ABSTRACT
A superstructured tetraphenylporphyrin with a covalently attached proximal imidazole axial base and three distal imidazole pickets has been developed as a model for the active site of terminal oxidases such as cytochrome c oxidase. The oxygen adduct of the Fe-only heme (at low temperature) has a diamagnetic NMR and is EPR silent, which taken together with a resonance Raman oxygen isotope sensitive band (nuFe-O) at 575/554 cm-1 (16O2/18O2) indicates formation of a six-coordinate heme-superoxide complex. Unexpectedly, the Fe/Cu complex, where the copper is in a trisimidazole environment approximately 5 A above the heme plane, displays similar characteristics: a diamagnetic NMR, EPR silence, and nuFe-O at 570/544 cm-1. This indicates the dioxygen adduct of this Fe/Cu system is also a superoxide. This contrasts with previously characterized partially reduced dioxygen intermediates of binuclear heme/copper complexes that form Fe/Cu mu-peroxo complexes.
Subject(s)
Copper/chemistry , Electron Transport Complex IV/chemistry , Heme/chemistry , Superoxides/chemistry , Biomimetic Materials/chemistry , Copper/metabolism , Electron Transport Complex IV/metabolism , Ferric Compounds/chemistry , Heme/metabolism , Models, Chemical , Nuclear Magnetic Resonance, Biomolecular , Oxidation-Reduction , Oxygen/chemistry , Oxygen/metabolism , Spectrum Analysis, Raman , Superoxides/metabolismABSTRACT
Cobalt(II) porphyrins were studied to determine the influence of distal site metalation and superstructure upon dioxygen reactivity in active site models of cytochrome c oxidase (CcO). Monometallic, Co(II)(P) complexes when ligated by an axial imidazole react with dioxygen to form reversible Co-superoxide adducts, which were characterized by EPR and resonance Raman (RR). Unexpectedly, certain Co porphyrins with Cu(I) metalated imidazole pickets do not form mu-peroxo Co(III)/Cu(II) products even though the calculated intermetallic distance suggests this is possible. Instead, cobalt-porphyrin-superoxide complexes are obtained with the distal copper remaining as Cu(I). Moreover, distal metals (Cu(I) or Zn(II)) greatly enhance the stability of the dioxygen adduct, such that Co superoxides of bimetallic complexes demonstrate minimal reversibility. The "trapping" of dioxygen by a second metal is attributed to structural and electrostatic changes within the distal pocket upon metalation. EPR evidence suggests that the terminal oxygen in these bimetallic Co-superoxide systems is H-bonded to the NH of an imidazole picket amide linker, which may contribute to enthalpic stabilization of the dioxygen adduct. Stabilization of the dioxygen adduct in these bimetallic systems suggests one possible role for the distal copper in the Fe/Cu bimetallic active site of terminal oxidases, which form a heme-superoxide/copper(I) adduct upon oxygenation.
Subject(s)
Cobalt/chemistry , Electron Transport Complex IV/chemistry , Metalloporphyrins/chemical synthesis , Binding Sites , Catalysis , Copper/chemistry , Electron Spin Resonance Spectroscopy , Kinetics , Models, Molecular , Molecular Conformation , Molecular Structure , Oxidation-Reduction , Oxygen/chemistry , Spectrum Analysis, RamanABSTRACT
We measured the redox stoichiometry and rate constants for the electrochemical reduction of ClO(2)(-) at pH 7, catalyzed by a series of metalloporphyrins of Mn, Fe, and Co with different proximal and distal environments. A clean four-electron reduction was observed. The catalytic activity correlates well with that observed in reduction of H(2)O(2). The axial imidazole and/or a redox-active distal metal (Cu or Co) increases the turnover frequency in several compounds. The metalloporphyrins were inert to ClO(x)(-) (x = 3,4) and IO(3)(-) but catalyzed facile two-electron reduction of IO(4)(-); six-electron reduction of BrO(3)(-) was also observed.
Subject(s)
Chlorides/chemistry , Metalloporphyrins/chemistry , Catalysis , Electrochemistry , Kinetics , Oxidation-ReductionABSTRACT
Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.
ABSTRACT
The existence of mixed complexes of the general formula Tl(CN)(m)()Cl(n)()(3)(-)(m)()(-)(n)() (m + n
