Waki Tsukasa, Nakanishi Ikuo, Matsumoto Ken-ichiro, Kitajima Junichi, Chikuma Toshiyuki, Kobayashi Shigeki
Division of Analytical Chemistry of Medicines, Showa Pharmaceutical University, 3-3165 Higashitamagawagakuen, Machida, Tokyo 194-8543, Japan.
Chem Pharm Bull (Tokyo). 2012;60(1):37-44. doi: 10.1248/cpb.60.37.
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.
采用2,2-二苯基-1-苦基肼自由基（DPPH自由基，DPPH(·)）清除法测定黄酮类化合物及其糖苷的抗氧化活性。结果表明，游离羟基黄酮类化合物的活性不一定高于O-糖苷。槲皮素和山奈酚的活性高于芹菜素。黄酮类化合物的C-糖苷和O-糖苷通常比苷元表现出更高的自由基清除活性；然而，山奈酚C3-O-糖苷（紫云英苷）的活性高于山奈酚。在黄酮类化合物的自由基清除活性方面，预计B环C3和C5位的OH取代、C2位的邻苯二酚取代以及C5位的OH与C3位的酮之间的分子内氢键会增加活性；然而，其原因尚待阐明。我们在此表明，黄酮类化合物的自由基清除活性受其作为电子态的绝对硬度（η）和绝对电负性（χ）的控制。山奈酚和槲皮素具有较高的自由基清除活性，原因如下：（i）C3和C5位的OH取代显著降低了黄酮的η；（ii）C3和C7位的OH取代降低了黄酮的χ和η；（iii）B环C2位的酚或邻苯二酚取代降低了黄酮的χ。作为电子态的坐标r(χ, η)必须较小才能提高黄酮类化合物的自由基清除活性。结果表明，化学性质较软的山奈酚和槲皮素比化学性质较硬的染料木黄酮和大豆苷元具有更高的DPPH自由基清除活性。
