Mechanism of immonoglobulin G adsorption on mica-AFM and electrokinetic studies

Adsorption of immunoglobulin G (IgG) from aqueous NaCl solutions of the concentration 10(-3)-0.15M on mica was studied. Initially, the kinetics was evaluated at pH 3.5 by direct AFM imaging. A monotonic increase in the maximum coverage of IgG with NaCl concentration was observed. These results were interpreted in terms of the theoretical model postulating an irreversible adsorption of the protein governed by the random sequential adsorption (RSA) model. Additionally, IgG adsorption and desorption was studied under in situ conditions, with streaming potential measurements. These measurements revealed that the maximum coverage of irreversibly adsorbed IgG varies from 0.37mgm(-2) for 10(-3)M, NaCl to 1.2mgm(-2) for 0.15M, NaCl. The significant role of ionic strength was attributed to the lateral electrostatic repulsion among adsorbed IgG molecules, positively charged at this pH value. These experimental results confirmed that monolayers of irreversibly bound IgG can be produced by adjusting ionic strength of the protein solution. In further experiments the stability and acid base properties of such monolayers were studied using the streaming potential method. It revealed that the monolayers were stable against pH cycling for the range from 3.5 to 9.5. The isoelectric point of mica supported IgG monolayers was 5.9, similar to derived from the micro-electrophoretic measurements in the bulk (5.8). Beside significance for basic sciences, the results indicate that thorough characteristics of IgG can be acquired via streaming potential measurements using microgram quantities of the protein.