Separation by Adsorption II: Ion Exchangers and Nonspecific Adsorbents

Abstract

Although Chapter 5 was a general one describing the principles of chromatography, ion exchangers were mentioned frequently, and much of the theory has been tested using ion exchange adsorbents. Proteins bind to ion exchangers by electrostatic forces between the proteins’ surface charges (mainly) and the dense clusters of charged groups on the exchangers. The substitution level of a typical diethylaminoethyl (DEAE-) cellulose or carboxymethyl- (CM-) cellulose may be as much as 0.5 mmol cm-3 (packed, swollen adsorbent), that is 0.5 M of charged groups. If such a concentration were evenly distributed in three dimensions, the average distance between each charged group would be 1.5 nm. A compact globular protein of MW 30,000 has a diameter of 4 nm. The charges are of course balanced by counterions such as metal ions, chloride ions, and sometimes buffer ions. A protein must displace the counterions and become attached; generally, the net charge on the protein will be the same sign as that of the counterions displaced—hence “ion exchange.” The protein molecules in solution also are neutralized by counterions; the overall effect in a given region of the adsorbent must be electrically neutral. This is illustrated in Figure 6.1.