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.

Chameleonic Nature of Hydroxyheme in Heme Oxygenase and Its Reactivity: A Density Functional Theory Study

“…We have also illustrated that the electronic and spin distribution, oxidation state, and reactivity in six-coordinate iron oxophlorin were related to the axial ligands. 26,27 The oxidation of 2 [(Py) 2 Fe III (PO)] 0 to 1,3 [(Py) 2 Fe III (PO • )] + occurred by removal of an electron from an a 2u orbital, in agreement with the experimental results (Scheme 2). Thus, the radical nature of the macrocycle in [(Py) 2 Fe III (PO)] 0 , at higher temperatures, and in [(Py) 2 Fe III (PO • )] + , at lower temperatures, makes them very reactive toward O 2 .…”

“…All calculations were performed using previously described methods, which have been briefly summarized here. ,, The density functional theory (DFT) method was employed using the B3LYP* and OPBE hybrid density functional method, as implemented in the Gaussian 2003 (A.B.3) program series. The B3LYP* and OPBE , /6-31+G* methods were used for the optimization of all structures.…”

“…Furthermore, Balch and co-workers have indeed shown that in the coupled oxidation process there is an ambiguity in view of the allotment of electrons distributed between the metal and ring of oxophlorin. 19−24 Experimental 25 and theoretical 26 investigations have also manifested that 2 [(Py) 2 Fe III (PO)] 0 undergoes two reversible, one-electron processes. We have also illustrated that the electronic and spin distribution, oxidation state, and reactivity in six-coordinate iron oxophlorin were related to the axial ligands.…”

“…Furthermore, Balch and co-workers have indeed shown that in the coupled oxidation process there is an ambiguity in view of the allotment of electrons distributed between the metal and ring of oxophlorin. − Experimental and theoretical investigations have also manifested that 2 [(Py) 2 Fe III (PO)] 0 undergoes two reversible, one-electron processes. We have also illustrated that the electronic and spin distribution, oxidation state, and reactivity in six-coordinate iron oxophlorin were related to the axial ligands. , The oxidation of 2 [(Py) 2 Fe III (PO)] 0 to 1,3 [(Py) 2 Fe III (PO • )] + occurred by removal of an electron from an a 2u orbital, in agreement with the experimental results (Scheme ). Thus, the radical nature of the macrocycle in [(Py) 2 Fe III (PO)] 0 , at higher temperatures, and in [(Py) 2 Fe III (PO • )] + , at lower temperatures, makes them very reactive toward O 2 .…”

Computational electrochemistry: prediction of liquid-phase reduction potentials

“…559 Another recent work involves a DFT study of the structure and redox behavior of iron oxophlorin and the role of electron transfer in the heme degradation process. 560 A combined experimental and theoretical (DFT) approach has been carried out to study the redox properties of two dinuclear Ru(II) complexes containing the planar dpt-ph-dpt This journal is © the Owner Societies 2014 bridging ligand where dpt-ph-dpt denotes 1 00 ,4 00 -bis(2,4-dipyrid-2 0 -yl-1,3,5-triazin-6-yl) benzene. 561 Redox potential calculations for the set of 21 iron-containing complexes in acetonitrile have been carried using density functional theory and the PCM implicit-solvent model.…”