American Chemical Society
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Copper(II) and Copper(I) Complexes with an Open-Chain N4 Schiff Base Ligand Modeling CuZn Superoxide Dismutase:  Structural and Spectroscopic Characterization and Kinetics of Electron Transfer

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posted on 2000-07-01, 00:00 authored by Jürgen Lange, Horst Elias, Helmut Paulus, Jörg Müller, Ulrich Weser
The structure of the complex [CuII(PuPy)](ClO4)2 (PuPy = L = 1,8-bis(2-pyridyl)-2,7-diazaoctadiene-1,7) and the structure of the corresponding copper(I) complex were determined. In CuIIL(ClO4)2, a model compound with CuZnSOD activity, the unit CuIIL2+ has a tetrahedrally distorted square-planar N4 coordination geometry. The copper(I) complex with L was found to be dimeric, (CuIL)2(ClO4)2·DMF (DMF = N,N-dimethylformamide). The binuclear unit (CuIL)22+ has a helical structure with two ligands L bridging the two copper atoms to provide tetrahedral N4 coordination of each copper(I). In solutions of (CuIL)2(ClO4)2·DMF, solvent-dependent dissociation occurs according to D ⇌ 2M (D = (CuIL)22+; M = CuILSx+; S = solvent). Stopped-flow spectrophotometry was used to determine the rate constants for the dissociation of the dimer D (kM) and dimerization of the monomer M (kD) for S = acetonitrile and DMF. Equilibrium constants Kdim = kM/kD were determined spectrophotometrically. In aqueous solution, the oxidation of the dimer (CuIL)22+ by CoIII(NH3)5Cl2+ and cis- and trans-CoIII(en)2Cl2+ follows a second-order rate law, rate = kox[(CuIL)22+][Co(III)]. Data for rate constant kox and for the activation parameters ΔH and ΔS are presented. In DMF, the oxidation of (CuIL)22+ by CoIII(NH3)5Cl2+ occurs via the monomer CuIL(DMF)x+ and the dissociation of (CuIL)22+ becomes rate-controlling. The reduction of CuIIL2+ by RuII(edta)H2O2- was found to be too fast to be resolved by stopped-flow spectrophotometry. The kinetic results are discussed mechanistically in terms of the redox switch aspects of the system.