Stopped-flow fluorescence quenching by the anaerobic addition of 48 Fe2+ to apo-variant #3 (curve a), of 12 Fe2+ to apo-variant #1 with 8 Zn2+ bound (curve b), of 12 Fe2+ to apo-variant #1 (curve c), and of 48 Fe2+ to apo-variant #1 (curve d). Each curve is the average of 6 to 9 runs. Final conditions: 0.5 μM protein in 25 mM Mops hemisodium salt, 12.5 mM NaCl, pH 7.15, 25 °C. The Fe2+ solution was prepared in anaerobic 25 mM NaCl, pH ~ 2.5.