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Corroborative Models of the Cobalt(II) Inhibited Fe/Mn Superoxide Dismutases

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posted on 2005-07-11, 00:00 authored by Marciela Scarpellini, Amy J. Wu, Jeff W. Kampf, Vincent L. Pecoraro
Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide the same set of donor atoms observed in the active site of these enzymes:  KiPr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five new CoII complexes (15) were obtained and characterized by X-ray crystallography, mass spectrometry, electrochemistry, magnetochemistry, UV−vis, and electron paramagnetic resonance (EPR) spectroscopies. The crystal structures of 1 and 35 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR data of complexes 3 and 4 agree with those of the CoII-substituted SOD, which strongly support the proposition that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 15 clearly demonstrates the stabilization of the CoII state in the ligand field provided by these ligands. The irreversibility displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement of the geometry around the metal center for complexes 1 and 35 that probably changes from a trigonal bipyramidal (high spin, d7) to octahedral (low spin, d6) as CoII is oxidized to CoIII, which is also expected to be accompanied by a spin-state conversion. As the redox potentials to convert the CoII to CoIII are high, it can be inferred that the redox potential of the CoII-substituted SOD may be outside the range required to convert the superoxide radical (O2•-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes reported here are the first corroborative structural models of the CoII-substituted SOD.

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