| The medicinal and edible plants are the main part of the medicinal and edible sources.Pectin is the main component of plant-derived polysaccharides and has been found to have a variety of biological activities.Therefore,pectin may be one of the main functional components in the medicinal and edible plants.In this study,through literature research,three common medicinal and edible plants including Portulaca oleracea L.,Lilium davidii var.unicolor and Chinese yam were selected.First,hot-water extraction was used to extract the water-extracted pectin,and then 0.5 M Na OH was used to extract alkali-extracted pectin from the residue at room temperature after water-extraction(hereinafter referred to as alkaline extraction);crude pectins were mainly purified by the combined method of graded alcohol precipitation,ion exchange and gel column chromatography;the pectin fractions were further analyzed by methods including HPAEC-PAD,methylation experiment combined with GC-MS,partial degradation,1D/2D NMR and HPSEC-MALLS to characterize the primary structure and solution conformation of pectins from three kinds of medicinal and edible plant sources.The main research results are summarized as follows:(1)The water-extracted pectin from P.oleracea was prepared,then purified by gel column,and the structure of purified fractions was elucidated.P.oleracea water-extracted pectin was mainly composed of POWP-1(17.4%)and POWP-2(82.6%).POWP-1 was rhamnogalacturonan I(RG-I)pectin,and the main chain of RG-I accounted for 21.2%,and the side chain included arabinogalactan II(AG-II)(36.8%),galactan(28.0%),arabinan(11.8%)and type I arabinogalactan(AG-I)(2.2%);in addition,the O-2 and O-3 site ofα-Rhap of RG-I main chain had single substitution of acetyl group.POWP-1 had the M_w of 1188.0 k Da,R_g of 67.3 nm and R_h of 49.6 nm,and it exhibited a compact and coiled conformation in solution.POWP-2 was a high methyl-esterified pectin and mainly contained homogalacturonan(HG).It had the degree of methyl esterification of 0.63,the degree of acetylation of 0.20 and the HG accounted for proportion of 70.0%.It also contained part of RG-I.There were O-6 methyl esterification,O-2/O-3 acetylation,or both methyl esterification and acetylation substitution occurring on the→4)-α-Galp A-(1→residue in HG main chain.The RG-I was composed of 2.4%main chain and 18.6%side chain that included AG-II,AG-I and arabinan structural domains.The M_w of POWP-2 was19.4 k Da and R_h was 8.7 nm.It had a relatively extended semi-rigid chain conformation in solution.(2)P.oleracea alkali-extracted pectin was prepared,and then purified by anion exchange and gel column chromatography,and the structure of main purified fractions(POAP-005 and POAP-02-1)was characterized.POAP-005 was composed of glucuronoxylan(GX)and RG-I pectin;the proportion of GX was 76.1%,the main chain wasβ-1,4-xylan,and the O-2 position was connected toα-Glcp A-(1→orα-Glcp A-4-O-Me-(1→side chain;the proportion of RG-I was 21.3%,and the main chain accounted for 3.2%;the side chain mainly consisted of arabinan.POAP-005 had the M_w of 13.2 k Da,R_h of 3.5 nm,and existed as a compact coiled structure in solution.POAP-02-1 consisted of RG-I and GX.RG-I accounted for 85.6%and GX was only 11.9%.The main chain proportion of RG-I was 21.3%,and the side chain was mainly composed of arabinan(48.1%),galactan(11.7%)and a small amount of AG-II side chain.In addition,RG-I and GX in POAP-02-1 might coexist in a non-covalent manner.The RG-I had the M_w of 113.5 k Da,R_g of 9.9 and R_h of 8.1 nm,respectively.It might exist as a compact coiled structure in solution.(3)The hot-water and alkali-extraction methods were applied to extract the water(LWP)and alkali-extracted polysaccharides(LAP)of L.davidii var.unicolor;the LWP was purified by the graded alcohol precipitation method,and the LAP was mainly purified by the combination method composed of graded alcohol precipitation,ion exchange and gel column methods;the structure of main purified fractions were elucidated.The LWP was mainly theβ-glucomannan with acetylated single substitution sites mainly at the O-2/O-3 sites of Manp.There were pectin components named LAP-0-60,LAP-01-2 and LAP-02-1 and one low-M_wα-glucan(LAP-80S).LAP-0-60 was mainly composed of RG-I pectin(65.3%)andβ-glucomannan(34.2%);the proportion of RG-I main chain was 7.3%,and the side chain included arabinan(29.7%),galactan(26.0%)and a small amount of AG-I and AG-II;the main chain of β-glucomannan was composed of alternateβ-Glcp andβ-Manp connected by 1,4-glycosidic bonds.LAP-01-2 was RG-I pectin with main chain proportion of 6.3%,and the side chain was connected to the O-4 position ofα-Rhap;the side chain proportion was 93.7%,which mainly included arabinan(50.6%)and galactan(40.8%)side chains;it had the M_w of 168.9 k Da,R_g of 11.5 nm and R_h of 9.1 nm,and adopted in a compact coiled conformation in solution.LAP-02-1 was RG-I pectin,but there was a significant difference in M_w from LAP-01-2;the RG-I backbone of LAP-02-1 accounted for 5.5%,while had abundant galactan side chain(52.4%),followed by arabinan(42.1%).It had the M_w of 1877.0 k Da,R_g of 34.2 nm,R_h of 36.0 nm,and showed a compact coiled conformation in solution.(4)The water-extracted pectin from Chinese yam was extracted and purified by ion exchange column combined with gel column to obtain the main component CYWP-03-3;the structure of CYWP-03-3 was characterized.CYWP-03-3 was a high methyl-esterified pectin rich in HG,and had the degree of methyl esterification of 0.33 and degree of branching of 0.22.The proportion of HG domain was about 63.4%;RG-I was the secondary domain of CYWP-03-3,and the main chain of RG-I was relatively minor;while the side chain was mainly AG-II(20.8%),and it also contained AG-I(6.2%)and arabinan side chain(4.8%).CYAP-03-3 had the M_w of 13.4 k Da and R_h of3.1 nm.It presented a random coil conformation in solution.(5)The alkali-extraction method was applied to prepare the alkali-extracted pectin of Chinese yam(CYAP);two main fractions named CYAP-03-1 and CYAP-03-2 were purified based on CYAP by the combination method included graded alcohol precipitation,ion exchange and gel column methods,and their structure were elucidated.CYAP-03-1 was RG-I pectin with 1,4-galactan proportion of 89.8%.CYAP-03-2 was also RG-I pectin,the main chain of RG-I was short with proportion of 4.3%,while the side chain accounted for 80.6%,which included arabinan(7.1%),AG-II(34.4%)and galactan(38.3%).CYAP-03-2 had the M_w of 113.5 k Da,R_g of 8.1nm,R_h of 8.3 nm,and it exhibited a relatively compact coiled conformation in solution.(6)Based on the comparison and discussion of the purification and structure study of pectin from different sources,it was found that:pectins were the main polysaccharide substances in water and alkali-extracted polysaccharides from three kinds of medicinal and edible plants;the purification of pectin could be carried out based on the raw material stage,extraction stage and crude polysaccharide stage;hot-water extraction was easy to prepare HG pectin,while alkali extraction could prepare the RG-I pectin with a large amount of arabinan,galactan or AG-II side chains;the M_w(<20 k Da)of HG pectin from different sources was lower than that of RG-I pectin(113.5～1877.0 k Da),and HG pectin exhibited the random coil or semi-rigid conformation,while all RG-I pectin showed a compact coiled conformation in solution;HG and RG-I pectins from different sources had the similarity in the types of domains overall,but there were obvious differences in fine structure. |
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