Immobilized metal-ion affinity
Affinity chromatography is the most selective and the most rapid technique for the purification of proteins, and it has been utilized successfully for purifiying a great number of different proteins, including antibodies, antigens, enzymes, hormones, receptors, etc. (Cuatrecasas et al., 1968; Porath and Kristiansen, 1975; Wilchek et al., 1984; Scopes, 1987a). The success of affinity chromatography is essentially due to its high efficiency which originates from the specific or pseudospecific recognition between a solid phaseimmobilized ligand and the molecule to be separated. Affinity purification methods can be divided into two large classes depending on whether the mechanism of protein/ligand recognition is biospecific or pseudobiospecific (Vijayalakshmi, 1989). Proteins usually bind biospecific ligands with high affinity (10−7 to 10−15M). As a consequence, protein desorption in biospecific ligand affinity chromatography often requires drastic conditions which are likely to result in ligand and/or protein denaturation. In contrast, pseudobiospecific ligand affinity chromatography involves interactions of medium affinity (Cuatrecasas et al., 1968; Scopes, 1987b).