Structural Characterization Of Pectic Polysaccharides From Four Medicinal Plants And Their Antioxidant Activities

Pectin is the main active ingredient of medicinal plants,which has various biological activities such as antioxidant,immune regulation,hypoglycemia,anti-tumor and so on.In-depth analysis of the structural characteristics of pectin is the premise and basis of studying its biological activity and structure-activity relationship.Therefore,in this paper,four kinds of medicinal plant pectin were selected as research objects to study their structures and antioxidant activities.The results are as follows:The total pectin fractions were obtained by using hot-water extraction,ethanol precipitation and anion exchange chromatography from four species of medicinal plants,including Emilia sonchifolia(Linn.)DC,Polygonum aviculare L.,Salvia plebeian R.Br,Lepidium apetalum Wild,respectively.Eight pectins with relatively homogenous charge and molecular weight distribution were separated and purified by ion-exchange chromatography and gel-filtration chromatography,which are respectively named as WESP-Aa,WESP-Ab,WPAP-Aa,WPAP-Ab,WSPP-Aa,WSPP-Ab,WLAP-Aa and WLAP-Ab.These fractions are mainly composed of galacturonic acid(GalA),rhamnose(Rha),galactose(Gal)and arabinose(Ara)monosaccharides.The analysis showed that WESP-Aa,WESP-Ab,WLAP-Aa and WLAP-Ab were mainly composed of rhamnogalacturonan-I(RG-I),while WPAP-Aa,WPAPAb,WSPP-Aa and WSPP-Ab were composed of three pectin domains,namely,homogalacturonan(HG),rhamnogalacturonan-I and rhamnogalacturonan-II(RG-I and II).The structural characteristics of four RG-I pectin(WESP-Aa,WESP-Ab,WLAP-Aa and WLAP-Ab)were analyzed by methylation,NMR,HPLC and mass spectrometry.These pectins had different branching degrees of 0.4,0.5,0.9,and 0.8,and the types and proportions of their side chains were also different.The neutral sugar side chains of WESP-Aa and WESP-Ab include predominantly arabinan,Arabinogalactan-I(AG-I),Arabinogalactan-II(AG-II)and/or galactan,while the neutral sugar side chains of WLAP-Aa and WLAP-Ab were mainly highly branched AG-II,but also contain a small amount of arabinan,AG-I and/or galactan.After four mixed pectins(WPAP-Aa,WPAP-Ab,WSPP-Aa and WSPP-Ab)were hydrolyzed by Endo-PG and separated by gel chromatography,three types of pectin structural units were obtained: RG-I pectin domain(molecular weight between 67.2 k Da and 77.0 k Da),RG-II pectin domain(molecular weight between 6 k Da and 14 k Da)and a series of galactouronic acid oligosaccharide fragments with polymerization degree of 1-3.Although the domain composition of these four mixed pectins was similar,their content was different.The content of HG pectin domains in WPAP-Aa(HG 77.3 %,RG-I 16.7 %,RG-II 6.0 %),WSPP-Aa(HG 70.1 %,RG-I 12.3 %,RGII 17.6 %),and WPAP-Ab(HG 46.1 %,RG-I 38.3 %,RG-II 15.6 %)was higher;In WSPP-Ab(HG 12.7 %,RG-I 57.8 %,RG-II 29.5 %),the RG-I pectin domain was the main component.After radical scavenging experiments,it was found that mixed pectin had stronger scavenging activity on DPPH free radicals,ABTS free radicals and hydroxyl free radicals than RG-I pectin.The Polygonum aviculare L.pectins(WPAP-Aa,WPAP-Ab)> Salvia plebeian R.Br pectins(WSPP-Aa,WSPP-Ab)> Emilia sonchifolia(Linn.)DC pectins(WESP-Aa,WESPAb)> Lepidium apetalum Wild pectins(WLAP-Aa,WLAP-Ab).The results also showed that the antioxidant activity of pectin polysaccharide was closely related to GalA content,and the higher the GalA content was,the stronger the free radical scavenging ability of pectin polysaccharide was.The antioxidant capacity of pectin polysaccharides in vitro was the result of synergistic effect of different structural units,in which the contribution of HG pectin domain was the largest,followed by RG-II pectin domain.The RG-I pectin domain with less GalA content,larger molecular weight and more branches had the lowest contribution to the antioxidant activity of pectin polysaccharide.In conclusion,the structure analysis and antioxidant activity of pectins from different medicinal plants were systematically analyzed in this study,and the differences in structural units and contents of these pectins as well as their antioxidant activity were demonstrated.The results of this study provide data support for the structure-activity relationship of medicinal plant pectin,theoretical basis and important reference for the development and utilization of medicinal plant pectin,and enrich their structure and activity information.