Interaction of IgG and albumin with functionalized silicas

Studies of the adsorption of IgG and albumin to modified silica particles are reported. The silica particles were functionalized with so-called thiophilic and other groups designed to promote the specific adsorption of IgG according to Oscarrson and Porath, (in T.T. Ngo (Ed.), Molecular Interactions in Bioseparations, Plenum, New York, 1993, pp. 403–414). Porous silica beads were first coated with dextran which had been substituted with a calculated amount of positively charged diethylaminoethyl (DEAE) functions. This treatment was designed to “passivate” the silica, i.e. to minimize nonspecific protein adsorption via the silanol groups. The DEAE-dextran coated silica was functionalized with 2-mercaptoethanol via the coupling agent divinylsulfone. This treatment provides the thiophilic groups of Oscarrson and Porath. Ligands consisting of cysteine methyl ester and divinylsulfone residues were also coupled to the passivated silica to provide related chemical functions. The adsorption isotherms of human IgG to these materials from buffer were determined. All the functionalized materials showed an increase in adsorption of IgG in comparison with the passivated silica. The apparent affinities of the functionalized materials for IgG binding were all similar, thus demonstrating the importance of the sulfone group in the so-called thiophilic interaction. The adsorption of IgG from serum onto the functionalized materials showed the same specificity of IgG for these materials but the amounts of adsorbed IgG were lower than from buffer, indicating that other proteins in serum compete with IgG. IgG adsorption from serum also showed the Vroman effect, i.e. a maximum in adsorption as a function of serum dilution, indicating that initially adsorbed IgG is displaced by proteins of higher affinity. Human albumin adsorption from buffer showed an increase compared to the control surface on only one of the functionalized materials, i.e. the thiophilic material. Of the three derivatized materials, the cysteine methyl ester material shows the greatest binding capacity for IgG both from buffer and serum, and essentially no specificity for albumin binding. It would thus probably be the most effective of the materials studied for chromatography of IgG.