A selective photobiological assay to detect and quantify psoralen in Psoralea plants (Leguminosae)

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Abstract
A simple and rapid bioassay involving the gram‐positive bacterium Bacillus brevis was developed to detect and quantify furocoumarins (psoralen and angelicin) in Psoralea plants (Leguminosae). Small paper discs, soaked with furocoumarin standards or the corresponding plant samples, are placed on B. brevis lawns in Petri dishes and irradiated for 24 h with UVA. The diameter of the growth inhibition zone around the discs is very well‐correlated with the quantity of furocoumarin standard in the disc. Because psoralen is more phototoxic than angelicin, its detection limit (5 × 10−8 g/disc) was lower than angelicin (5 × 10−7 g/disc). Consequently, it is possible to dilute a crude plant extract in order to reach a detectable psoralen concentration whilst maintaining the angelicin concentration below its lower detection limit. In this condition, angelicin does not interfer with psoralen detection, and the growth inhibition is entirely due to psoralen. The bioassay was validated using a classical high pressure liquid chromatographic method and the correlation between the two techniques was satisfactory with plant samples. Therefore, the bioassay can be considered as a sensitive, simple, rapid and selective method to quantify psoralen in Psoralea plants.