Study On Antioxidant Structure-activity Relationship And Mechanism Of Phenolic Acids

Phenolic acids are not only plant secondary metabolites,but also important phenolics directly ingested by humans and animals.They have bioactivities such as anti-cancer,anti-virus,anti-inflammatory and anti-oxidation.To further estimate their action mechanisms,especially the relationship between dose-effect and structure-activity in antioxidants,several plant materials were selected and the correlation between their total phenolic contents?TPC?and antioxidant activity was analyzed.Then antioxidant activities of several typical phenolic acids were determined through experiments.Their antioxidant mechanisms were speculated by Density Functional Theory?DFT?.The main results were as follows:?1?Fifty-four plant materials were selected,TPC and total flavonoids contents,total antioxidant capacity?FRAP?and free radical scavenging ability of plant materials were measured and correlated.Based on TPC,54 plant materials were divided into 5 groups:group A?n=4?with total phenolic contents>200 mg/g;group B?n=8?with total phenolic contents of 100-200 mg/g;the total phenolic contents in group C?n=11?,group D?n=27?and group E?n=3?were all<100 mg/g.Total phenolic contents was significantly correlated with FRAP,and the linear fit is high,r=0.987.These results showed that there was a dose-effect relationship between TPC and antioxidant activity,and the Rosaceae plant:blackberry leaves,red raspberry leaves and walnuts of walnut seed coat would be used as a potential plant polyphenol material.?2?The effects of common substituent groups methyl?-CH₃?,methoxy?-OCH₃?and amino?-NH₂?on the antioxidant activity of phenolic acids were investigated based on salicylic acid?2-H-B?.Twelve 2-H-B derivatives were selected,and their antioxidant capacities were determined through experiments.The bond dissociation energy?BDE?,ionization potential?IP?,proton dissociation enthalpy?PDE?,proton affinity?PA?,electron transfer enthalpy?ETE?and highest occupied molecular orbital?HOMO?energy of phenolic acids were calculated by the B3LYP method in DFT theory.The results of antioxidant activity assays showed that the addition of these three substituent groups had improved the antioxidant activity of 2-H-B,the order of the was:-NH₂>-OCH₃>-CH₃,and the promotion effect of antioxidant activity on substituent located in the-OH ortho and para positions of2-H-B was obviously greater than that of meta position.The values of BDE and IP in the gas phase,ETE and HOMO in the polar solvent were matched well with the experimental results.The addition of three substituents-CH₃,-OCH₃ and-NH₂ all reduced the BDE,IP,ETE and HOMO absolute value of 2-H-B as follows:-NH₂>-OCH₃>-CH₃.In the gas phase,the hydrogen atom transfer?HAT?mechanism was dominant,whereas in the polar solvent,the sequential proton electron transfer?SPLET?mechanism was dominant.?3?The effect of o-hydroxy,o-methoxy and carboxylic acid groups on the antioxidant activity of phenolic acids was investigated,based on hydroxybenzoic acid.Eighteen phenols acids were selected,the antioxidant activity was measured and the corresponding thermodynamic parameters were calculated by the B3LYP and M06-2X methods.The results showed that-CH₂COOH and-CH=CHCOOH could increase the antioxidant activity of phenolic acid when the other substituents on the benzene ring were same,compared with-COOH.The introduction of o-hydroxyl group and o-methoxyl group could promote the antioxidant activity of phenolic acids,and the antioxidant activity was increasing with the increase of the amount.Meanwhile,dehydrodimer formations of sinapic acid and ferulic acid in DPPH-ethanol system were detected by HPLC-MS.Compared with-COOH,the BDE,IP and ETE of phenolic hydroxyl were reduced by the introduction of-CH₂COOH and-CH=CHCOOH,and also reduced by the introduction of o-hydroxyl and o-methoxy.According to experiment and calculation,it was speculated that both the HAT and SPLET mechanisms were occurred in the DPPH reaction system,and the SPLET mechanism was considered to be the main reaction mechanism in the FRAP system.?4?To determine the structure of dehydrodimer formation of sinapic acid?SA?and ferulic acid?FA?in DPPH-ethanol system,3 sinapic acid 8-8 dimer and 3 ferulic acid 8-8dimer were synthesized and used to determine their antioxidant activities.The BDE and other parameters were calculated by M06-2X.The results showed that the dehydrodimer formations of SA and FA in DPPH-ethanol system were 8-8 dilactone dimer?DSA-1 and DFA-1?.The order of antioxidant activity was:DSA-3>DSA-2>DSA-1,DFA-3>DFA-2>DFA-1,respectively.The results of theoretical calculation and experiment were consistent.Meanwhile,in DSA-2 and DFA-2,it was found that the antioxidant activity of the phenolic hydroxyl group was increasing with the double bond on the side chain.