The Oxidation And Deterioration Mechanism Of Phytosterols Induced By Light Based On Density Functional Theory

Phytosterols(PS),a group of natural compounds,are widely distributed in foods.They have been proved to have various bioactive properties,including reducing serum total and low-density lipoproteins(LDL)-cholesterol concentration and showing protection against various chronic ailments.However,PS are prone to be oxidized with the formation of phytosterols oxidation products when exposed to heat and light,losing the original functions and even doing harm to human health.At present,researches are mainly focus on the thermal-oxidation of PS,rather than photooxidation.And the mechanisms and the key intermediates of PS photooxidation still remain unclear.To understand and control the photooxidation of PS,it is very important to explore the processes of PS photooxidation to form POPs.Theoretical studies of reaction mechanism through computational methods have been greatly facilitated by the innovation of computer technology,making the study of complicated reaction details come true.What's more,the methods help to obtain the structures of intermediates and transition states and explain the processes and stereoselectivity.The aim of this study is to clarify the mechanism of PS photooxidation from both the microscopic reaction and macroscopic performance,and to establish the reaction pathway network by density functional theory and evaluate the important factors that may improve or inhibit the occurrence and development of PS photooxidation.The main contents and results of this research are summarized as follows:1.The active reaction sites of PS in photooxidation process were analyzed by quantum chemical calculation method combined with Multiwfn and VMD program.At M06-2X/6-311+G**-CPCM(eps=3.202)//B3LYP/6-31G*level,the active reaction site of PS was the double bond on the B ring by MEP,NPA and FMO analysis.It indiated that PS was easy to be attacked by electrophile and prone to be photooxidized with singlet oxygen through ene reaction mechanism.2.The reaction process of PS photooxidation was studied based on density functional theory.Then the reaction pathways of different products,the chemical selectivity,and regioselectivity were constructed and discussed.At M06-2X/6-311+G**-CPCM(3.202)//B3LYP/6-31G*level,the dominant intermediate of PS photooxidation was the 5-OOH,while the secondary intermediate was 6-OOH.What's more,5-OOH was formed through a hydrogenabstraction step followed by free radical coupling,with a unique product of a conformation.6-OOH was formed through an open-chain diradical mechanism,leading to the ? conformation as the main product,prior to the a conformation.3.The rearrangement and isomerization processes of hydroperoxide were established,clarifying the mechanisms of chirality change.The rearrangement from 5a-OOH to 7a-OOH was through the peroxyl dissociation-readdition mechanism,involving a peroxyl precursor that generated a doublet intermediate pair.The pair could collapse to the doublet rearrangement radical or diffuse into solution.The selectivity of the rearrangement thus resulted from a cage effect wherein the diffusion of the allyl radical-oxygen pair was prevented due to caging by the solvent.The isomerization from 7a-OOH to 7?OOH occurred by dissociation into allyl and 3O2 fragments.But the separation between the fragments was larger,allowing more extensive oxygen exchange with the atmosphere,and the diffusion of the dioxygen to the opposite face of the sp2 carbon radical center.4.The mechanisms established above was verified by the performance of PS photooxidation in the oil matrix.And the effects of light intensity,photosensitizers and the unsaturation degree of fatty acids methyl esters were investigated in simulation conditions.The amount of POPs was sequenced as:7?-hydroxy>7a-hydroxy>5?,?-epoxy>7-keto>5?.6?-epoxy>6?-hydroxy.The results showed that the 7-hydroxyl/ketone were the main products,accounting for more than 2/3 of total POPs,while the 6-hydroxyl were secondary products,accounting for only 5.2%,with the dominant conformation was ?-type.The calculation results were consistent with these conclusions,indicating that the mechanisms of PS photooxidation established in this paper were reliable.Mathematical modeling of the PS photooxidation all followed the first order dynamics.The intensity of light was negatively correlated with photo-stability of phytosterols.Both the existence and higher concentration of photosensitizers could promote the photooxidation of PS while an inhibitory effect of the unsaturated matrix against photooxidation was found in the model system.And such effect seems to be further promoted by the increasing of unsaturation degree.The mechanisms of PS photooxidation established in this study enrched the mechanisms of oil photooxidation,suggesting that it was a powerful exploration and successful practice of applying quantum chemical calculation to the theoretical research of food chemistry.Methods were also provided for the control and prevention of PS photooxidation in the process of food industry production and sales.