title: A superoxo-ferrous state in a reduced oxy-ferrous hemoprotein and model compounds.
abstract: Cryoreduction of the [FeO2]6 (n = 6 is the number of electrons in 3d orbitals on Fe and pi* orbitals on O2) dioxygen-bound ferroheme through irradiation at 77 K generates an [FeO2]7 reduced oxy-heme. Numerous investigations have examined [FeO2]7 centers that have been characterized as peroxo-ferric centers, denoted [FeO2]per7, in which a ferriheme binds a dianionic peroxo-ligand. The generation of such an intermediate can be understood heuristically if the [FeO2]6 parent is viewed as a superoxo-ferric center and the injected electron localizes on the O-O moiety. We here report EPR/ENDOR experiments which show quite different properties for the [FeO2]7 centers produced by cryoreduction of monomeric oxy-hemoglobin (oxy-GMH3) from Glycera dibranchiata, which is unlike mammalian "globins" in having a leucine in place of the distal histidine; of frozen aprotic solutions of oxy-ferrous octaethyl porphyrin; and of the oxy-ferrous complex of the heme model, cyclidene. These [FeO2]7 centers are characterized as "superoxo-ferrous" centers ([FeO2]sup7), with nearly unit spin density localized on a superoxo moiety which is end-on coordinated to a low-spin ferrous ion. This assignment is based on their g tensors and 17O hyperfine couplings, which are characteristic of the superoxide ion coordinated to a diamagnetic metal ion, and on the absence of detectable ENDOR signals either from the in-plane 14N ligands or from an exchangeable H-bond proton. Such a center would arise if the electron that adds to the [FeO2]6 superoxo-ferric parent localizes on the Fe ion, to make a superoxo-ferrous moiety. Upon annealing to T > 150 K, the [FeO2]sup7 species converts to peroxo/hydroperoxo-ferric ([FeO2H]7) intermediates. These experiments suggest that the primary reduction product is [FeO2]sup7 and that the internal redox transition to [FeO2]per7/[FeO2H]7 states is driven at least in part by H-bonding/proton donation by the environment.
authors.affiliation: Department of Chemistry, Northwestern University, 2156 Sheridan Road, Evanston, Illinois 60208-3113, USA.
authors.forename: Roman
authors.initials: R
authors.lastname: Davydov
authors.forename: James D
authors.initials: JD
authors.lastname: Satterlee
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujii
authors.forename: Alexandra
authors.initials: A
authors.lastname: Sauer-Masarwa
authors.forename: Daryle H
authors.initials: DH
authors.lastname: Busch
authors.forename: Brian M
authors.initials: BM
authors.lastname: Hoffman
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005296:Ferrous Compounds / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D005296:Ferrous Compounds; D006420:Hemeproteins; C034236:ferrous oxide