Comparing antioxidant activities of flavonols from Annona coriacea by four approaches

Abstract Antioxidants protect cells from the injuries caused by oxidative reactive species, especially under biotic and abiotic stressful conditions. Each kind of free radical presents different oxidative sites and, therefore, different assays could be done to better evaluate the antioxidant capacity of a compound. Little is known about secondary metabolites from Annona coriacea Mart. (Annonaceae), especially related to their antioxidant capacity. Eleven flavonols were isolated from leaves: Quercetin-3- O -gentiobioside ( 1 ), Quercetin-3- O -robinobioside ( 2 ), Rutin ( 3 ), Hyperin ( 4 ), Isoquercitrin ( 5 ), Biorobin ( 6 ), Nicotiflorin ( 7 ), Keioside ( 8 ), Narcissin ( 9 ), Cacticin ( 10 ) and Isorhamnetin-3- O -glucoside ( 11 ). The isolated compounds and their commercial aglycones were evaluated in DPPH, ABTS +• radical scavenging, ferric reducing antioxidant power (FRAP) and oxygen reduction antioxidant capacity (ORAC). Two patterns of response were observed. In the first, found in DPPH and FRAP assays, quercetin and its derivatives (1–5) showed higher antioxidant activity than kaempferol (6–7) and isorhamnetin derivatives (8–11). On the other hand, in the second pattern, observed in ABTS +• and ORAC assays, isorhamnetin showed higher or similar activities than quercetin derivatives. These differences were attributed to the B-ring substitution pattern of flavonols and to the distinct mechanisms of antioxidant activity in each assay. Different assays must be performed to determine the mechanisms of antiradicalar action of a compound.