Structure and Redox Behavior of Iron Oxophlorin and Role of Electron Transfer in the Heme Degradation Process

Abstract

Iron-oxophlorin is an intermediate in heme degradation, and the metal oxidation number can alter spin, electron distribution, and the reactivity of the metal and the oxophlorin ring. The role of electron transfer in the structure and reactivity of [(Py)(2)Fe(III)(PO)] (PO is the oxophlorin trianion) in different redox states has been investigated using the B3LYP and OPBE methods with the 6-31+G* and 6-311+G** basis sets. A computation study has shown that [(py)(2) Fe(III)(PO)] loses one electron from its a(2u) orbital. Thus the oxidized species, [(Py)(2)Fe(III)(PO(•))](+) (where PO(•) is the oxophlorin dianion radical), has an open-shell-singlet ground state with a d(xy)(2) d(xz)(2) a(2u)(1) d(yz)(1) electronic configuration with closely lying triplet and quintet states which are populated at ambient temperature. The aforementioned complex is highly reactive toward O(2). The reduced species [(Py)(2)Fe(II)(POH)] (where POH is the hydroxyheme) has the closed-shell-singlet ground state (π(xz) π(yz))(4) a(2u)(2) d(xy)(2) electronic configuration in which pyridines have a more π-accepting character and, thus, are tightly bound to iron. This reduced form is considerably less reactive toward O(2). The axial ligands effects (Im, t-BuNC) have also been studied in redox reactions of iron oxophlorin complexes. Complex [(Im)(2)Fe(III)(PO)] shows facile oxidation to form a cation radical and a reduction to form hydroxy while the [(t-BuNC)(2)Fe(II)(PO(•))] has high positive oxidation potential.