Effect of Zn²⁺ on binding of FXIIa to HRG monitored by SPR. Biotin-HRG was adsorbed onto a streptavidin-coated sensor chip to 50 RU. Increasing concentrations of FXIIa (0-256nM) were injected at a flow rate of 45 μL/min in the presence of 2mM ethylenediaminetetraacetic acid (A) or 12.5μM ZnCl₂ (B). FXIIa concentrations are indicated adjacent to each tracing, with the exception of 0.5 and 0.25nM, which were omitted for clarity. FXIIa association to the chip surface was monitored for 8 minutes followed by injection of HBS to monitor dissociation. RU values were corrected for background and plotted versus time. (C) b-HRG (800 RU) was immobilized on a streptavidin chip, and 10nM FXIIa was injected in the presence of 0 to 20μM ZnCl₂ for 200 seconds (a), followed by Zn²⁺-containing buffer lacking FXIIa for 600 seconds (b). Dissociation was then observed in buffer lacking Zn²⁺ for 400 seconds (c). The micromolar concentrations of ZnCl₂ are noted on each tracing.