Novel Fractional Purification Approach of Crude Polysaccharides via Boronic Acid-Tagged Thermoresponsive Triblock Copolymers

A novel fractional purification approach of crude polysaccharides with a boronic acid-tagged thermoresponsive triblock copolymer PEG113-b-PVBA49-b-PNIPAM105 was designed in this paper. The fractional precipitation of polysaccharides was realized through simple temperature regulation based on the difference in the lower critical solution temperatures of the triblock copolymer–dextran complex. The precipitated polysaccharides from the extracting solution was in a descending order of molecular weight. Under optimal conditions, four astragalus polysaccharide fractions (APS1, APS2, APS3, and APS4) with narrow distribution of molecular weights were obtained with the temperature-controlled fractional precipitation. The structural properties, monosaccharide compositions, and molecular weights of APS1, APS2, APS3, and APS4 were characterized by the Fourier transform infrared spectrum, Congo red spectrophotometric method, gas chromatography, and gel permeation chromatography. The differences in the biological activities of various APS indicate that the molecular weights of polysaccharides have an influence on the activities of APS. This novel temperature-controlled fractional precipitation method has a favorable prospect in the fractional purification of polysaccharides.