Electrochemical Reduction of Nickel(II)—Dihydroxyanthraquinone Sulfonate: How Complex Formation Affects Biochemical Behavior?

Abstract

Electrochemical behavior of the Ni^II complex of sodium 1,4-dihydroxy-9,10-anthraquinone-2-sulfonate Na₂[Ni^II(LHSO₃)₂Cl₂] · 2H₂O was studied by cyclic voltammetry in non-aqueous and aqueous buffer. The aim of the present work was to understand the behavior of the semiquinone radical anion formed after reduction of the free quinone centre of a ligand for which the other quinone centre binds a metal ion (Ni^II), not having a stable low oxidation state. In dimethylformamide solution an additional reduction peak appeared at a much more negative potential due to adsorption. The complex Na₂[Ni^II(LHSO₃)₂Cl₂] · 2H₂O undergoes diffusion controlled one-electron reduction unlike the two-electron reduction observed for most hydroxy-9,10-anthraquinones inspite of having two quinone centres in the complex (one on each ligand). In aqueous media a kinetic effect was observed during electron transfer at the electrode surface. The values of the electron transfer rate constants during the reduction of the complex at the electrode were evaluated and found to be dependent on the scan rate. Electrochemical behavior of the chosen hydroxy-9,10-anthraquinone (NaLH₂SO₃) was significantly modified in the complex with quinone. This result is important for biochemical studies, since NaLH₂SO₃ resembles the core moiety of anthracycline antitumor drugs. Several studies have reported that in case of anthracyclines complexation can help to reduce the formation of semiquinone moieties, leading to a lesser generation of superoxide responsible for drug’s cardiotoxicity. The electrochemical behavior of the title complex actually justifies such claims and tries to explain why a reduction in semiquinone formation helps to decrease the formation of superoxide.