Flavone C-glycosides from Capsicum annuum L.: relationships between antioxidant activity and lipophilicity

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Three flavonoid C -glycosides isolated from pepper fruit ( Capsicum annuum L.), var. Capel Hot (luteolin 6- C -glucoside, luteolin 6,8-di- C -glucoside and apigenin 6- C -glucoside-8- C -arabinoside), were investigated to determine their antioxidant activity and lipophilicity. The antioxidant activity was evaluated using in vitro methods to generate free radicals in hydrophilic (superoxide radical) and lipophilic (2,2-diphenyl-1-picrylhydrazyl (DPPH) and peroxide radicals) media. Parameters characterising the lipophilicity (log P ) of the studied compounds were calculated using three commonly available software programs. The relationships between the calculated log P of the investigated compounds and the experimentally determined antioxidant activities are discussed. The chemical activity of the flavonoid C -glycosides and aglycones was variable depending on the type of assay used. Luteolin and its C -6 glucoside showed the greatest ability to scavenge superoxide radicals (generated in the enzymatic and non-enzymatic systems) and DPPH radicals and also had the strongest activity in inhibiting xanthine oxidase activity, followed by apigenin- C -glycoside and apigenin. A high positive correlation coefficient was found between the ability of tested C -glycosides to inhibit xanthine oxidase and the calculated log P values. The highest correlation coefficient was obtained for the ACD/log P (Advanced Chemistry Development, Inc.) method ( R = 0.790). In contrast, negative correlations were obtained between the abilities to scavenge superoxide, DPPH and lipid peroxyl radicals. The highest negative correlation coefficient was obtained in a β-carotene/linoleic acid model system and the Ghose–Crippen method ( R = −0.725).