Dialysis and concentration of protein solutions

Conventional dialysis separates small molecules from large molecules by allowing diffusion of only the small molecules through selectively permeable membranes. Dialysis is usually used to change the salt (small-molecule) composition of a macromolecule-containing solution. The solution to be dialyzed is placed in a sealed dialysis membrane and immersed in a selected buffer; small solute molecules then equilibrate between the sample and the dialysate. Concomitant with the movement of small solutes across the membrane, however, is the movement of solvent in the opposite direction. There are several simple and relatively inexpensive methods for concentrating protein solutions. Dialysis against Aquacide 11A (Calbiochem), which removes water through the dialysis tubing, may be used. After concentration, the solution must be redialyzed into the appropriate buffer. Another method is to use Immersible-CX Ultrafilters (Millipore) which, when connected to a vacuum, remove everything below their molecular weight cutoff (MWCO). Alternatively, centrifugal concentrators, which are operated with the aid of ordinary laboratory centrifuges may be used.