The molecular structures of calycosin,calycosin-O-β-D-glucopyranoside,formononetin and ononin are calculated by using the Density Functional Theory B3LYP method with 6-311G(d,p)basis set The four compounds having good antioxidant activity to scavenge harmful free radicals for health are isoflavones from astragalus. It is found that the more positive charge on H atom of C3· -OH or C7-OH, the lower bond dissociation energy of the hydroxyl, the higher energy level of the HOMO and the smaller ΔE( lumo-homo) are of benefit are of to the antioxidant activity of isoflavone compounds. The C3· -OH is the highest active site of calycosin and calycosin-O-p-D-glucopyranoside, while the C7-OH is also an active site but possessing lower activity relatively. The substitution of the H atom of C7-OH by glucoside decreases the'antioxidant activity for losing the phenolic hydroxyl in calycosin-O-β-D-glu-copyranoside and ononin. The antioxidant activity of the four compounds goes along with the series Calycosin>Calycosin-O-β-D-glu-copyranoside﹥Formononetin﹥﹥Ononin. The transition state of the reaction of formononetin scavenging hydroxyl radical is calculated.%本文采用密度泛函理论中的B3LYP方法在6-311G(d,p)水平上对黄芪中的四种异黄酮类化合物毛蕊异黄酮、毛蕊异黄酮苷、芒柄花素、芒柄花苷进行了优化计算,从四个分子的几何结构、酚羟基氢原子上的NBO电荷、酚羟基解离能、HOMO和LUMO能级以及其能级差△E(LUMO-HOMO)等方面分析了四种黄芪异黄酮类化合物清除自由基的活性.C3.位的酚羟基为毛蕊异黄酮苷元及其苷分子的最大可能活性位点,C7位酚羟基也具有一定的活性,可以增加分子本身的抗氧化活性,C7位酚羟基为芒柄花素分子的活性位点.C3,位或C7位上酚羟基氢原子带正电荷越大、酚羟基的解离能越小、△E(LUMO-HOMO)越小、HOMO能级相对越高分子的抗氧化活性越高.糖苷取代C7位酚羟基上的H原子,可以提高HOMO、LUMO的轨道能级,但是分子失去了7位酚羟基,从而降低了毛蕊异黄酮苷分子的抗氧化活性.结果表明,四种黄芪异黄酮类化合物的抗氧化能力大小为:毛蕊异黄酮＞毛蕊异黄酮苷＞芒柄花素＞＞芒柄花苷.对芒柄花素和羟基自由基反应的过渡态进行了计算研究.
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