Electrochemical studies of isolapachol with emphasis on oxygen interaction with its radical anions

Cyclovoltammetric studies, in aprotic medium (DMF+0.1 mol l−1 TBAP or DMSO+0.1 mol l−1 TEAP), on glassy carbon and/or platinum electrodes were performed with isolapachol [2-hydroxy-3-(3-methyl-1-butenyl)-1,4-naphthoquinone], in the absence and presence of oxygen, which were aimed at investigating its electrochemical reduction mechanism and possible oxygen interaction with its radical anion. The electrochemical behaviour is complex and similar to that observed for lapachol, and the first peak is related to the semiquinone formation, although complicated by the occurrence of self-protonation mechanisms and hydrogen-bonded intermediates formation. The observed positive shift in the potential of the first wave of isolapachol, in comparison to lapachol, is related to the higher acidity of the enolic group. The cyclovoltammetric results obtained in the presence of O2 clearly indicate the consumption of the semiquinone anion-radicals of isolapachol by oxygen, in an EC type reaction, generating the deprotonated form of isolapachol and HOO•. The observed generation of the superoxide radical, after electron transfer can be related to the mechanism of biological action and toxicity of isolapachol.