Oxoiron(IV) in Chloroperoxidase Compound II Is Basic: Implications for P450 Chemistry

With the use of x-ray absorption spectroscopy, we have found that the Fe-O bond in chloroperoxidase compound II (CPO-II) is much longer than expected for an oxoiron(IV) (ferryl) unit; notably, the experimentally determined bond length of 1.82(1) Å accords closely with density functional calculations on a protonated ferryl (Fe^(IV)-OH, 1.81 Å). The basicity of the CPO-II ferryl [pK_a > 8.2 (where K_a is the acid dissociation constant)] is attributable to strong electron donation by the axial thiolate. We suggest that the CPO-II protonated ferryl is a good model for the rebound intermediate in the P450 oxygenation cycle; with elevated pK_a values after one-electron reduction, thiolate-ligated ferryl radicals are competent to oxygenate saturated hydrocarbons at potentials that can be tolerated by folded polypeptide hosts.