Disproportionation and nuclease activity of bis[2-ethyl-2-hydroxybutanoato(2-)]oxochromate(V) in neutral aqueous solutions

Complex 1, [Cr(V)O(ehba)₂]^- (ehba = 2-ethyl-2-hydroxybutanoate(2-)) is the most studied model compound of relevance to the biological activity of Cr(V) with regard to Cr-induced cancers. The first detailed kinetic study of disproportionation of 1 under neutral pH conditions (pH 6.0-8.0, [NaClO₄] = 1.0 M, 37 °C) is reported. Kinetic data were collected by stopped-flow and conventional UV-vis spectroscopies and processed by the global analysis method. The disproportionation, which follows the stoichiometry 3Cr(V) → 2Cr(VI) + Cr(III) (1), leads to release of 5 mol of H⁺/3 mol of Cr(V). Reaction 1 is accelerated by phosphate, but is not affected by acetate, HEPES, or Tris buffers. Initial rates of Cr(V) decay are directly proportional to [Cr(V)]₀ (0.020-1.0 mM); they increase with an increase in the pH values and decrease in the presence of a large excess of ehba ligand. The first direct evidence for the formation of Cr(IV) intermediates in reaction 1 has been obtained; however, their UV-vis spectral properties were different from those of the well-characterized Cr(IV)-ehba complexes. The Cr(III) products of reaction 1 in phosphate buffers differ from those in the other buffers. A mechanism is proposed for reaction 1 on the basis of kinetic modeling. Influences of the reaction time and conditions on the extent of plasmid DNA cleavage induced by 1 have been studied under conditions corresponding to those of the kinetic studies. A comparison of the kinetic and DNA cleavage results has shown that direct interaction of 1 with the phosphate backbone of DNA is the most likely first step in the mechanism of DNA cleavage in neutral media. Small additions of Mn(II) ((0.01-0.1)[Cr(V)]₀) did not affect the rate and stoichiometry of reaction 1, but suppressed the formation of Cr(IV) intermediates (presumably due to the catalysis of Cr(IV) disproportionation). However, much higher concentrations of Mn(II) ((0.1-1.0)[Cr(V)]₀) were required to inhibit DNA cleavage induced by 1. Thus, contrary to previous reports (Sugden, K. D.; Wetterhahn, K. E. J. Am. Chem. Soc. 1996, 118, 10811-10818), inhibition by Mn(II) does not indicate a key role of Cr(IV) in Cr(V)-induced DNA cleavage.