Structure—function correlation of Cu(II)-and Cu(I)-di-Schiff-base complexes during the catalysis of superoxide dismutation

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Aquo-[ N,N′-bis(2-(6-methyl-pyridyl) methylene)-1,4-butanediamine] ( N,N′ ,N″,N‴)-copper(II) perchlorate, [Cu (II)-(Pu-6-MePy)-(H 2O)] (ClO 4) 2, a copper(II)-di-Schiff base was synthesized and reduced to the Cu(I) form. The successful preparation was controlled by IR and in the case of the Cu(II) complex additionally by EPR spectroscopy. [Cu(II)-(Pu-6-MePy)(H 2O)] ClO 4) 2 has an electronic absorption maximum at 717 nm of ε 717 = 108 M -1 cm −1. The EPR parameters are g ⊥ = 2.059 and g ⊥ = 2.235 which are very close to those of the respective values of Cu(II) in intact Cu 2Zn 2 superoxide dismutase (SOD). The electronic absorption maximum of the Cu(I) complex is at 474 nm ( ε 717 = 12 123 M −1 cm −1) and, as expected, no EPR is seen. The crystal structure of [Cu(II)-(Pu-6-MePy) (H 2O)](ClO 4) 2 shows a monomer while the reduced Cu(I) form reveals a dimer. Both complexes comprise the same SOD mimetic activity of 0.3% compared to that of the cytosolic enzyme. As the reduction of the Cu(II) complex proceeds within 10–40 min it is unlikely that the Cu(I) dimer is formed during the reduction oxidation cycle of superoxide dismutation which occurs at a timescale of milliseconds. The Cu(I) dimer is a promising superoxide dismutase mimicking compound in an aerobic reducing environment as, for example, in cytosol.