Comparison studies on antioxidant capacities of flavonoids by different assays and the structure-activity relationship implication

The antioxidant activities of four flavonoids( apigenin,kaempferol,luteolin,quercetin) and their glycosides were studied by using three chemical assays( DPPH,FRAP and ORAC assays) and cellular antioxidant activity( CAA) method.Both DPPH and FRAP assays showed that apigenin and its glucosides with only one-OH on B ring but without C- OH showed extremely low antioxidant capacity. Flavonoids with 3',4'- OH on B- ring showed significant higher antioxidant activity than those had only one 4'-OH when they had same structure of C ring. For example,luteolin and quercetin had higher antioxidant activity than that of apigenin and kaempferol,respectively.In the mean time,ORAC assay gave a contrary result.All three chemical assays showed that the antioxidant activities were decreased when C- 3- OH was substituted by glycosyl,which suggested that 3- OH on C- ring also contributed to antioxidant capability of flavonoids.Antioxidant capabilities of flavonoids were increased when A-ring was glycosylated at 6 or 8 position and formed C-glycosides.CAA results showed that apigenin and its glycosides with only one OH on B ring but no C-OH had no cellular antioxidant activity.Luteolin with two-OH on B-ring but no C-3-OH showed higher antioxidant activity.Unlike those chemical methods,C-glycosylation on A ring decreased the CAA of its aglycone. The antioxidant activity effect of O-glycosylation on C- ring was more complicated compared with chemical assays.