Abstract:

Six different natural flavonoids with similar structures, quercetin, kaempferol, luteolin, naringenin, baicalein, and apigenin were selected to examine the redox system between flavonoids and Ce(Ⅳ) using ultraviolet spectrophotometry and microtitration methods. According to their responding difference to Ce(Ⅳ), the mechanism of the reaction and the contribution of phenolic hydroxyls on different sites of the chromone to the antioxidation activity were investigated. The result indicates that the 3-OH of the pyrone ring is a sensitive site to react with Ce(Ⅳ) involving the proton dissociation, and the obtained product has a strong absorbance at 294 nm. The 3′-OH and 4′-OH on the B ring of the 2-phenyl-chromone are also the sensitive sites to oxidants since 4′-OH dissociates and forms intramolecular hydrogen bonding. The hydroxyl on ring C is more sensitive to Ce(Ⅳ) than that on ring A. The antioxidant capacity order of the six compounds is:Quercetin＞Kaempferol＞Luteoin＞Apigenin＞Baicalein＞Naringenin, which is consistent with the sensitivity of spectral response of the six compounds to Ce(Ⅳ).

Key words: Ce(IV), flavonoids, antioxidant capacity, UV-Vis spectrum