Study On Free Radical Scavenging Mechanism Of Several Polyphenols From Cherry

The pericarp and flesh of cherries are rich in natural polyphenols,which have high antioxidant,antibacterial and anti-inflammatory abilities.These active substances enter the cherry wine through different degrees of impregnation during the microbial fermentation process,making the cherry wine a comprehensive fruit wine that is“easy to drink”,“nourish”and“health”.In order to improve the content and stability of active substances in cherry wine during microbial fermentation,it was explored the antioxidant mechanism of scavenging free radicals with polyphenolic active substances in cherry and identified the antioxidant activity of different active substances.Through the research,it provided theoretical and technical support and exploration direction for the microbial fermentation of cherry wine.In this paper,we mainly studied the difference contents of antioxidant active substances in different cherry cultivars in Yantai region and analyzed the antioxidant mechanism of three kinds of 14 polyphenols which have been separated and identified in cherry from the perspective of quantum chemistry by Density Functional Theory?DFT?.Meanwhile,providing an analytical model and theoretical basis for studying the mechanism of antioxidant activity of polyphenols,the detailed research contents and conclusions are as follows:?1?The objective of the study was to throw light on the contents of the basic physiochemical indexes and the active substances of 8 sweet cherry cultivars,including Pioneer,Hongdeng,Early big,Tieton,Black pearl,Brooks,Lapins,Summit.All of them were determined and analyzed the correlation between the contents with the free radical scavenging rate.The results indicated that there were certain differences in the sugar content,titratable acid,pH value and Vc content of different varieties of cherries.Their total phenol content of the sweet cherry Early big and Black pearl,was relatively high,about 62.00mg/100 g.The total flavanol content was around 2.7 mg/g among the eight sweet cherry cultivars.Especially,the cherry Brooks had the highest content of total flavonoids,at 5.98mg/g.The content of total anthocyanin in different cherry varieties was significantly different,and the cherry Black pearl had the highest content of total anthocyanin was 57.12mg/100g.The hydroxyl radical scavenging rate of different varieties of cherries was determined.Brooks and Black pearl had the highest scavenging rate,which was 59.18%and 60.16%respectively.But the cherry Hongdeng had the lowest clearance rate at 40.11%.Correlation analysis was carried out on the 10 indicators.The total sugar content was positively correlated with the sugar acid ratio,total phenols,total flavonoids,total flavanols and total anthocyanins,and was significantly positively correlated with the hydroxyl radical clearance rate?P<0.05?.The total flavanols were significantly positively correlated with the total flavonoids?P<0.05?.The hydroxyl radical clearance rate was positively correlated with sugar acid ratio,total phenols,total flavonoids,total flavanols and total anthocyanins,and was significantly positively correlated with total sugars?P<0.05?.?2?In this section,we used density functional theory to optimize the calculation of Neochlorogenic acid?3-CQA?,Crypto chlorogenic acid?4-CQA?,Chlorogenic acid?5-CQA?and Isochlorogenic acid A?3.5-DICQA?in cherry at the level of M06-2X/6-311G++?d,p?respectively in water and gas phases,and to explore the antioxidant mechanism of scavenging free radicals.The results showed that the four molecules were more stable in gas phase than in water phase.The main active sites of chlorogenic acids were found to be located on the phenolic hydroxyl groups at C_5',C_5''positions by molecular geometry,by calculating the geometric configuration parameters of four chlorogenic acids molecules,the charge distribution of phenolic hydroxyl group,the bond dissociation energy of phenolic hydroxyl group and the map of Front-line molecular orbital energy level.Among the four phenolic acids,the antioxidant activity of 3.5-DICQA was the strongest,while the activity of 3-CQA,5-CQA and 4-CQA decreased in turn.At the same time the antioxidant activities of 3-CQA,4-CQA and 5-CQA are similar.The results were consistent with the experimental results,which proved that the model of this study was feasible.?3?In this section,density functional method is used to optimize the calculation of five species of Cyanidin,Peonidin and their derivatives in cherry.The antioxidant activities of five anthocyanins were in the order of Cyanidin-3-O-glucoside>Cyanidin-3-O-rutinoside>Cyanidin-3-O-sophoroside>Peonidin-3-O-glucoside>Peonidin-3-O-rutinoside.On the B ring,the C_4'position of phenolic hydroxyl groups is the most susceptible site for hydrogen abstraction reaction and has the highest activity in the five anthocyanin compounds.On the other hand,the A ring has certain activity.The phenolic hydroxyl group at position C_5',was easy to form intramolecular hydrogen bond with C_4'phenolic hydroxyl group,which reduced its partial antioxidant activity.From the energy level diagram of Front-line molecular orbital energy level,it is found that the phenolic hydroxyl groups on the C₃ sites of glucoside,rutinoside and sophoroside have no electron density distribution,so there was no antioxidant activity.?4?In this section,the optimization calculation of the five kinds of flavonoids in cherry were carried out to explore its antioxidant mechanism.The results showed that the antioxidant activities of the five flavonoids were in the order of Epicatechin>kaempferol>Quercetin>quercetin-3-O-glucoside>quercetin-3-O-rutinoside.The most active sites of epicatechin,quercetin and kaempferol were located at C₃ site.The energy gap of frontier molecular orbitals of kaempferol,quercetin and epicatechin are smaller and more reactive than their glucoside derivatives.Among them,the epicatechin had the minimum value:?E_LUMO-HOMO=3.6273 eV,this demonstrated that its antioxidant activity is the strongest.In the five flavonoids of the cherry,the C_4'and C_5'phenolic hydroxyl groups on the B ring were the main active sites.The antioxidant activity of the phenolic hydroxyl group at the C₅and C₇ positions of the A ring was smaller than that of the C₃ phenolic hydroxyl group.According to the frontier orbital of epicatechin molecule and the orbital diagram nearby,electron densities of ring A,B and C were all distributed,but there was a gap in the ability to accept electrons.The ring B was the most likely to accept electrons,while ring A and C have a weak ability to accept electrons due to conjugate effect.In order to further explore the mechanism of antioxidant scavenging free radicals,it was found through the study of kaempferol C₃ hydroxyl as the attack object of free radicals that when the distance between free radical oxygen atom and phenol hydroxyl hydrogen atom d=0.1124 nm and the length of phenol hydroxyl bond l=0.1280 nm,it was the critical point for kaempferol to react with free radicals.The free radical only needs to overcome the potential energy of-85.6 Kcal/mol to react with the C₃ phenol hydroxyl group successfully.