Modification Of Citrus Pectin By UV-Catalyzed Hydrogen Peroxide Oxidation Reaction And Its Structure And Bioactivities

Modified citrus pectins?MCPs?are a series of degraded forms of citrus pectin?CP?that possess diverse health-promoting functions,such as immune regulation,anti-cancer,detoxification and inhibiting fibrotic diseases,etc.It is a promising candidate to be developed as a functional ingredient for dietary supplyment or anti-cancer drugs.In this thesis,ultraviolet-catalyzed and hydrogen peroxide oxidation?UHP?process was applied in CP degradation to prepare MCPs.The advantages of UHP process for CP degradation and the depolymerization dynamics were analyzed systematically.The degradation mechanism was explored by analyzing the variation of side chain during the degradation process.The structure of MCPs prepared by UHP under different p H conditions was compared,and their in vitro activities of anti-inflammation and anti-proliferation were investigated.The main results are as follows:?1?Evaluation of the advantages of CP degradation by UHP and degradation kinetics analysis.The optimum hydrogen peroxide loading was determined to be 30?L/100mg,which was around 85%?99%lower than other similar oxidation methods,suggesting the obvious advantage of UHP with low cost.The molecular weight?Mw?of raw CP was reduced from641.17 k Da to 47.31 k Da under reaction time for 5 h,with the degradation rate of?6.93±0.44?×10^-6min^-1.The recovery rates of final product MCPs prepared under different processing condition were higher than 95.3%,indicating the benefits of UHP degradation of CP in efficiency and yield.The p H value of reaction solution is a critical factor in affecting the degradation process.Under alkaline condition?p H 12?,the reaction proceeded rapidly in the initial stage of reaction?<1 h?with the molecular weight decrease from 524.88 k Da to 182.37k Da.In contrast,the reaction rate in acid condition?p H 4?was elevated to a high level after reaction for 15 h,with the degradation rate constant of?1.53±0.07?×10^-5min^-1.?2?The degradation on the side chain of CP and pectin degradation mechanism with UHP.The changes of the degraded products,such as total carbohydrate,galacturonic acid and reducing sugar released into the degradation medium,with the reaction time,reflect a gradual degradation mode of both main and side chains.The content of double bond varied with p H,indicating that the?-elimination reaction was a dominant process under acidic and neutral conditions?p H 4 and p H 7?.The decrease in the degree of esterification was more significant under alkaline conditions than neutral or acidic p H.Monosaccharide analysis showed that the total amount of neutral sugar in the solution increased gradually with reaction time and resulted in higher content than the uronic acid,indicating that the structure of CP was changed from linear to branched form.The mechanism of UHP degradation of CP exhibited differently with p H conditions.An initial random degradation and selective oxidative degradation of the main chain were dominant under acidic and neutral conditions,while?-elimination reaction,random scission and chain-end scission was the main pathway under alkaline condition.?3?Structural characterization of MCPs and their anti-inflammatory and anti-tumor activities in vitro.Two modified citrus pectin?MCP4-P and MCP12-P?prepared by UHP under p H 4 and p H 12 were purified by means of ion-exchange and gel chromatography.Their structures were characterized by chemical analysis and spectral analysis?GPC,FT-IR,HPAEC-PAD and 1D and 2D NMR?.Results indicated that purified MCP12-P with average-weight molecular weight 38.3 k Da was composed of a short homogalacturonan?HG?domain with low degree of methoxylation?24%?and a enriched rhamnogalacturonan-I?RG-I?region with galactans and arabinogalactans highly branched to rhamnose residue.In comparison,MCP4-P has molecular weight of 31.5 k Da,exhibited as a linear-dominant structure composed of enriched HG domain with high degree of methoxylation?46%?and less RG-I region with lower neutral sugar branches.Compared to MCP4-P,MCP12-P showed higher anti-inflammatory activity in suppressing the expression of NF-?B and the production of pro-inflammatory factors TNF-?and IL-1?in THP-1 cells upon lipopolysaccharide?LPS?stimulation.It also possessed stronger anti-proliferative activities towards Caco-2 cells,which is likely due to terminal galactan residues in the RG-I domain.