Mononuclear and Dinuclear Monoperoxovanadium(V) Complexes with a Hetero Ligand. 1. Self-Decomposition Reaction, Detection of Reactive Oxygen Species, and Oxidizing Ability

A mononuclear peroxovanadium(V) complex with histamine-N,N-diacetate (histada), K[VO(O2)(histada)], and a
dinuclear peroxovanadium(V) complex with 2-oxo-1,3-diaminopropane-N,N,N‘,N‘-tetraacetate (dpot), Cs3[(VO)2(O2)2(dpot)], were prepared and characterized. The self-decomposition reaction was examined for these peroxovanadium(V) complexes as well as for K[VO(O2)(cmhist)] (cmhist = N-carboxymethylhistidinate). The reaction profiles depicted
by the absorbance change in the UV−vis spectrum show a sigmoid shape with an induction period. The induction
period is reduced by the addition of acid, fluoride, thiocyanate, VO2+, VO2+, and trolox compared to the solution
containing perchlorate. On the other hand, the induction period was elongated by the addition of chloride, bromide,
and 2-tert-butyl-p-cresol. These behaviors are discussed on the basis of a radical chain mechanism. The self-decomposition reactions have also been followed by the 1H and 51V NMR and EPR spectra. These spectral studies
as well as the UV−vis spectral study indicate that vanadium(V) is partly reduced to vanadium(IV) in the self-decomposition process. The histada complex yields a mixed-valence dinuclear complex in a concentrated solution,
and the dpot complex yields a mixed-valence tetranuclear complex. The reduction of vanadium ion suggests that
the peroxo ligand may act as a reducing agent. In order to know the fate of the peroxo ligand, we tried to detect
superoxide anion and hydroxyl radical, which were anticipated to be produced in the self-decomposition process.
The formation of superoxide anion was spectrophotometrically confirmed using two independent methods, including
the reduction of cytochrome c and the reduction of sodium 4-[3-(iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1). The formation of hydroxyl radical was confirmed by an EPR spin trapping technique.
The oxidizing abilities of the peroxovanadium(V) complexes toward bovine serum albumin (BSA) were also evaluated.
In the protein carbonyl assay, it was found that the total amount of protein carbonyl in BSA was increased by the
reaction with the peroxovanadium complexes in the concentration-dependent manner. In addition, the oxidation of
sulfhydryl group in BSA induced by the peroxovanadium complexes was confirmed.