A novel dialysis procedure for the crystallization of proteins

Various dialysis methods are commonly employed for the crystallization of proteins. Typical procedures include the use of dialysis bags, dialysis buttons or Zeppezauer microdiffusion cells. The general principle involved is that the protein solution is gradually brought to a point of supersaturation by imposing a gradient of ionic strength or organic solvent concentration across the wall of the dialysis membrane. However, in some cases, the imposition of this gradient across the dialysis membrane can result in the formation of a large number of crystal nucleation sites, thereby giving rise to a reduction in the maximum size of the crystals which can be obtained. A novel 'double-dialysis' procedure which incorporates a second dialysis membrane, thus reducing the rate of equilibration in the crystallization experiment, has been developed in our laboratory. The system has been employed successfully on the delta toxin of Staphylococcus aureus resulting in a useful increase in crystal size. A more quantitative analysis of the technique has been carried out on rat liver malic enzyme. The results of a limited series of crystallization trials with this protein have shown that employment of the 'double-dialysis' technique allows a fine control of the rate of crystal nucleation and therefore provides a mechanism for the controlled growth of large crystals.