Structure and quantum chemical characterization of chloroperoxidase compound 0, a common reaction intermediate of diverse heme enzymes

We have determined the crystal structure of the chloroperoxidase (CPO) hydroperoxo reaction intermediate (CPO compound O) at 1.75-angstrom resolution. The intermediate was generated through controlled photoreduction of the CPO oxygen complex during x-ray data collection, which was monitored by recording of the crystal absorption spectra. Initially, the peroxo-anion species was formed and then protonated to yield compound O. Quantum chemical calculations indicate that the peroxo-anion species is not stable and collapses instantaneously to compound O. Compound O is present in the ferric low-spin doublet ground state and is characterized by a long O-O bond length of 1.5 angstrom and a Fe-O bond distance of 1.8 angstrom, which is also observed in the crystal structure.