Abstract: Objective: The polysaccharides from Spirulina platensis were extracted, purified and thermally degraded. The chemical properties and fine composition of different polysaccharide fractions were analyzed and compared. Methods: Five polysaccharide fractions (P0, P0.2, P0.4, P0.6 and P0.8) and their degradation products were obtained by sequential protease hydrolysis, alcohol precipitation, anion exchange chromatography and thermal degradation. High performance size exclusion chromatography (HPSEC), ion chromatography (IC), high performance liquid chromatography (HPLC) and hydrophilic interaction liquid chromatography-Fourier transform mass spectrometry (HILIC-FTMS) were used to analyze the chemical properties and fine composition of different polysaccharide fractions. Results: The content of sulfated groups in P0, P0.2, P0.4 and P0.6 ranged from 6% to 7%, lower than that in P0.8 (14.46%). The monosaccharide composition analysis showed that P0 was a glucan. P0.2 was a heteropolysaccharide mainly composed of glucose, galactose, rhamnose, fucose and other neutral sugars, while P0.4, P0.6 and P0.8 were mainly composed of rhamnose, glucuronic acid, glucose, xylose and galactose. Through HILIC-FTMS analysis, it was found that the oligosaccharides derived from the thermal degradation of P0.2 were mainly composed of hexose monosaccharide, pentose oligosaccharides, deoxyhexose-pentose oligosaccharides and deoxyhexose-uronic acid oligosaccharides with sulfate groups being in the deoxyhexose, pentose and hexose moieties, and that the composition of the oligosaccharide chain was complicated. However, the composition of P0.4 and P0.6 was very similar, mainly composed of rhamnose-pentose oligosaccharides, rhamnose-glucuronic acid oligosaccharides, pentose disaccharides and trisaccharides with sulfate groups being in the rhamnose and pentose moieties. Conclusion: There are many complex sulfated polysaccharides with different structures in S. platensis, including sulfated glucouronic acid-rhamnan as a unique sulfate polysaccharide.

Key words: Spirulina platensis; sulfated polysaccharide; rhamnan; thermal degradation; composition analysis