Theoretical Study On Antioxidant Activity Of Curcumin Analogues

The DFT method of quantum chemistry at B3LYP/6-31+G(d) basis setlevels was employed to investigate the antioxidant activities of curcumincompounds or phenol compounds in this thesis. The parameters including atomicnet charge, electron spin density, dipole, O-H BDE, Mulliken charge distribution,highest occupied molecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO), et al were obtained. It was found that structure-activityrelationship of the curcumin kind of antioxidants by density functional theorymethod. This provides theoretical basis for searching for and designing a newhigh-efficiency antioxidants, as well as accelerating screening process ofcurcumin kind of antioxidants and providing the theory basis to synthesize ofnew high-efficiency antioxidants.The main results in this thesis can be summarized as follows.1.Antioxidant activity of curcumin, demethoxycurcumin,bisdemethoxycurcumin was investigated. The results showed that, with thenumber of methoxy groups were increased, O-H BDEs were reduced, dipolemoment were increased, energy gap Egapvalues were reduced, namely the orderof their antioxidant activity was curcumin> demethoxycurcumin>bisdemethoxycurcumin, which is in accordence with the results of experiment. Itshowed that methoxy in ortho position had played a key role in enhance antioxidant activity. The calculated O-H BDEs gave a reasonable explanation onthe free redical-scavenging activity diffierence of curcumin compounds.2.A series of curcumin derivatives that they have seven carbon atoms tobridge chain were investigated. The results showed that antioxidant activity ofcurcumin derivatives not only related to the number of hydroxyl groups, andwhether having ortho-methoxyphenolic and ortho-hydroxylpenolic structuralcharacteristics are closely related. Analysis by△E that, when phenolic hydroxyladjacent a hydroxyl or methoxy etc power supply groups are present, curcumincompounds released protons from4-or4’-hydroxyl and generated on the freeradicals,which is the most stable free radicals were played the best antioxidantactivity effect.The order of antioxidant activity of curcumin derivatives wascompoundsⅠandⅡwhich bear ortho-diphenoxyl functionality possesssignificantly highest antioxidant activity, compounds Ⅳand Ⅴ which bear4-hydroxyl-3-methoxyphenyl group possess significantly higher antioxidantactivity than other compounds such as compound Ⅲ, which is in accordencewith the results of experiment.3.A series of bridge chain of five carbon atoms of the symmetric andasymmetric compounds were studied.Curcumin compounds with the numbers ofhydroxyl and hydroxyl ortho methoxy groups were increased, electrophilic indexω was bigger, the chemical hardness η was smaller, the molecular stability waslower, the reaction activity of the compound was greater; Molecular minimumO-H BDE was lower, the dipole moment was increased, energy gap Egap valueswere reduced, the highest occupied molecular orbit energy ΕHOMOvalues wereincreased, electronic spin density values were smaller, single electron in a molecule in the atomic density distribution was more uniform, so thesemiquinonoid free radicals was relatively more stable, the antioxidant activity ofcompounds molecular was stronger. That is hydroxyl ortho methoxy includelowered molecular hydroxyl dissociation enthalphy, increased the dipole activityand electrophilic index ω, reduced chemical hardness η, had a great contributionto enhancement of molecular antioxidant activity. In short, the order of theirantioxidant activity was1-1>1-2>1-3>1-4>1-6>1-7>1-8>1-9, which alsoagree with experimental results.