Antioxidant mechanisms and products of four 4,5,7-trihydroxyflavonoids with different structural types

4 ',5,7-OHs are common substituents of natural flavonoids, a type of effective phenolic antioxidant. However, the antioxidant processes between 4 ',5,7-trihydroxyflavonoids with different structural types have not been compared systematically, and the antioxidant products are challenging to determine. This study compared four 4 ',5,7-trihydroxyflavonoids, including apigenin, genistein, kaempferol, and naringenin. In quantum chemical analyses, the four 4 ',5,7-trihydroxyflavonoids showed different thermodynamic properties, and the C-4 '-OH (or C-3-OH of kaempferol) possessed the strongest activity. Moreover, the reaction rate constants were larger when a hydrogen atom was transferred from C-4 '-OH (or C-3-OH of kaempferol) than from C-5-OH. When different atoms were linked to 2,2-diphenyl-1-picrylhydrazyl radical (DPPH), the C-3 '-DPPH adducts showed the smallest energy. In experimental assays, the scavenging ability for neutral free radicals, radical cations, and radical anions was negatively correlated with the corresponding theoretical parameters. Finally, mass spectroscopy detected the apigenin-DPPH, genistein-DPPH, and naringenin-DPPH adduct peaks. In conclusion, the structural type of 4 ',5,7-trihydroxyflavonoids can affect the antioxidant ability, site, and speed, but not the mechanism. After hydrogen abstraction at C-4 '-OH, 4 ',5,7-trihydroxyflavones, 4 ',5,7-trihydroxyisoflavones, and 4 ',5,7-trihydroxyflavanones will produce antioxidant products via C-3 '-radical linking.