Electrochemical study of Hypocrellin B in acetonitrile and aqueous micellar media. Comparison with Hypocrellin A

Abstract

The electrochemical behaviour of Hypocrellin B (HBH₂) has been investigated in acetonitrile and in aqueous micellar media. In organic medium, HBH₂ behaves similarly to Hypocrellin A (HAH₂) since two successive reversible one-electron transfers are observed in the cathodic region. Exhaustive electrolyses have led to the formation of different reduced forms of HBH₂, which have been characterized by spectroscopic investigations. Contrary to HAH₂, EPR experiments demonstrated that the one-electron transfer is weakly coupled with protonation phenomenon, leading mainly to the formation of HBH₂˙⁻. Added protons in the medium are required to obtain HBH₃˙. In aqueous micellar medium, the first reduction step, whose potential depends on the pH value, is equally observed. This has allowed a diagram of E_1/2vs. pH to be established and the determination of the two pK_a values of HBH₂, whereas the pK_a value of the one-electron reduced form of HBH₂ has been estimated. The ability of this radical species to produce superoxide anion is discussed and compared with what was previously described for the one-electron reduced forms of HAH₂.