Ion exchange interaction biochromatography
Fractionation of protein mixtures by chromatography requires solid phases on which proteins are adsorbed reversibly without deterioration of their biological properties. Proteins are large size multifunctional polyelectrolytes and as such they can be adsorbed by electrostatic or coulombic interactions onto ionizable resins of opposite sign. This principle is extensively used and represents the most popular means to separate proteins from complex mixtures. Its effectiveness depends on a number of factors which have been studied in depth at theoretical and practical levels (Vermeulen et al., 1984; Fernandez et al., 1996). Since the interaction is based on electrical charges that can vary in sign, and strength, the possibilities of ion exchange in protein separation are numerous, and special conditions can be found to optimize operational parameters to obtain the best separation results for a particular protein mixture. A number of solid phase media are available today; they are made using various matrices to which ionizable groups are attached. Such diversity is firstly caused by the specificity that may vary from one matrix to another for the same ionic chemical group. Secondly, ion exchangers are also prepared to enhance various separation parameters such as binding capacity (Boschetti et al., 1995), sorption kinetics (Muller, 1990), or speed (Afeyan et al., 1990). Separation efficiency, mechanical stability or even the enhancement of diffusion (Coffman et al., 1998) have recently been at the basis of new ion exchangers to reach better or more rapid separations.