Preparation of low viscosity polysaccharides from Plantago asiatica by sulfation and their structural characterization and antioxidant activity
Polysaccharides are abundant in animals, plants, and microorganisms, and are widely used in the food and pharmaceutical industries due to their low toxicity and excellent biological activity. Modern research has shown that polysaccharides have various functions such as immune regulation, anti-tumor, anti-inflammatory, antiviral, antioxidant, anti radiation, hypoglycemic, lipid-lowering, and liver protection. In recent years, various polysaccharide derivative methods have been developed to enhance the biological activity and optimize the physicochemical properties of polysaccharides, such as sulfation, acetylation, carboxymethylation, and phosphorylation. The sulfation of polysaccharides can significantly enhance their structural characteristics, improve their biological activity, and even give them new biological activities. Yang et al. studied the sulfation of natto polysaccharides using the chlorosulfate pyridine method, and the antibacterial activity of sulfated natto polysaccharides was enhanced. Chen et al. prepared sulfated yam polysaccharide derivatives using the chlorosulfate pyridine method, and the results showed that sulfation modification optimized the physicochemical properties of yam polysaccharides, exhibiting superior immunomodulatory activity. Arokiarajan et al. improved the solubility of seaweed polysaccharides in water and further increased the extraction rate of polysaccharides by sulfation modification. The decisive factor for the biological activity of biological structural compounds, Tao et al. compared the anti-tumor activity of sulfated derivatives of natural fungal polysaccharides in vivo and in vitro using the chlorosulfonic acid pyridine method, and found that the sulfated derivatives had a higher in vitro inhibition rate on HepG2 cells than the original polysaccharides.
Plantago ovata Forsk, a dried and mature seed of the Plantago genus, is a traditional medicinal plant used for food and medicine in Xinjiang, China. Plantago asiatica has the effects of stopping diarrhea, diuresis, relieving pain, reducing heat, reducing swelling, and clearing throat. It is mainly used to treat diseases such as nephritis edema, urinary system infections, frequent urination, painful urination, bloody urination, and diarrhea. The polysaccharides from Plantago asiatica have high viscosity, which endows it with physiological functions such as moistening the intestines, promoting bowel movements, and promoting intestinal health. However, it also brings inconvenience to its production, research, and application. This issue has been widely concerned, but progress has been slow. Therefore, it is necessary to develop new methods to effectively reduce the viscosity of polysaccharides from Plantago asiatica while ensuring low cost, structural integrity, and activity, in order to promote further development and utilization of Plantago asiatica polysaccharides and improve economic benefits. So far, there have been no reports on the structural modification of polysaccharides from Plantago asiatica and their biological activity and structural characteristics.
Therefore, this study mainly focuses on the polysaccharides of Plantago asiatica as the research object. Low viscosity Plantago asiatica polysaccharides were prepared by sulfation method, and then isolated and purified to prepare homogeneous polysaccharides. The structural characteristics were preliminarily studied, and their antioxidant activity was evaluated by in vitro antioxidant experiments. To our knowledge, this is the first study on the sulfation of polysaccharides from Plantago asiatica to reduce viscosity.

Due to the high viscosity characteristics of polysaccharides from Plantago asiatica, purification and activity detection cannot be carried out. In this study, sulfation was used to modify polysaccharides from Plantago asiatica var. asiatica (POFP), and the viscosity of the modified polysaccharides was significantly reduced. Two homogeneous polysaccharides were prepared by separating and purifying POFP-S. The degree of substitution and purity of polysaccharides, glucuronic acid, and sulfate groups in POFP-S1 were 38.96%, 2.91%, 0.364%, and 94.20%, respectively; The degree of substitution of polysaccharides, uronic acids, and sulfate groups in POFP-S2 was 52.55%, 3.68%, 1.080%, and 98.89%, respectively, and no protein was detected in either polysaccharide; Among them, POFP-S1 has a relatively large molecular weight and abundant monosaccharide components. UV and FT-IR spectroscopy revealed that both POFP-S1 and POFP-S2 exhibit typical sulfated polysaccharide characteristic absorption peaks, indicating successful sulfation modification; X-ray diffraction (XRD) and Congo red experiments revealed that POFP-S2 is a polysaccharide with a unique helical structure that coexists both crystalline and amorphous phases. In terms of antioxidant activity, POFP-S1 polysaccharide has a stronger ability to scavenge DPPH and ABTS radicals compared to POFP-S2, which may be related to its larger molecular weight and monosaccharide composition (glucose). POFP-S2 has a stronger ability to scavenge hydroxyl radicals, which may be due to the higher degree of substitution of sulfate groups in POFP-S2 polysaccharide. This study provides a literature basis for further investigating the structure and activity of polysaccharides from Plantago asiatica, as well as a theoretical basis for developing potential medicinal and edible products, and provides ideas for the sulfation research of polysaccharides.