Application of Badger's Rule to Heme and Non-Heme Iron-Oxygen Bonds: An Examination of Ferryl Protonation States

摘要

To gain insight into the protonation state of enzymatic ferryl species we have examined the applicability of Badger's rule to heme and non-heme iron-oxygen bonds.Using density functional theory we have calculated r_e and v_e for the Fe-O bonds of complexes with different axial ligands,iron-oxidation,oxygen-protonation,and spin states.Our results indicate that Badger's rule holds for heme and non-heme oxo and hydroxo complexes.We find that the long Fe-O bonds that have been reported in the crystal structures of the ferryl forms of myoglobin,horseradish peroxidase,cytochrome c peroxidase,and catalase deviate substantially from the values predicted by Badger's rule,while the short Fe-O bonds obtained from X-ray absorption measurements are in good agreement with Badger's rule.In light of our analysis we conclude that the ferryl forms of myoglobin,horseradish peroxidase,and cytochrome c peroxidase are authentic iron(IV)oxos with Fe-O bonds on the order of 1.66 A and pKa's < 4.