Preparation, Structural Characterization and Biological Activity of Phosphorylated Polysaccharide from Undaria pinnatifida Suringar

The polysaccharide purified from Undaria pinnatifida Suringar was phosphorylated using phosphate as a crosslinking agent. The structure of the phosphorylated polysaccharide was characterized by ultraviolet (UV) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), periodate oxidation and Smith degradation, Congo red test, β-elimination reaction, and iodine-potassium iodide test, and the free radical scavenging capacity and hypoglycemic activity of the raw and modified polysaccharide were evaluated. The results showed that the substitution degree of phosphate was 9.26, and the characteristic absorption peaks of P=O and P–O–C appeared at 1216 and 886 cm^-1, indicating that the phosphorylation modification was successful. The maximum relative molecular mass of the phosphorylated polysaccharide was 94.4 × 10³. It had a triple-stranded helical structure and was composed of glucopyranose unis linked together by β-glycosidic bonds. The polysaccharide and amino acids were linked together mainly by –O– glycopeptide bonds. The main linkages between monosaccharides were 1→3, 1→2,1→4 and 1→6 glycosidic bonds, the molar ratio was 0.494:0.504:0.002, and the sugar chain had a branched structure. The scavenging capacity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, superoxide anion radical (O₂^-·) and hydroxyl radical (·OH) and the α-glucosidase inhibitory activity of the polysaccharide were significantly increased by 34.76%, 12.30%, 76.05% and 3.70% after the phosphorylation modification (P < 0.05), indicating that phosphorylation modification enhanced the free radical scavenging capacity and hypoglycemic activity of the polysaccharide from Undaria pinnatifida Suringar.