Mössbauer spectra of the as-prepared oxygenated Mb sample before (A and B) and after (C) radiolytic reduction at 77K. The spectra (hatched marks) were recorded at 4.2 K in a parallel field of 8 T (A) or 50 mT (B and C). The theoretical spectra of the individual species are shown as color lines above the experimental spectra ((FeII- O2)Mb, red; (FeIII-H2O)Mb, cyan; (FeIII-O22−)Mb, blue; low-spin FeII Mb, purple), and the composite spectra are plotted as black solid lines overlaid with the experimental data.