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Surface plasmon resonance (SPR) is an optical phenomenon used in certain commercial instruments to measure the kinetics of interaction between macromolecules. One of the interacting counterparts is immobilized on a sensor chip surface, whereas the other is present in the solvent above the surface. The SPR response is correlated to changes in refractive index at the sensor chip surface caused by concentration changes, e.g., when the analyte binds to the immobilized ligand. The SPR signal is monitored continuously which makes it possible to measure both association and dissociation rate constants (kon and koff). Once these are established, the equilibrium binding constant (ka) can be calculated. [ABSTRACT FROM AUTHOR]