FIGURE 2.

XPR kinetics of the FeFe cofactor. A, iron XANES spectra of CtR2_FeFe^ox (squares) and CtR2_FeFe^red (open circles) of CtR2_FeFe samples initially in the oxidized (ox) state after x-ray exposure at 20 K for ∼90 min (triangles) and ∼260 min (solid circles), and of iron oxidation state references (Fe^IIN₄O₂, dashed line; Fe^IIIN₅O, solid line; see Table 3). Upper inset, K-edge spectra around the 50% level of CtR2_FeFe^ox and CtR2_FeFe^x260, the arrows mark the edge downshift (ΔE) and corresponding fluorescence intensity increase (ΔF) for an excitation energy of 7123 eV. Lower inset, expanded view of isolated pre-edge peaks in the XANES (asterisk). B, K-edge energies (triangles, left y axis) and x-ray fluorescence intensities at 7123 eV (circles, right y axis) at 20 K, together with the fit curves calculated using parameters in Table 3. Inset, time scan trace of the x-ray fluorescence intensity at 7123 eV at 80 K, K-edge data at 20 K, and fit curves (Table 3) on a logarithmic axis. Traces were normalized to unity amplitude after offset subtraction on the basis of the fit results. The given dose values (in Gy, gray = J kg⁻¹) are approximate and correspond to the incident dose, the absorbed dose in 1 mm of water for an energy of 7125 eV is ∼77% of the incident dose. Dose values were calculated for an energy of 7125 eV = 1.14 × 10⁻¹⁴ joule, a flux (upper limit) of 10¹¹ photons s⁻¹, and an irradiated volume of 0.2 mm³ corresponding to 2.15 × 10⁻⁷ kg (aqueous buffer plus R2 protein contributing about 0.15 × 10⁻⁷ kg at 1.8 mm and a molecular mass of ∼43 kDa).