Cryoradiolytic reduction of chloroperoxidase. (a) UV-Vis spectra of CPO crystals recorded during X-ray exposure at 12 keV with a flux of 6.3 × 1010 photons s−1, yielding a dose rate of 0.028 MGy s−1. The beam was defocused to 100 µm × 100 µm and two 200 µm Al absorbers were inserted. A new peak at 563 nm rapidly appears. No shift of the Soret band is observed. (b) Time and dose dependency of peak formation at 563 nm. (c) Active site of CPO. The 1.75 Å resolution σA-weighted 2mFo − DFC map is shown in grey with a contour level of 1σ. The mFo − DFc omit map of the low-dose data set is shown in red and the corresponding density of the high-dose data set is drawn in black. Both difference maps are contoured at 3σ.