Isolation, Purification and Structural Characterization of Polysaccharides from Semen Astragali complanati

Polysaccharides from Semen Astragali complanati (SACP) were obtained by hot water extraction followed by ethanol precipitation and separated by fractional precipitation with 20% and 25% ethanol into two fractions: SACP-20 and SACP-25, which was the major one with higher purity. Their physicochemical properties were determined by colorimetric methods, and their molecular mass distribution, conformation in solution and monosaccharide composition were analyzed by size exclusion chromatography coupled with muti-angle laser light scattering (SEC-MALLS) and high-performance anion exchange chromatography coupled with pulsed amperometric detection (HPLC-PAD). Results indicated that SACP-25 was a homogenous polysaccharide with total sugar content of 87.40% and was composed of galactose (42.92%) and mannose (57.08%). Meanwhile, it showed a random coil conformation in 0.1 mol/L NaNO₃. Analysis by methylation combined with gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy revealed that the main chain of SACP-25 was composed of β-1,4-linked-mannopyranose (Manp) with galactopyranose (Galp) side chains attached to C-6 of the Manp residues. This study indicated that Semen A. complanati is good natural source of galactomannans as a water-soluble dietary fiber. The results of this study may provide theoretical support for the ordered structure of polysaccharides and also provide a theoretical foundation for the development and application of SACP in the functional food and pharmaceutical industries.