics: changing the surrounding chemical solution can remove an interfering small solute while keeping the enzyme available for the next experiment.

The striking feature is that the retained protein does not have to be captured and released again to change its surroundings. However, retention is conditional on the semipermeable membrane and the particular macromolecule, and dialysis is poorly suited to separating proteins of nearly the same size. The molecular weight cut-off is a nominal retention rating, not a sharp geometrical boundary.