Ion exchange in purification.

Biochemicals have large complex three-dimensional structures with multiple weak acid and weak base groups on their surfaces. The ionization of these groups results in heterogeneous charge structures and complex ion-exchange equilibria. In this chapter the ion-exchange equilibria of amino acids are reviewed first. The ionization of the alpha-COOH group has a strong effect on the apparent equilibrium affinities of amino acids. Salt concentration and pH can also significantly affect the apparent affinities and valences. The implications on the ion-exchange equilibria of peptides and proteins are discussed. The second focus of this chapter is on the local equilibrium models of multicomponent chromatographic processes. The differences between unidirectional and two-way flow conditions, linear and nonlinear systems, elution and displacement processes, and isocratic and gradient operations are discussed. Fundamental analysis of these phenomena is applied to formulate strategies to shorten cycle time, improve resolution, raise product concentration, and increase throughput.