| Onion(Allium cepa L.),one of the most widely planted herbaceous plant in the world,is a perennial herb in Liliaceae.Its bulb is often used for medicine,which has the functions of strengthening stomach,digesting food,clearing heat and diuresis,lowering blood sugar,reducing inflammation and inhibiting bacteria.In clinic,onion can be used to the treatment of hypertension,hyperlipemia,esophageal cancer,coronary heart disease,atherosclerosis,diabetes,osteoporosis,etc.Meanwhile,onion can also regulate the physiological balance and metabolism of the human body.Studies have reported that onion contained flavonoids,sulfur-containing,nitrogen-containing,sterols,and simple phenolics.Pharmaologically,onion has a variety of biological activities,such as antioxidant,anti-tumor,blood sugar lowering,antispasmodic and anti-hypertensive effects.In this thesis,the 70% ethanol extract of onion peels was successively extracted with ethyl acetate and n-butanol to obtain two fractions.The antioxidant assay showed that the ethyl acetate fraction was more effective.Therefore,the ethyl acetate fraction was selected for further chemical composition study.The ethyl acetate fraction was separated and purified by silica gel,Sephadex LH-20,ODS and preparative Rp-HPLC.Fifity-two compounds were isolated and purified.The chemical structure of the isolated compounds were then identified by spectroscopic methods,such as nuclear magnetic resonance,infrared,ultraviolet,CD,and etc,in combination with their physicochemical properties.They were identified as: quercetin(1),isorhamnetin(2),quercetin 3-O-β-D-glucopyranoside(3),quercetin 7-O-β-D-glucopyranoside(4),quercetin 4′-O-β-D-glucopyranoside(5),4′-methoxyl-quercetin-3′-glucopyranoside(6),quercetin 3,4′-O-β-D-diglucopyranoside(7),(2R,3S)-3,5,7,3’,5’-pentahydroxydihydroflavonol(8),(2S,3S)-taxifolin(9),(2S,3S)-3′-O-methyltaxifolin(10),(2S,3R)-taxilfolin 4′-O-β-D-glucopyranoside(11),(2S,3S)-taxilfolin 4′-O-β-D-glucopyranoside(12),(2R)-naringenin(13),1,3,6,8-tetrahydroxyxanthone(14),3-[3-(1-ethyl ester-2,4,6-trihydroxyphenyl)]-2,3-epoxyflavanone(15*),3-[4-(1,3,6,8-tetrahydroxyxanthone)]-2,3-epoxyflavanone(16*),2-O-β-D-glucopyranosyl-6-hydroxy-benzoic 4-hydroxyphenethyl ester(17*),phloroglucinoyl-3,4-dihydroxybenzoate(18),methyl 2-{[(3,4-dihy-droxyphenyl)carbonyl]oxy}-4,6-dihydroxybenzoate(19),jaboticabin(20),lunularic acid(21),3,4’-dihydroxy-3’-methoxybibenzyl-2-carboxylic acid(22),lunularic acid 4’-O-β-D-glucopyranoside(23),3-hydroxy-4-O-β-D-glucopyranoside methyl benzoate(24),3-hydroxy-4-O-β-D-glucopyranoside ethyl benzoate(25),methyl 2,4,6-trihydroxybenzoate(26),ethyl 2,4,6-trihydroxybenzoate(27),1,3,8,9-tetrahydroxydibenzo[b,d]pyran-6-one(28),3,4-dihydroxybenzoic acid(29),methyl 3,4-dihydroxybenzoate(30),ethyl 3,4-dihydroxybenzoate(31),(2E)-3-phenyl-2-propenoic acid(32),(E)-ethyl-3-(4-methoxyphenyl)-2-propenoate(33),diisobutyl phthalate(34),dibutyl phthalate(35),4-hydroxyphthalide(36),(-)-epiloliolide(37),(2α,3β,4α)-2,3,23-trihydroxyursa-12,20(30)-dien-28-oic acid(38),7,11,15-Trimethyl-3-methylene-1,2-hexadecanediol(39),methyl hexadecanoate(40),ethyl hexadecanoate(41),methyl(Z)-11-octadecenoate(42),ethyl(Z)-11-octadecenoate(43),methyl linoleate(44),ethyl linoleate(45),hexahydrofarnesyl acetone(46),tetradecanoic acid(47),palmitic acid(48),cetostearic acid(49),ergosterol peroxide(50),β-sitosterol(51)and β-daucosterol(52),respectively.Among them,there are 16 flavonoids(1-16),a triterpenoid(38),3 sterols(50-52),2 phenylpropanoids(32-33),11 aliphatic compounds(39-49)and 17 other compounds(17-31,34-37),of which 15,16,17 were new compounds.In order to concentrate the flavonoids in the extract,the total flavonoids in the ethyl acetate layer were enriched by polyamide resin.By UPLC analysis,compound 1(quercetin)was the highest content of onion 70% ethanol extract and ethyl acetate fraction,which was 131.9±19.5 mg/g and 286.7±19.0 mg/g,respectively.In addition,the total flavonoid content in the 70% ethanol extract of onion was 211.9 mg/g,and was 504.1 mg/g in the ethyl acetate fraction.After the ethyl acetate layer was enriched through the polyamide column,the total flavonoid content in the 60% methanol eluted portion has reached 902.5 mg/g.The results showed that the total flavonoids in the onion were effectively enriched by the polyamide column.The fractions obtained from the total extract of onion peels,each extract fraction,purified compounds and the elutes from polyamide column were tested for in vitro antioxidant activities,including DPPH radical scavenging capacity,superoxide anion free radical scavenging ability,oxygen radical absorbance capacity(ORAC)and ferric reducing/antioxidant power.In the DPPH free radical scavenging assay,compared with the positive control vitamin C(24.60±1.47 μmol/L)and gallic acid(13.38 ± 0.61 μmol/L),compounds 1(14.34±0.08 μM),3(21.52±0.84 μM),4(23.14±0.61 μM),9(21.91±2.15 μM),16(26.31±0.62 μM),28(10.37±1.53 μM),and 31(1.98±0.16 μM)showed strong DPPH free radical scavenging activity.In addition,the 70% ethanol extract,the ethyl acetate fraction,and the 80% elution fraction of the polyamide column and the MeOH elution fraction also showed strong DPPH free radical scavenging activity.In the superoxide radical scavenging activity assay,the 20% elution fraction(61.57±1.52 μg/mL)and the 80% elution fraction(65.04±5.79 μg/mL)of the polyamide column showed strong activity.In the oxygen free radical activity test,Compounds 1,5,7,9,and 14,the ethyl acetate fraction and the 80% elution fraction of the polyamide column showed strong activity,of which Compound 1(25.90 ± 8.09 μmol TE/g)was the most active.In the iron reducing ability test,compounds 1,2 and 3,and the ethyl acetate fraction,the eluted fractions of the polyamide column showed strong activity.Based on the results of above four antioxidant activity test,it was shown that the flavonoids and the flavonoid-rich fractions showed strong antioxidant activity,and the simple phenolic compounds had weak antioxidant activity. |
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