Effect of Extraction pH on the Structure of Glycyrrhiza Polysaccharide with Acid-extraction
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摘要: 本研究以甘草渣为原料,采用不同pH酸溶液(HCl、pH1[P1]、pH3[P3]、pH5[P5])为提取液制备甘草多糖,采用离子色谱及高性能尺寸排阻色谱法分别测定单糖的组成和相对分子量,通过红外光谱、原子力显微镜、核磁共振、扫描电镜及X射线衍射等技术对甘草多糖结构进行分析和比较。结果表明,3种甘草多糖均为杂多糖,主要由不同比例的Fuc(岩藻糖)、Rha(鼠李糖)、Ara(阿拉伯糖)、Gal(半乳糖)、Glc(葡萄糖)和GalA(半乳糖醛酸)组成;P3重均分子量(Mw)呈多组分分布,Mw为682 kDa约占7.3%(w/w),Mw为7110 kDa约占92.7%(w/w),Mw最大;红外与核磁谱图验证了3种甘草多糖可能均是以→4)-β-D-Glcp-(1→连接单元为主;刚果红实验显示P1和P5可能存在三股螺旋构象;扫描电镜显示P3和P5具有相似的形态结构,其表现为大量的圆球形颗粒状堆积成表面粗糙的网状结构,而P1呈片状结构,具有不平坦的表面和孔隙结构;原子力显微镜显示3种不同提取方法的甘草多糖呈相互分支和纠缠结构,有少量较小的球状聚集体的微观结构形;X射线衍射分析表明P1具有晶体和非晶态结构,而P3和P5无晶体结构。因此,不同pH对酸提甘草多糖的结构影响显著。Abstract: In this study, the Glycyrrhiza polysaccharides were prepared from Glycyrrhiza residues with different pH acid solutions (HCl, pH 1[P1], pH 3[P3], pH 5[P5]) as extraction solution. The monosaccharide composition and relative molecular weight of Glycyrrhiza polysaccharides were determined by Dionex system and high performance size exclusion chromatography. The structures of Glycyrrhiza polysaccharides were analyzed and compared by FT-IR spectroscopy, atomic force microscope (AFM), nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and X-Ray diffraction (XRD). The results showed that three Glycyrrhiza polysaccharides were heteropolysaccharides, which were mainly composed of Fuc (fucose), Rha (rhamnose-rhamnose), Ara (arabose), Gal (galactose), Glc (glucose) and GalA (galacturonic acid) in different proportions. The weight average molecular weight (Mw) of P3 exhibited a multicomponent distribution. Mw(682 kDa) accounted for about 7.3% (w/w), and Mw(7110 kDa) was about 92.7% (w/w), accounting for the largest proportion. The FT-IR spectroscopy and NMR verified that the backbone of Glycyrrhiza polysaccharides could be composed of→4)-β-D-Glcp-(1→. Congo red experiment showed that P1 and P5 might have a triple helix conformation. The SEM showed a similar morphological structure both in P3 and P5, which was a network structure with a rough surface cumulated by a large number of spherical particles. While P1 was a lamellar structure with an uneven surface and multiple pores. The AFM revealed a mutually branched and intertwined structure of Glycyrrhiza polysaccharide, and a small amount of Glycyrrhiza polysaccharide displayed a microstructure of spherical aggregates. The XRD showed that P1 had crystal and amorphous structure, while P3 and P5 had no crystal structure. Therefore, the structure of Glycyrrhiza polysaccharide by acid extraction was significantly affected by different pH.
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Keywords:
- Glycyrrhiza polysaccharide /
- pH /
- acid extraction /
- structure
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甘草(Glycyrrhiza uralensis Fisch)属于豆科(Leguminosae)多年生草本植物,是传统常用中草药之一,其性甘平,味甘,具有清火解毒、补脾养气等功效,在中国主要产于甘肃、内蒙古、新疆等地,分布面积广,蕴藏含量较多[1-2]。甘草在医药[3]、烟草[4]、食品[5]和养殖行业[6]等行业具有广泛应用,这使其成为具有巨大发展前景的经济作物。目前,对于甘草中有效成分的利用主要有甘草酸、甘草次酸、甘草黄酮和甘草多糖,提取后的副产物甘草渣中仍含有大量的黄酮、多糖及木质纤维素等[7-8],当前甘草渣的综合利用还不够广泛和深入,只用于牛羊饲料或燃烧成肥料,造成了严重的环境污染和生物资源浪费[9]。因此,对甘草渣综合利用进行研究具有一定的现实意义及可观的经济效益。
甘草多糖的提取方法有溶剂提取、超声辅助提取、微波辅助提取和酶解提取等[10-11]。目前,应用最广泛的提取技术是热水提取和超声辅助提取。由于甘草渣是工厂经传统热水提取甘草浸膏等后剩余的固体残渣,再次用热水几乎不能从甘草渣中提取出多糖,因此选用酸碱溶剂提取甘草渣中的多糖。张晓晶[12]通过单因素和回应面试验对影响甘草渣碱提多糖得率的因素进行了考察,确定甘草渣碱提多糖的最佳提取工艺条件为碱液(NaOH)浓度5.2%,提取时间3.20 h、液料比(mL/g)26.85:1,多糖得率为6.44%±0.02%。酸提法因其操作简便,是食品工业中提取果胶类多糖应用最广泛的方法,这种提取方法受多种因素的影响,如pH、温度和料液比等。Bai等[13]通过研究秋葵多糖的理化性质,发现提取pH对多糖的性质和功能有重要影响。且碱提多糖的Mw小于酸提多糖的Mw,在海带多糖[14]和仙人掌多糖[15]中也发现了类似的现象。另外,不同提取液的pH对多糖结构及性质有显著的影响,并且多糖的生物活性是由其结构决定的,因此,系统地对从甘草渣中提取的多糖结构进行解析十分必要。综上,本研究以甘草渣为原料,采用不同pH(1.0、3.0及5.0)酸性溶剂提取甘草多糖,并对其初级结构(分子量、单糖组成等)、高级结构(FT-IR光谱、AFM、SEM等)进行解析,研究结果有望为研究多糖提取方法与结构之间的关系提供新的见解,并对甘草的综合利用具有一定的指导意义。
1. 材料与方法
1.1 材料与仪器
甘草渣 新疆阿拉尔新农甘草产业有限责任公司提供,在55 ℃干燥至恒重;单糖标准品(纯度≥98%):D-葡萄糖(Glc)、L-阿拉伯糖(Ara)、L-岩藻糖(Fuc)、L-鼠李糖(Rha)、D-半乳糖(Gal)、D-木糖(Xyl)、D-甘露糖(Man)、D-葡萄糖醛酸(GlcA)和D-半乳糖醛酸(GalA) 上海默克公司。
Waters e269高效液相色谱 美国Waters公司;ICS-5000+离子色谱仪 美国Thermo Fisher公司;UV-2600紫外可见分光光度计 日本Shimadzu公司;Thermo傅里叶变换红外光谱仪 美国赛默飞公司;DD2-600核磁共振波谱仪 美国Agilent公司;D8 Advance X-射线衍射仪 德国Bruker公司;Cypher-S原子力显微镜 美国Asylum Research公司;SU-8010扫描电子显微镜 日本日立公司;
1.2 实验方法
1.2.1 甘草多糖的提取及总糖含量测定
将各30 g甘草渣分别浸泡在装有900 mL(30%,w/v)不同pH酸性溶液的烧杯中(pH分别为1、3、5、3 mol/L HCl调配),55 ℃水浴搅拌3 h,提取液冷却后经400目滤布过滤,收集滤液,并用6 mol/L NaOH溶液调节pH至中性,加3倍体积的无水乙醇进行醇沉,置于4 ℃过夜。收集沉淀,并用80%乙醇重复淋洗1~2次,加入适量蒸馏水复溶,8000 r/min,离心10 min,上清液置于截留分子量为3500 Mw的透析袋内用蒸馏水透析3 d,真空冻干后得到甘草多糖,得率按下式计算,pH1、pH3、pH5条件下提取的甘草多糖分别命名为P1、P3、P5。
采用硫酸苯酚法测定总糖含量[16],取样品300 μL,加入5%苯酚300 μL,混匀后加入浓硫酸1.5 mL,静置后测其在490 nm处的吸光值。以D-葡萄糖为标准品并绘制标准曲线(y=8.53x+0.0603,R2=0.9947),比较3种甘草多糖的总糖含量,所有测定均三次重复。
1.2.2 单糖组成测定
采用Hu等[17]高压离子色谱方法分析单糖的组成。将2 mg多糖用4 mol/L三氟乙酸在110 ℃下水解8 h,在氮气下除去三氟乙酸,然后用双去离子水稀释消化物,并在分析前过滤。制备的水解物或单糖标准物使用Dionex系统,脉冲电化学检测器和Carbopac™PA10分析柱,在30 ℃下测量。分离方法为18 mmol/L NaOH等容洗脱15 min,然后18 mmol/L NaOH在100 mmol/L NaOAc中洗脱35 min。
1.2.3 分子量测定
本实验采用多角度激光光散射仪与尺寸排阻色谱法(SEC-MALLS)联用测定多糖样品相对分子质量及其分布,方法参照Hu等[17]和Wu等[18]。样品直接分散在纯净水(5 mg/mL)中,通过0.22 μm过滤膜过滤两次。流动相为0.15 mol/L NaCl溶液,含0.02% NaN3((pH7.0)),流速为0.5 mL/min。
1.2.4 紫外可见光谱(UV-Vis)和红外光谱(FT-IR)分析
多糖溶解于蒸馏水(2 mg/mL)中,在25 ℃下用紫外可见分光光度计在220~300 nm范围内扫描;1 mg样品与150 mg光谱级KBr于玛瑙研钵中研磨混合后,并制成1 mm的薄片。采用傅里叶变换红外光谱仪进行光谱采集,扫描范围为4000~500 cm−1,分辨率为4 cm−1,扫描次数为32次[19]。
1.2.5 核磁共振(NMR)分析
称取多糖30 mg,加入500 µL D2O中,超声使其充分溶解,在核磁共振仪上对多糖进行测定。仪器参数为:频率400 MHz,温度 296.9 K[20]。
1.2.6 刚果红实验
2 mL多糖(0.5 mg/mL)和2 mL刚果红(80 µmol/L)的混合物中加入1 mol/L NaOH溶液,使NaOH的最终浓度分别为0、0.1、0.2、0.30、0.4和0.5 mol/L,并在各个NaOH浓度下用紫外分光光度计进行全波长扫描,记录各个NaOH浓度下混合溶液的最大吸收波长。以蒸馏水为空白对照,NaOH浓度为横坐标,最大吸收波长为纵坐标绘图[21]。
1.2.7 扫描电镜(SEM)分析
取充分干燥的多糖样品,将其均匀且薄地粘在专用双面胶上,用洗耳球吹掉未粘住的样品。用离子溅射喷镀仪在其表面喷金,喷金后将其放入扫描电镜内,调整放大倍数为200~1000倍观察样品的微观样貌[22]。
1.2.8 原子力显微镜(AFM)观测
甘草多糖(1 mg/mL)溶于去离子蒸馏水中,直至样品浓度为0.01 mg/mL。然后将5 µL多糖溶液(0.01 mg/mL多糖+0.1 mol/L NH4OAc,pH7)滴在新的云母片上,室温风干,样片干燥后即可进行AFM观测[23]。
1.2.9 X射线衍射谱(XRD)分析
采用衍射仪对多糖进行晶体特性的测定。测定条件为:采用Cu靶,Ka辐射源,电压40 kV,电流40 mA,4°~40° 2θ连续扫描,扫描速度2°/min,粉末衍射法进行测定[24]。
1.3 数据处理
数据由IBM SPSS statistics 23.0软件进行分析,显著性水平为P<0.05,所有实验均平行三次,绘图软件分别采用Origin 2022,MestReNova及MestReNova等。
2. 结果与分析
2.1 甘草多糖的得率、总糖含量及单糖组成分析
采用不同pH(pH为1、3及5)的酸性溶液从甘草渣中提取甘草多糖,不同pH下的得率如表1所示,以甘草渣干重为基准,P1的得率最高(3.12%),其次为P3得率(2.08%),得率最低的为P5(1.83%),pH对酸提甘草多糖的得率影响显著(P<0.05),可能原因为pH越低,细胞壁破坏程度越大,多糖的析出越多[25]。3种甘草多糖的紫外可见光谱(图1A)显示在260和280 nm处没有吸收峰,说明甘草多糖样品中不存在核酸和蛋白质。通过苯酚硫酸法测定甘草多糖,得出P1总糖含量最高,P3总糖含量较低。通过对比甘草多糖色谱图与单糖标准品的保留时间,确定甘草多糖的单糖组成,如表1所示,3种甘草多糖均为杂多糖,由不同比例的Fuc、Rha、Ara、Gal、Glc和GalA组成,其中Glc(≥62.19 mol%)是所有甘草多糖中的主要单糖,这表明3种甘草多糖是一种主要基于葡萄糖链连接的多糖。另外,不同pH酸提法不改变甘草多糖的单糖类型,但会引起单糖比例的变化。
表 1 甘草多糖的得率、总糖含量、单糖组成及相对摩尔比Table 1. The extraction yields and monosaccharide composition and of Glycyrrhiza polysaccharides名称 得率(%) 总糖(%) 单糖组成(mol%) Fuc Rha Ara Gal Glc GalA P1 3.12±0.76a 85.75±0.16a 1.77±0.02a 6.93±0.21a 17.54±0.41a 8.58±0.58a 62.19±1.08b 3.00±0.41a P3 2.08±0.44b 83.28±0.39b 1.31±0.02c 7.00±0.19a 10.10±0.5b 7.35±0.65b 69.78±2.03a 4.45±0.13a P5 1.83±0.58c 84.12±0.75a 1.66a±0.01b 7.52±0.13a 11.85±0.35b 8.98±0.62a 65.59±1.53a 4.39±0.52a 注:同列不同字母表示存在显著差异(P<0.05)。 2.2 分子量分析
P1、P3和P5的分子量及其分布如表2所示,P1的重均分子量(Mw)为413 kDa,对于P3,约7.3%(w/w)的Mw为682 kDa,约92.7%(w/w)的Mw为7110 k Da,而对于P5,约7.5%(w/w)的Mw为572 kDa,约92.5%(w/w)的Mw为6120 kDa,甘草多糖P1的Mw最小,可能原因为pH低多糖的糖苷键水解严重,故分子量变小[26]。多分散性指数(PDI)被用作显示分子量分布的工具,多糖的分子量分布受聚合方式和聚合因素的影响,PDI值越大分子量分布越广,PDI值接近于1,则分子量分布较窄,P1的PDI为8.97,P3组分1和组分2的PDI分别为8.07和1.13,比P5略低(12.19和1.15),这表明P1、P3组分1分子量和P5组分1分子量均为分子量广泛分布的多分散多糖。
表 2 甘草多糖的分子量测定结果Table 2. Molecular weight determination results of Glycyrrhiza polysaccharides峰值(%) Mw(kDa) Mn(kDa) Mp(kDa) PDI(kDa) P1 100 413 46 25.5 8.97 P3 7.3 682 84.5 45 8.07 92.7 7110 6310 7940 1.13 P5 7.5 572 47 26.7 12.19 92.5 6120 5310 6380 1.15 注:Mw:重均分子量;Mn:数均分子量,Mp:峰平均分子量,PDI:多分散性指数。 2.3 紫外线(UV)和FT-IR光谱分析
红外光谱(IR)是多糖结构分析最有用的技术之一,可提供多糖官能团及糖环构型等结构信息,如图1B所示,3种甘草多糖的吸收峰相似,3393.138 cm−1左右处的宽峰是O-H的伸缩振动引起的,约2940.912 cm−1处的吸收峰是C-H的伸缩振动。在约1627.626 cm−1处对应的吸收峰是由-COOH或-CO基团的C=O的非对称拉伸振动引起的[8],在约1428.512 cm−1处出现了C-H形变振动[10],上述峰均为多糖的典型红外吸收峰。由1000~1200 cm−1处的3个三处特征吸收峰,可推断吡喃型糖苷键的存在[27],甘草多糖在700~800 cm−1范围内有2个小吸收峰表明其可能含有α-和β-糖苷键[28]。三种多糖在1730 cm−1处无峰,说明它们几乎不含糖醛酸。
2.4 核磁共振(NMR)分析
甘草多糖的1H NMR谱如图2所示,质子信号几乎都出现在δ 3.31~5.35 ppm范围内,为典型的多糖信号,α-糖苷类异位质子信号的化学位移一般在δ 5.0~5.5 ppm范围内,而β-糖苷类信号的化学位移一般在δ 4.5~5.0 ppm范围内[29-31]。如图2A所示,P1的H-1质子有8个异位氢(约δH 5.28、5.22、5.16、5.04、4.97、4.85、4.39和4.30 ppm)、P3的H-1质子约为δH 5.25、5.11、5.04、4.95、4.83、4.28、4.17和4.08 ppm(见图2B);P5的H-1质子约为δH 5.24、5.19、5.14、5.08、5.01、4.95、4.83和4.50 ppm(见图2C)。在约δ 5.25 ppm附近的信号(P1, 5.28, P3, 5.25, P5, 5.24)的信号来自葡萄糖残基,在1.80 ppm附近的信号归属于GalA的O-2和O-3部位的乙酰基H信号[17]。在高场区,P1样品的H信号峰更加丰富,提示P1含有更加丰富的结构特征。
3种甘草多糖的13C NMR谱数据如图2所示,多糖的异位碳信号出现在90~110 ppm的范围内。α型糖苷键的化学位移为97~101 ppm,β型糖苷键的化学位移为103~107 ppm[32]。3种甘草多糖在δ 90~110 ppm之间都有不同的异位碳,在约102 ppm处的信号是β-D-Glc异位氢的特征峰,而在92~95 ppm处的较弱信号是α-D-Gal异位氢的特征峰。根据相关文献[33-37],P1 (δ 5.28/102.42 ppm),P3 (δ 5.25/102.44 ppm),P5 (δ 5.24/102.42 ppm)处异位氢和碳的强化学位移信号可能证实了有→4)-β-D-Glcp-(1→连接单元的存在。
2.5 三螺旋构象分析
在碱性介质中,研究多糖-刚果红复合物的最大吸收波长(λmax)可以得到多糖构象的有关信息,根据所研究多糖的水溶液与刚果红形成配合物的λmax的变化情况,可判断出所研究多糖是否具有三螺旋结构。甘草多糖与刚果红结合后在不同浓度的NaOH条件下,其最大吸收波长变化如图3所示,在0.0~0.5 mol/L的NaOH浓度范围内,P1/刚果红合络合物和P5/刚果红络合物的最大吸收波长发生了红移,表明这2种甘草多糖可能具有三螺旋的结构,而P3不具有稳定三螺旋的结构。多糖的三螺旋结构主要受糖苷键连接方式、支化程度及取代基团的静电排斥作用的影响[38]。
2.6 SEM观测结果
3种甘草多糖在1000倍放大倍率下的SEM显微图如图4所示,P3和P5具有相似的形态结构,表现为大量的链状和圆球形颗粒状堆积成表面粗糙的网状结构,P1的形态结构与P3和P5截然不同,呈片状结构,具有不平坦的表面和孔隙结构,孔隙结构结构表明多糖分子间存在排斥作用[39]。从甘草渣中经酸提取的多糖表观形态与从乌拉尔甘草根提取的多糖的表观形态不同[33]。
2.7 AFM观测结果
AFM用于直接表征多糖样品的表面分子形态,甘草多糖P1、P3和P5的AFM显微镜图像如图4所示,呈相互分支和纠缠结构,有少量较小的球状聚集体,与P3和P5相比,P1显示出更多高度分支结构的多糖链,多糖分子出现聚集,这可能是由于分子间的氢键缔合作用强,多糖聚合较紧密[40]。实验结果与Li等[41]的报道相似。AFM显微镜图像直接证实了甘草多糖P1、P3和P5具有高支化多糖的化学结构。
2.8 甘草多糖晶体特性分析
XRD是一种用于分析材料晶体结构的方法。晶体表现为尖而窄的特征峰,而非晶态物质表现为宽而弥散的衍射峰[42]。如图5所示,P1的特征衍射峰出现在27.28和31.72(2θ)处,表明所有P1均具有结晶和非晶态结构,然而,P3和P5没有特征衍射峰,则认为P3和P5没有晶体结构。
3. 结论
在不同pH(1、3和5)条件下从甘草渣中提取甘草多糖,P1的提取率最高而重均分子量(Mw)较小,P3和P5的Mw呈多组分分布,分子量较大,3种甘草多糖是以葡萄糖为主要单糖组成的杂多糖,结合单糖组成、红外及NMR综合分析表明,3种甘草多糖的主链连接可能以→4)-β-D-Glcp-(1→连接单元的存在。P1、P5可能含有三螺旋构象,P3和P5具有相似的相貌,但与P1的相貌不同,甘草多糖P1、P3和P5具有高支化多糖的化学结构,P1具有晶体和非晶态结构,而P3和P5无晶体结构。因此,不同pH对酸提多糖的结构影响显著。另外,酸法从甘草渣中提取的多糖结构与采用水浸法从甘草根中提取的多糖的结构不同[33-34],提取方法同样影响甘草多糖的结构。
本文基于前期研究结果,创新性地以甘草渣为原料,经不同pH的酸提取、乙醇沉淀及冷冻干燥等,制备了3种甘草渣多糖,并系统对其结构进行研究。另外,3种甘草多糖的功能还需要进一步研究,进而建立酸提取的pH、结构、生物活性三者之间的联系。这些结果为研究提取方法对多糖结构的影响提供了新的思路,并为设计新型功能多糖提供了可能。
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表 1 甘草多糖的得率、总糖含量、单糖组成及相对摩尔比
Table 1 The extraction yields and monosaccharide composition and of Glycyrrhiza polysaccharides
名称 得率(%) 总糖(%) 单糖组成(mol%) Fuc Rha Ara Gal Glc GalA P1 3.12±0.76a 85.75±0.16a 1.77±0.02a 6.93±0.21a 17.54±0.41a 8.58±0.58a 62.19±1.08b 3.00±0.41a P3 2.08±0.44b 83.28±0.39b 1.31±0.02c 7.00±0.19a 10.10±0.5b 7.35±0.65b 69.78±2.03a 4.45±0.13a P5 1.83±0.58c 84.12±0.75a 1.66a±0.01b 7.52±0.13a 11.85±0.35b 8.98±0.62a 65.59±1.53a 4.39±0.52a 注:同列不同字母表示存在显著差异(P<0.05)。 表 2 甘草多糖的分子量测定结果
Table 2 Molecular weight determination results of Glycyrrhiza polysaccharides
峰值(%) Mw(kDa) Mn(kDa) Mp(kDa) PDI(kDa) P1 100 413 46 25.5 8.97 P3 7.3 682 84.5 45 8.07 92.7 7110 6310 7940 1.13 P5 7.5 572 47 26.7 12.19 92.5 6120 5310 6380 1.15 注:Mw:重均分子量;Mn:数均分子量,Mp:峰平均分子量,PDI:多分散性指数。 -
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