Electron-transfer disproportionation of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH˙) occurred in the presence of Sc(3+) acting as a strong Lewis acid in deaerated acetonitrile. In contrast, in the case of weaker Lewis acids than Sc(3+), such as Mg(2+) and Li(+), external protons from acetic acid were required for the disproportionation of DPPH˙ to occur.

The antioxidant activities of flavonoids and their glycosides were measured with the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH radical, DPPH · ) scavenging method. The results show that free hydroxyl flavonoids are not necessarily more active than O-glycoside. Quercetin and kaempferol showed higher activity than apigenin. The C-and O-glycosides of flavonoids generally showed higher radical scavenging activity than aglycones; however, kaempferol C3-O-glycoside (astragalin) showed higher activity than kaempferol. In the radical scavenging activity of flavonoids, it was expected that OH substitutions at C3 and C5 and catechol substitution at C2 of B ring and intramolecular hydrogen bonding between OH at C5 and ketone at C3 would increase the activity; however, the reasons have yet to be clarified. We here show that the radical scavenging activities of flavonoids are controlled by their absolute hardness (η) and absolute electronegativity (χ) as a electronic state. Kaempferol and quercetin provide high radical scavenging activity since (i) OH substitutions at C3 and C5 strikingly decrease η of flavones, (ii) OH substitutions at C3 and C7 decrease χ and η of flavones, and (iii) phenol or o-catechol substitution at C2 of B ring decrease χ of flavones. The coordinate r(χ, η) as the electron state must be small to increase the radical scavenging activity of flavonoids. The results show that chemically soft kaempferol and quercetin have higher DPPH radical scavenging activity than chemically hard genistein and daidzein.Key words antioxidant activity; chemical hardness; flavonoid; glycoside; 2,2-diphenyl-1-picrylhydrazyl radical Reactive oxygen species (ROS) and free radicals have been noted as a cause of many diseases; for instance, inflammation, cancer, diabetes mellitus, and neurodegenerative diseases such as Parkinson and Alzheimer diseases.1-4) ROS and other reactive species are related to about 90% or more of age-related diseases; therefore, the reduction of oxidative damage that contributes to the mechanisms of age-related disease and cancer is connected with saving many people from many diseases. Excess production of superoxide anion radical (O 2 −· ) and hydroxyl radical ( · OH), which is classed as ROS, has doublet spin multiplicity and seems to be the most dangerous factor in the biological system. In particular, · OH, which has high redox potential [E 0 ( · OH/H 2 O)=+2.32 V vs. normal hydrogen electrode (NHE), pH 7.0], 5,6) oxidatively damages the cell membranes, proteins, and DNA (and RNA) in organs in vivo.7) To defend the organs from strong · OH attack, potential radical scavengers or antioxidants such as vitamin E (VE) and vitamin C (VC), and flavonoids such as flavones and flavonols in many plant products play an important role in our health. 8,9) Although the flavonoids, especially flavones and flavonols, have been studied as antioxidants containing polyphenolic hydroxides, for instance, it is not yet clear why kaempferol is more active than apigetrin in radical-scavenging activity. Additionally, the ...

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Mg(2+) enhanced the scavenging activity of (+)-catechin and quercetin against the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH˙), while Al(3+) decreased their activity. Such effects of Mg(2+) and Al(3+) were not observed for kaempferol. Na(+) and Ca(2+) with large ionic radii showed little effect on the DPPH˙-scavenging activity of these three flavonoids.