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

The reactions of seven-coordinate [Fe(III)(dapsox)(H(2)O)(2)]ClO(4).H(2)O (1), [Fe(II)(H(2)dapsox)(H(2)O)(2)](NO(3))(2).H(2)O (2), and [Mn(II)(H(2)dapsox)(CH(3)OH)(H(2)O)](ClO4)2(H2O) (3) complexes of the acyclic and rigid pentadentate H(2)dapsox 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 [Mn(II)(Me(2)[15]pyridinaneN(5))(H(2)O)(2)]Cl(2).H(2)O (4) complex with the flexible macrocyclic Me(2)[15]pyridinaneN(5) ligand (Me(2)[15]pyridinaneN(5) = 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 2-4 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) x 10(6), (3.9 +/- 0.5) x 10(6), (1.2 +/- 0.3) x 10(7), and (5.3 +/- 0.8) x 10(6) M(-1) s(-1) for 1-4, respectively. The cytochrome c McCord-Fridovich (McCF) assay in an aqueous solution at pH = 7.8 resulted in the IC(50) values (and corresponding kMcCF constants) for 3 and 4, 0.013 +/- 0.001 microM (1.9 +/- 0.2 x 10(8) M(-1) s(-1)) and 0.024 +/- 0.001 microM (1.1 +/- 0.3 x 10(8) M(-1) s(-1)), respectively. IC(50) values from a nitroblue tetrazolium assay are found to be 6.45 +/- 0.02 and 1.36 +/- 0.03 microM 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.