Determination of refractive index increment ratios for protein–nucleic acid complexes by surface plasmon resonance

Three nucleic acid–protein complexes of 1:1 stoichiometry were analyzed by surface plasmon resonance on a Biacore biosensor to test whether or not proteins and nucleic acids yielded similar refractive index increments on binding. The expected maximum response in resonance units, (RUexp)max, and the observed one, (RUobs)max, on saturation of immobilized targets by interacting partners were compared to determine the ratio of (δn/δC)protein to (δn/δC)nucleic acid, where n is the refractive index at the surface and C is the concentration of one partner. Our results suggest that proteins and nucleic acids behave similarly and that the discrepancy between the expected and observed maximum responses for such complexes reflects inaccurate evaluation of the binding responses. Therefore, no correction of the instrument response is required for protein and nucleic acid interaction studies on a Biacore biosensor.