Real-time monitoring of antigen-antibody recognition on a metal oxide surface by an optical grating coupler sensor.

Real-time monitoring of intermolecular interactions can provide a direct and rapid estimate of the affinity and kinetics of interactions between biomolecules. Optical methods based on the measurement of changes of refractive index in the immediate vicinity of a liquid-solid interface are particularly convenient because they require no radioactive, fluorescent or other labelling of the molecules under study. In the present work we have followed the specific interaction of protein molecules on a SiO2/TiO2 surface with the help of the optical grating coupler sensor instrument BIOS-1. This instrument allows the determination of the absolute mass of protein adsorbed to the sensor surface and, therefore, the calculation of the molar ratio of the components partaking in an intermolecular interaction. For example, about 3 ng avidin/mm2 surface area could be adsorbed. This amount closely corresponds to a monolayer composed of densely packed globular avidin molecules. A dimeric, biotinylated leucine zipper peptide was bound to this avidin layer at a molar ratio of 1:1 (1 peptide molecule/4 biotin binding sites of tetrameric avidin). An average of 1/2.6 peptides was recognized by a peptide-specific monoclonal antibody. Even though avidin was not covalently bound to the sensor surface, the avidin-coated chip could be used repeatedly to measure the time course of antibody binding as a function of the concentration of the antibody. From such measurements it was possible to calculate the association and dissociation rate constants assuming that the interaction of the antibody with the surface-bound antigen can be described by a simple Langmuir binding model. The limits of the Langmuir model are discussed. The same antigen-antibody reaction was also analyzed by a surface plasmon resonance biosensor (BIAcoreTM, Pharmacia). The results obtained with the two instruments, which register different optical phenomena and employ different surface chemistry, were in good agreement.