Photochemical conversion of sanguinarine to oxysanguinarine

Abstract

The photochemical properties of the alkanolamine form of sanguinarine, a benzophenanthridine plant alkaloid, were studied using absorption, fluorescence and high pressure liquid chromatography techniques. An excitation time dependent decrease in the fluorescence intensity of its emission maximum, along with changes in the emission and absorption spectral pattern was observed when the solution of alkanolamine form was excited under stirring. The rate of the reaction was dependent on the number of photons and the process was found to be irreversible. The irreversibility of the fluorescence excitation, emission and absorption spectra of the alkanolamine solution on excitation confirmed that this form of the alkaloid was undergoing a photochemical conversion in the excited singlet state. Further evidence for a photo-oxidation reaction was obtained from the experiments performed in the absence of molecular oxygen. On saturation of the solution with nitrogen gas, the photochemical conversion was not found occur indicating an absolute requirement of molecular oxygen in the reaction. The photoproduct was isolated and identified as oxysanguinarine. On the basis of these observations, it is concluded that the sanguinarine alkanolamine form undergoes an irreversible photo-oxidation in the excited singlet state to produce oxysanguinarine.