Quantitation of Affinity, Avidity, and Binding Kinetics of Protein Analytes with a Dynamically Switchable Biosurface

​A label-free method for the analysis of interactions of proteins with surface-tethered ligands is introduced. Short DNA levers are electrically actuated on microelectrodes by ac potentials, and their switching dynamics are measured in real-time by fluorescence energy transfer. Binding of proteins to ligands attached to the top of the DNA levers is detected by time-resolved measurements of the levers’ dynamic motion. We demonstrate the quantitation of binding kinetics (k_on, k_off rate constants), dissociation constants (K_D in the pM regime), and the influence of competitive binders (EC₅₀ values). Moreover, the “switchSENSE” method reveals avidity effects and allows discriminating between analytes with one or more binding sites. In a comparative study, interactions of six hexa-histidine-tagged proteins with tris-nitrilotriacetic acid (NTA₃) ligands are quantitated. Their binding kinetics and affinities are found to vary over up to 2 orders of magnitude, evidencing that the proteins’ individual chemical environments significantly influence the His₆–NTA₃ interaction.