Concentration dependence of IgG–protein A affinity studied by wireless-electrodeless QCM

The binding affinity between human immunoglobulin G (IgG) and protein A was studied by the homebuilt wireless-electrodeless quartz crystal microbalance (QCM). Protein A was immobilized on the electrodeless AT-cut quartz plate of 0.05 mm thick and its fundamental resonance frequency near 34 MHz was measured by a noncontacting manner using a line antenna. The vibrational analysis was performed to ensure higher sensitivity of the electrodeless QCM. A flow-cell system was fabricated to continuously measure the resonance frequency during the injection sequence of the IgG solutions with concentrations of 1–20,000 ng/mL. The exponential frequency changes were recorded to determine the affinity based on the Langmuir kinetics. The equilibrium constant KA significantly varied between 6×106 and 6×1010 M−1, depending on the IgG concentration, which is attributed to various formations of IgG–protein A complexes.