Density Functional Theory Calculation on Principle Vitamin Eliminating Free Radicals
|Course||Physical and chemical|
|Keywords||the Density Functional Theory vitamin antioxidant activity molecular orbital infrared spectrum|
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.