Figure 5.

4.2-K/53-mT (parallel field) Mössbauer spectrum showing accumulation of the Fe₂^III/IV activation intermediate, X, in Ec RNR-β₂-Y122F mediated by the Cld/ClO₂⁻ system. A solution containing 2.6 mM Ec β₂-Y122F dimer, 7.41 mM ⁵⁷Fe^II, 10 mM ascorbate and 12.5 μM Cld was mixed with 0.25 equivalent volumes of 50 mM ClO₂⁻, and the reaction was freeze-quenched after 0.30 s. The solid line is the theoretical spectrum of X generated with published parameters (72) and plotted at 70% (2.0 mM) of the total absorption area of the experimental spectrum. The remaining contributions from the Fe₂^II/II (red) and Fe₂^III/III (blue) complexes represent 15% (each) of the total absorption area.