Electrochemical oxidation of berberine and mass spectrometric identification of its oxidation products

Electrochemical oxidation of the isoquinoline alkaloid berberine in aqueous medium was studied by cyclic and differential pulse voltammetry at a glassy carbon electrode (GCE). Two anodic peaks of the quaternary form of berberine were observed at + 1.2 V and + 1.4 V (vs. SCE) in acidic and neutral solutions. When the anodic polarization exceeded the value of + 1.1 V, the redox active film is formed on the GCE surface. The formation of adsorbed film was well-documented by quasireversible redox couple at + 0.25 V which was studied in redox cycling experiments. In alkaline medium, a new anodic peak at + 0.5 V appeared due to oxidation of berberine pseudobase to 8-oxoberberine. Solutions of berberine at different pH were subjected to controlled potential electrolysis on platinum gauze electrode and analyzed using liquid chromatography (HPLC) equipped with electrospray ionization/quadrupole time-of-flight mass spectrometry. The main water soluble monomeric product of berberine oxidation under physiological-near experimental conditions, OP1, was identified as demethyleneberberine cation (2,3-dihydroxy-9,10-dimethoxy-5,6-dihydroisoquinolino[3,2-a]isoquinolin-7-ium).

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► Electrochemical oxidation of the isoquinoline alkaloid berberine in aqueous medium has been studied.
► In this study cyclic and differential pulse voltammetry at a glassy carbon and platinum electrode was used.
► The main water soluble product of berberine oxidation (OP1) was identified as demethyleneberberine cation.
► The electrochemical approaches presented here could be useful for the study of oxidation of berberine and its derivatives.