chapter  3
38 Pages

ION-EXCHANGE EQUILIBRIA

Equilibria between ion exchangers (ion-exchange resins) and

solutions are one of the most important factors in ion-exchange

chromatography. The equilibrium relationship determines the

distribution of solutes (proteins) between gel phase (stationary

phase) and outer solution (mobile phase). It is influenced by

various factors, such as ionic strength of the solution, protein

concentration, pH, and types of ion exchangers. The depend-

ency of the distribution coefficient of protein on ionic strength

and pH is closely related to the elution pattern, that is, peak

position, peak width, and so on (see Chap. 2). Thus, equi-

librium should be expressed in a quantitative fashion as a func-

tion of the system variables in order to predict the elution pro-

file in column operations. However, experimental as well as

theoretical approaches to equilibria with proteins are considerably

hindered in comparison with those for low-molecular-weight in-

organic ions (e.g., Helfferich, 1962), since the ion-exchange

chromatography of proteins is a relatively new technology and

equilibria with macromolecules, such as proteins, are much more

complicated than those with inorganic ions. Special regard

should be paid to the properties of proteins. Although proteins

are biopolymers composed of distinct monomer units, that is,

amino acids linked by peptide bonds, their physical properties

are usually much different from those of the simple assembly of

individual amino acids. In an extreme case, those proteins with

the same net charge and molecular weight but with different

configurations exhibit different behaviors against ion exchangers.