Density Functional Theory Calculations On Scavenging Radicals Activity Of Several Natural Flavonoids

Quantum chemistry is based on the quantum mechanics, using the relevant theory and methods tostudy chemical problems. Density Functional Theory as a quantum-chemistry calculation method based onits very strict theoretical basis, has been widely used to analysize chemical properties, including electronicstructure, thermochemistry, spectrums, reaction mechanisms, drug molecular design, material science andlife science. Flavonoids which widely distributed in the plant kingdom, can scavenge free radicals andreveal excellent antioxidant activity. Because of their non-toxic and safety to human body, they areattracting much more attention as a kind of natural antioxidant.Luteolin generally exists in natural herbal, vegetable and fruit as a representational and natural flavone,and has various pharmacological actions on anti-oxidation, anti-bacterial and anti-tumor. The free radicalsscavenging activity of phenolic hydroxyls has been analyzed based on the static and dynamic propertiescalculated by using the density functional theory (DFT) method. The activity of the phenolic hydroxyls ondifferent sites of luteolin is discussed by the molecular structure parameters, including the dissociationenergy of the phenolic hydroxyl bonds, the electron spin density distribution and the correlative property ofthe frontier molecular orbitals(HOMO and LUMO). The reaction process of the three phenolic hydroxylson C4’, C5and C7sites scavenging hydroxyl radical(·OH) have been discussed respectively based onenergy of changing molecular structures in detail. The results show that there is not energy barrier needingto get over for the C4’-O-H during scavenging hydroxyl radical(·OH). So the transition state structuresaccording to the other two phenolic hydroxyls on C5and C7sites to scavenge OH free radical have beenoptimized by UB3LYP/6-311G(d,p) method. The energy barriers between the transition state and reactioncomplex are18.6and0.97KJ·mol-1separately. The C4’-OH has the highest antioxidation activity and theC5-OH has the lowest activity.The antioxidant activity of flavonoids depends on their chemical structures. Three luteolin derivativeswere designed by structural modification of luteolin. The structures of the derivatives have been structuraloptimized and calculated using density functional theory at the level of B3LYP/6-311+G (d,p)//B3LYP/6-311G (d,p). It is found that aminomethyl groups have different effect on the dissociationenergy(BDE) of O-H bonds of luteolin derivatives. The BDE values of C7-OH on the 8-aminomethylluteolin is obviously lower than luteolin, so the C7-OH of8-aminomethylluteolin has thehigher antioxidation.Kaempferol as an important natural flavone, exists widely in plant kingdom and exhibit manybioactivities and pharmacological function. The structures of kaempferol and eight possiblekaempferol-H2O complexes in the gas and liquid phases have been calculated by usingB3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) method. The free radicals scavenging activity of phenolichydroxyls on kaempferol has been analyzed based on several theoretical parameters. The results show thatthe C3-OH has the highest radical scavenging activity and the C5-OH reveal the lowest activity. Theintermolecular hydrogen bond between kaempferol and H2O are discussed based on the parameters ofkaempferol-H2O complexes. It showed that the distance between the H atom of C7-OH and the O atom ofH2O is the shortest, they have much higher interaction energy.Four flavonoids from Artemisia Ordosica, which are Apigenin-7,4’-dimethylether, genkainin,Hydroxygenkwanin and Rhamnetin, were geometrically optimized and calculated by using density functiontheory (DFT) B3LYP/6-311G(d,p)//B3LYP/6-31G (d,p) method. The properties of the flavonoids havebeen discussed in detail based on their molecular structures, NBO charges on hydrogen atoms of Phenolichydroxyl, the dissociation energy of O-H bonds, the energies of the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO), the energy gap between HOMO andLUMO, and etc. The results showed that compounds having more Phenolic hydroxyl groups andintramolecular hydrogen bonds reveal higher antioxidant activity. The C4’-OH has the highestantioxidation activity and the C5’-OH reveals the lowest activity，the C3-OH is favorable for the moleculeto form a conjugate system and improve its antioxidant activity. As a result, the antioxidant ability order ofthe four flavonoids compounds is Rhamnetin> Hydroxygenkwanin> genkainin> Apigenin-7,4’-dimethylether based on this work. In the end, the dynamic method has been adopted to analysis theRhamnetin molecular activity of scavenging free radicals.