Bioassay-guided isolation and EPR-assisted antioxidant evaluation of two valuable compounds from mango peels

Bioassay-guided HPLC/ESI-MS2 analysis of the ethyl acetate extract of mango peels (MPE) led to the isolation of two major valuable compounds, ethyl gallate (2) and penta-O-galloyl-glucoside (5). In this work, EPR spin-trapping technique was utilised for their antioxidant evaluation on the level of reactive oxygen species. Results demonstrated MPE, 2 and 5 possessed potent scavenging effects on hydroxyl radicals (OH)(OH), superoxide anions (O2-) and singlet oxygen (1O2). With respect to OHOH, 5 has much stronger inhibitory activity than 2, owing to more galloyl groups and the occurrence of intramolecular reactions in 5. Synergistic effects could be observed and explained the strongest scavenging property of MPE (IC50 = 4.08 μg/ml, kA = 3.51 × 1015 M−1 S−1) in terms of more potent inhibition of 5, which was caused by the inhibitory effects of 2 on 5 consumption. Regarding 1O2, 2 has a stronger quenching activity than 5, partially revealing that gallate type compounds quenched 1O2 with different mechanisms from those proposed against OHOH.