Effect of Extraction pH on the Structure of Glycyrrhiza Polysaccharide with Acid-extraction

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.