MULTIPOINT BINDING AND HETEROGENEITY IN IMMOBILIZED METAL AFFINITY-CHROMATOGRAPHY

Studies carried out using engineered proteins clearly demonstrate that adsorption to derivatized surfaces involves multiple interactions between functional groups on the protein and complementary sites distributed on the surface. The fact that adsorption involves multipoint interactions has important implications for the design of separations processes and for the interpretation of heterogeneity in biological recognition phenomena. Increasing the density of surface metal sites (immobilized copper ions) is found to be functionally equivalent to increasing the number of metal-coordinating groups on the protein (histidines and deprotonated amines), in that both processes increase the likelihood of simultaneous interactions between the protein and the surface. A consequence of multiple-site interactions is a significant increase in protein binding affinity that depends on the arrangement of surface sites. A protein will show the highest affinity for arrangements of surface sites which best match its own pattern of functional groups and will show lower affinity for less optimal arrangements, resulting in binding that is inherently heterogeneous. We have found that reversible protein adsorption in immobilized metal affinity chromatography (IMAC) is described by the Temkin model, which characterizes binding heterogeneity by a uniform distribution of binding energies over the population of surface binding sites. (C) 1995 John Wiley & Sons, Inc.