Biocatalytic Synthesis Of P-coumaric Acid Ester Derivatives And Their Antioxidant Activities

P-coumaric acid(PCA)is one of the derivatives of cinnamic acid series.It has many physiological functions,such as antioxidant,bacteriostatic,anti-inflammatory,lowering blood lipid and anti-cancer,etc.However,due to its low lipid solubility,PCA is difficult to play a good antioxidant role in emulsion and other systems.Esterification/transeserification could not only improve its stability,but also enhance its physiological activity.The current chemical methods have many problems,such as producing more by-products,high pollution and residual organic solvents.What‘s more,the direct esterification conversion of PCA is demonstrated to be low.Therefore,to solve the problems that existing in PCA esterification,this study explored the synthesis of PCA ester derivatives through enzymatic transesterification of methyl p-coumarate(Methyl-PCM)with fatty acid alcohols.The reaction conditions and conversion rates of enzymatic esterification in organic phase and ionoic liuids were explored and compared.Two kinds of efficient transesterification reaction systems for synthesising of PCA ester derivatives were established.Furthermore,PCA ester derivatives with different chain lengths were synthesized and antioxidant activity in different systems was investigated.Studies on the enzymatic transesterification of methyl-PCM in organic medium showed that both the solvent and the water content of the reaction system had a great influence on the synthesis of Octyl p-coumarate(Octyl-PCM).A series of factors affecting the catalytic transformation were investigated,and the optimal reaction conditions were determined as follows:lipase dosage 40 mg/m L,the ratio of pyridine-cyclohexane(1:9,v/v),the molar ratio of Methyl-PCM to octanol 1:8,the dosage of 4?molecular sieve 0.325 g/m L,the temperature50?,and the rotation speed 180 r/min.When the reaction time was 24 h,the conversion rate was up to 99%,and the initial rate was 6.9 mmol/L·h.Under these conditions,p-coumarate derivatives with different chain lengths were synthesized.The results of HPLC,LC-MS and¹³C-NMR analysis showed that the required ester derivatives were synthesized and the conversion rates of the reaction substrates for coumarate derivatives were 91.2%?98.7%.Ionic liquids were tested as reaction medium to replace traditional organic solvents.Among the tested ionic liuids,C₄MIm·BF₄ gave the best conversion rate in the reaction with pure ions as medium,and the catalytic synthesis of Butyl-PCM and Lauryl-PCM could reach53.1%and 44.2%,respectively.The addition of pyridine or 2-methyltetrahydrofuran to the pure ionic liquid can reduce the viscosity of the system and promote the esterification reaction.In the C₄MIm·BF₄-2-methyltetrahydrofuran system,the substrate conversion of p-butyl coumarate and decyl ester can be increased to 64%and 65.3%,respectively.Further more,different antioxidant evaluation systems were used to evaluate the antioxidant activities of coumarate derivatives synthesized with different chain lengths.The results showed that the antioxidant activity of p-coumarate was correlated with its HLB value.In the fat-soluble and pure oil systems,the antioxidant activity of decyl p-coumarate and laurel p-coumarate was significantly enhanced compared with p-coumaric acid,even higher than that of antioxidants such as BHT.The antioxidation effect of lauryl p-coumarate on corn oil was better than BHT,BHA and TBHQ.In water-soluble systems,it was found that the introduction of alkyl chain into coumaric acid structure had little effect on its antioxidant activity in water-soluble systems.Therefore,p-coumarate derivatives are expected to be used as new oil-soluble antioxidants in different food,drug and cosmetic systems.This study not only has high theoretical value,but also provides a new route for the research and development of new natural antioxidants.