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Seven-Coordinate Iron and Manganese Complexes with Acyclic and Rigid Pentadentate Chelates and Their Superoxide Dismutase Activity

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posted on 15.10.2007, 00:00 by Gao-Feng Liu, Miloš Filipović, Frank W. Heinemann, Ivana Ivanović-Burmazović
The reactions of seven-coordinate [FeIII(dapsox)(H2O)2]ClO4·H2O (1), [FeII(H2dapsox)(H2O)2](NO3)2·H2O (2), and [MnII(H2dapsox)(CH3OH)(H2O)](ClO4)2(H2O) (3) complexes of the acyclic and rigid pentadentate H2dapsox ligand [H2dapsox = 2,6-diacetylpyridinebis(semioxamazide)] with superoxide have been studied spectrophotometrically, electrochemically, and by a submillisecond mixing UV/vis stopped-flow in dimethyl sulfoxide (DMSO). The same studies were performed on the seven-coordinate [MnII(Me2[15]pyridinaneN5)(H2O)2]Cl2·H2O (4) complex with the flexible macrocyclic Me2[15]pyridinaneN5 ligand (Me2[15]pyridinaneN5 = trans-2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene), which belongs to the class of proven superoxide dismutase (SOD) mimetics. The X-ray crystal structures of 24 were determined. All complexes possess pentagonal-bipyramidal geometry with the pentadentate ligand in the equatorial plane and solvent molecules in the axial positions. The stopped-flow experiments in DMSO (0.06% of water) reveal that all four metal complexes catalyze the fast disproportionation of superoxide under the applied experimental conditions, and the catalytic rate constants are found to be (3.7 ± 0.5) × 106, (3.9 ± 0.5) × 106, (1.2 ± 0.3) × 107, and (5.3 ± 0.8) × 106 M-1 s-1 for 14, respectively. The cytochrome c McCord−Fridovich (McCF) assay in an aqueous solution at pH = 7.8 resulted in the IC50 values (and corresponding kMcCF constants) for 3 and 4, 0.013 ± 0.001 μM (1.9 ± 0.2 × 108 M-1 s-1) and 0.024 ± 0.001 μM (1.1 ± 0.3 × 108 M-1 s-1), respectively. IC50 values from a nitroblue tetrazolium assay are found to be 6.45 ± 0.02 and 1.36 ± 0.03 μM for 1 and 4, respectively. The data have been compared with those obtained by direct stopped-flow measurements and discussed in terms of the side reactions that occur under the conditions of indirect assays.

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