Electron Transfer Reaction Studies of Usnic Acid and Its Biosynthetic Precursor Methylphloroacetophenone

Abstract

This study aims to investigate the electrochemical properties of usnic acid (UA), a secondary metabolite commonly biosynthesized by a variety of lichen species, and its biosynthetic precursor methylphloroacetophenone (MPA). During cyclic and differential pulse voltammetry, well-defined anodic peaks were observed for UA and MPA in 0.04 M Britton–Robinson buffer solution (pH 5) containing 20% (v/v) acetonitrile. The absence of cathodic peaks during the reverse voltammetric scans revealed that both oxidation reactions are chemically irreversible. Scan rate studies demonstrate that UA oxidation is an adsorption-controlled process, whereas the oxidation of MPA molecules occurs as a diffusion-controlled process. For both molecules, the number of electrons transferred during the oxidation was calculated to be 3. Differential pulse voltammetry results demonstrate that the anodic peak for the two molecules is markedly influenced by the solution pH and the same numbers of protons and electrons are involved in the oxidation process of the molecules. Based on the evidence generated by the electrochemical studies, oxidation mechanisms are proposed for UA and MPA, which involves a two-step electron loss with a hydration reaction taking place in between. This study provides an understanding of the bioactivity mechanisms of these two natural products.