【作者】 周敏娜；

【导师】 吕庆章；

【作者基本信息】 河南师范大学 ， 物理化学， 2011， 硕士

【摘要】 抗氧化剂是一种重要的食品添加剂,对人类健康至关重要。它是指具有传递电子作用、与氧化剂发生氧化--还原反应,从而清除自由基或抑制自由基形成的物质(还原剂)。量子化学是以量子力学为基础,利用量子力学的原理和方法来研究化学中的问题的一门理论科学。从量子化学理论出发,人们能对化学物质的结构、功能、反应做出新的设计和预测。采用密度泛函理论方法的量子化学计算是当前量子化学计算中应用最为广泛的方法。维生素是具有化学活性的非酶类抗氧化剂。维生素A能够维持人类正常的视觉反应、维持上皮组织的正常形态与功能、以及维持正常的骨骼发育,对人体健康有着极大的作用。运用HPERCHEM7.5上的AM1半经验量子化学计算方法进行初步的分子结构优化,得到化合物的一些理化参数。接着运用Gaussian03量子化学程序,利用密度泛函理论方法B3LYP对维生素A进行分子结构优化并进行了振动频率计算。结果显示计算的振动频率无虚频,维生素A的偶极矩和脂水分配系数较大,可以看出其为脂溶性的物质,从前线轨道电子云分布可以看出,维生素A容易失去共轭双键和羟基的电子从而表现出还原性,从而发挥抗氧化作用。大多数类胡萝卜素是维生素A原,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发生反应,形成无害的产物。运用密度泛函理论方法对虾青素、β-胡萝卜素、玉米黄素和叶黄素分子进行了分子结构优化和单点能计算。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⊿ E (LUMO-HOMO)变小,光谱移至可见区,从而使各种类胡萝卜素显出不同的颜色。从原子的NBO电荷值,前线轨道⊿E方面分析可见,抗氧化活性高低次序理论上应为:虾青素>玉米黄素≈β-胡萝卜素>叶黄素。此外,玉米黄素和叶黄素结构中的羟基会增强其与水溶性或脂溶性氧化物反应,导致其清除自由基的能力大于β-胡萝卜素。运用密度泛函理论方法优化计算了维生素C的分子结构及其红外光谱。从分子中O-H的键长、O-H上H原子的正电荷值、以及分子和自由基的生成能、前线分子轨道结构特征等方面讨论了维生素C的抗氧化活性位点和还原性特征。抗氧化剂清除自由基的活性与O-H键的强度成反比,O-H的键的强度越弱则抗氧化剂的活性越高。对比维生素C分子的前线轨道能量和其去掉H原子以后的自由基前线轨道能量,可以看出失去H原子后,导致其电子最高占据轨道能级降低,从而使其失电子能力降低,即还原性降低。从理论上得出维生素C分子具有还原性的特征是由于它容易失去H自由基。运用Gaussian03进行构型优化和频率计算,得到维生素C分子及其去掉H原子以后的自由基的红外光谱。对计算的LVC分子的光谱、标准LVC分子及水溶液中测定的LVC分子的红外光谱进行了对比;对计算的几种自由基的红外光谱与标准LVC分子红外光谱也进行了对比。结果表明,只有失去H18或者H19后得到的自由基的红外光谱与标准维生素C红外光谱频率最为相近,可以预测标准维生素C最可能的存在形式是失去H18或者H19后的自由基。更多还原

【Abstract】 As important food additive, antioxidant is essential to human health. It can eliminate free radicals by transmitting electron and form harmless products (reducing agent). Quantum chemistry, a kind of theoretical sciences, is based on the principles of quantum mechanics to study the chemical problems. On the basis of quantum chemical theory, people can make new designs of chemical reactions and predictions on chemical substances structures and functions. Now the Density Functional Theory is the most popular method used in quantum chemistry calculations.Vitamin is a kind of antioxidants which owns biologic activity but not enzymes. Vitamin A plays an important role in human health. Vitamin A can sustain human normal visual response, maintain the normal form and function of the epithelial tissue, and maintain the normal growth of bone. First, the semi-empirical quantum chemistry calculation method AM1in HPERCHEM was used to optimize the molecular structure of Vitamin A, and some quantum chemical parameters of the compound were obtained. The results showed that the dipole moment and lipid-water partition coefficient of Vitamin A are both very large, so it is fat-soluble substance. Then, the Density Functional Theory B3LYP method in Gaussian03 was used to optimize its geometry structure ulteriorly and calculate its vibration frequencies. The results showed that the compound can exist stably for no imaginary vibrational frequencies appearing in its IR spectrum. Based on the structure property of the frontier molecular orbital, one can see that vitamin A can lose its electrons of the conjugateπbond and hydroxide easily. That is just the reason of Vitamin A showing reductive and anti-oxidative properties.It is well-known that there is a conjugatedπbond in the carotenoid molecular structure. It can eliminate free radicals in the organisms by reacting with free radicals through the conjugatedπbond to form harmless products or destroy the chain reaction of involving radicals. In this work, the Density Functional Theory was adopted to optimize the molecular structures of astaxanthin,β-carotene, zeaxanthol and lutein, and then B3LYP / 6-311 (d) method was applied to calculate their Single Point Energys (SPEs). The carotenoids’molecular structure, total energy, NBO charge value of the atoms and the structures of the frontier molecular orbitals were analyzed in detail. The results showed that in the conjugated system, the molecular conjugated effects make the C-C bond lengths unified, the total energy and the energy differences⊿E (LUMO-HOMO) reduced, and the absorptive spectrum shifted to the visible spectrum area. Based on the electronic parameters of the NBO charges and the structures of the molecular frontier orbitals, one can concluded theoretically that the antioxidation activity goes along with the series as astaxanthin >β-carotene≈zeaxanthol > lutein. But the hydroxide radicals in the structures of zeaxanthol and lutein enhance their hydrophilicity and make them easy to react with water-soluble or fat-soluble oxides. So their ability of eliminating free radicals is higher than that ofβ-carotene.The structures and its several free radicals losing one H atom of O-Hs of Vitamin C were optimized by using Density Functional Theory (DFT) B3/LYP method at 6-311++G (2d, p) basis set. The reductive feature and active sites of the vitamin C were investigated based on the length of O-H bonds, the charge value on the H atoms of the O-H, the total energies of the Vitamin C molecule and its free radicals, and the structures of their frontier orbital. The results showed that the activity of antioxidant to scavenge free radicals in organism is negative to the strength of the O-H bonds. The lower the O-H bond strength is, the higher the anti-oxidative activity is. For the highest occupied orbital energy of the free radicals is lower than that of Vitamin C, their ability of losing electron is lowered, it means that the reducibility of Vitamin C decreases. So it can be concluded theoretically that the vitamin C possesses the reductive ability due to its losing H atom easily.The IR spectrum of the vitamin C molecule and its free radicals after losing H atoms were calculated in this work yet. The infrared spectrums of the LVC molecular calculated, spectrums of standard LVC sample, and spectrums of LVC measured in aqueous solution were compared. Also, the infrared spectrums of several free radical molecules calculated were compared with the standards LVC IR. The results showed that only the infrared spectra of the free radicals by losing H18 or H19 were most similar to that of the standard vitamin C. So we guessed that the vitamin C usually exists in the form of free radicals by losing H18 or H19.更多还原