  Iron-peroxide intermediates are central in the reaction cycle of many
iron-containing biomolecules. We trapped iron(III)-(hydro)peroxo species in
crystals of superoxide reductase (SOR), a nonheme mononuclear iron enzyme that
scavenges superoxide radicals. X-ray diffraction data at 1.95 angstrom
resolution and Raman spectra recorded in crystallo revealed iron-(hydro)peroxo
intermediates with the (hydro)peroxo group bound end-on. The dynamic SOR active
site promotes the formation of transient hydrogen bond networks, which
presumably assist the cleavage of the iron-oxygen bond in order to release the
reaction product, hydrogen peroxide.
