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类胡萝卜素分子结构中含有一个共轭大π键,可能是这种特殊的结构使它容易与自由基起反应,形成无害的产物,或通过破坏自由基链反应,将自由基清除。运用密度泛函理论方法(density functional theory,DFT)在B3LYP/6-31(d)水平上对虾青素、β-胡萝卜素、玉米黄素和叶黄素四种类胡萝卜素分子进行了优化计算,之后在B3LYP/6-311(d)水平上进行了单点能(single pointenergy,SPE)计算。文章从分子的几何结构、总能量、原子的NBO电荷值、以及前线轨道结构方面详细讨论了它们性质。研究表明,分子的共轭效应使C-C单双键的键长均匀化,同时降低了分子的能量,最高占据轨道与最低空轨道能级间隔ΔE(LUMO-HOMO)变小,光谱移至可见区,从而显出多种颜色。从原子的NBO电荷值,前线轨道ΔE方面分析可见,抗氧化活性理论上虾青素>玉米黄素≈β-胡萝卜素>叶黄素。玉米黄素和叶黄素结构中的羟基的亲水性会增强其与水溶性或脂溶性氧化物反应,导致其清除体内自由基的能力大于β-胡萝卜素。
The carotenoid molecular structure contains a conjugated π bond. It may be that this particular structure makes it easy to react with free radicals to form harmless products or to destroy free radicals by disrupting the free-radical chain reaction. Six kinds of carotenoid molecules of astaxanthin, β-carotene, zeaxanthin and lutein were optimized by density functional theory (DFT) at B3LYP / 6-31 (d) B3LYP / 6-311 (d) level single point energy (single pointenergy, SPE) calculations. The article discusses their properties in detail from the geometry of the molecule, the total energy, the NBO charge of the atom, and the frontier orbital structure. The results show that the conjugate effect of molecules makes the bond lengths of CC single and double bonds uniform and reduces the molecular energies. The LUMO-HOMO interval between the highest occupied orbital and the lowest empty orbitals decreases and the spectrum shifts to the visible region , Which shows a variety of colors. From the atomic NBO charge value, the frontier orbital ΔE analysis shows that the antioxidant activity of astaxanthin theoretically> zeaxanthin ≈ β-carotene> lutein. Hydrophilicity of the hydroxyl groups in the zeaxanthin and lutein structures enhances their ability to react with water-soluble or fat-soluble oxides, resulting in their ability to scavenge free radicals more than the beta-carotene.