DFT Study On The Quenching Mechanism Of Singlet Oxygen By Lycopene

Lycopene,a kind of carotenoids,widely distributed in fruits and vegetables,is a kind of natural food ingredients which is one of the strongest antioxidants in natural at present.It has many good physical activity and it could induce an inhibitory effect on low-density lipoprotein oxidation and HMG-CoA reductase activity,impact cholesterol biosynthesis,and lower blood triglyceride and cholesterol levels and so on.It is its multiple conjugated double bonds leading to a great variety of configurations.Among them,the all-trans lycopene has the vast majority and is the most predominant geometrical isomer in those configurations.However,the mechanism of scavenging singlet oxygen by either all-trans lycopene or cis-isomers is still unclear which has restricted the extensive development and application of lycopene in many fields.For singlet oxygen,it plays a key role in bio-oxidation process and the extinguished process of life,however,its disadvantages have outweighed the benefits.Therefore,the singlet oxygen and its quenching by lycopene stay at the forefront of scientific researches.Density functional theory has been applied to optimize the configurations of ground and excited states of lycopene and oxygen respectively.The quenching mechanism of singlet oxygen by lycopene is analyzed based on the calculations at B3LYP/6-311+G(d,p)level.In short,the quenching of singlet oxygen by lycopene is a process of electron exchange with energy transfer.That is to say,the excited oxygen in singlet state turns into ground state in triplet and gives out energy,lycopene in ground state absorbs the energy changing into excited lycopene at the same time.The quenching process involves exchange of two electrons in different spin coming from lycopene and singlet oxygen respectively.The quenching is spontaneous according to the calculated energy and Gibbs free energy change.From the point of energy level and the construction of frontier orbitals,electron exchanges simultaneously with energy transfer and different spin.In addition,it also can be concluded that oxygen and lycopene can pass electric charge to each other in the meantime,however,it is respectively dominated by one to the other in different stages.All in all,the close energy levels and well symmetry matching of the reactants' frontier orbitals,no electron spin-flip in the quenching system,and the negative Gibbs free energy difference all together make the quenching process very easy.In addition,this paper also putsemphasis on the discussion of four common lycopene isomers,which are 5-cis,9-cis,13-cis and 15-cis lycopene isomers respectively.Likewise,we optimize the configurations of the four common lycopene isomers and calculate the frequency of the corresponding lycopene isomers at B3LYP/6-311+G(d,p)level with density functional theory.On that basis,we analyze the energy,the Gibbs free energy and frontier molecular orbital of them in order to prove that cis lycopene isomers could quench singlet oxygen and analyze the quenching capacity of them.This paper proves and analyzes the quenching mechanism of singlet oxygen by lycopene,and also the performed work would certainly provide the theoretical basis and support for the further application of lycopene in food,chemistry,pharmaceutical industry,medical aspect and other fields.However,the quenching mechanism has not been confirmed with experiments.So,only researchers persistently figure out the mechanisms of scavenging singlet oxygen even the other free radicals,can the true value of lycopene be obtained a valid and accurate assessment.And the applied range of lycopene can be extended in the mean time.