Antioxidant activity-guided separation of coumarins and lignan from Melicope glabra (Rutaceae)

The ethyl acetate and methanol bark extracts of Melicope glabra were evaluated for their antioxidant capacities by 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity and β-carotene bleaching/linoleic acid system. Both extracts exhibited strong inhibition against the DPPH radical (IC50 values of 24.81 and 13.01 μg ml−1, respectively) and strong antioxidant activity in β-carotene bleaching assay. Both samples were found to have high phenolic content with values of 39 and 44 mg GAE/g as indicated by Follin–Ciocalteau’s reagent. Antioxidant TLC assay-guided isolation on the methanol extract led to the isolation of a new pyranocoumarin, glabranin (1), umbelliferone (2), scopoletin (3) and sesamin (4), and their structures were determined by spectroscopy. Compounds (1–3) showed significant activities on DPPH free radical with the IC50 of 240.20, 810.02 and 413.19 μg ml−1, respectively. However, in β-carotene bleaching assay, sesamin (4) showed higher inhibitory activity (1 mg ml−1, 95%) than glabranin (1) (1 mg ml−1, 74%), whilst umbelliferone (2) and scopoletin (3) were slightly pro-oxidant.