CoxB (constant final concentration of 25 μM) was mixed with different amounts of ScoI Cu2+ (37.5, 75, or 125 μM final concentration) at 25°C in 20 mM Mops-NaOH (pH 7.0) and 50 mM NaCl using a SX20 stopped-flow instrument (Applied Photophysics), and complex formation was monitored by the increase in absorbance at 520 nm. Each dataset was recorded six times and averaged. The absorbance traces were then globally fitted with DynaFit 4 according to a mechanism with reversible formation (k1) and dissociation (k2) of an encounter complex that irreversibly reacts to the final complex (k3). Indicated errors of rate constants correspond to errors from the fits performed with DynaFit.