The structural features of the sulfated heteropolysaccharide (ST-1) from Sargassum thunbergii and its neuroprotective activities

Polysaccharide (ST) was prepared from Sargassum thunbergii using hot water. Two fractions (ST-1 and ST-2) were prepared using anion exchange chromatography. One desulfated polysaccharide (ST-1-DS) was also prepared. Electrospray ionization mass spectrometry (ESI-MS) performed on ST-1-DS showed that the desulfated polysaccharides contained methyl glycosides of mono-sulfated and di-sulfated galacto-fucooligosaccharides. This result suggested that ST-1 might contain sulfated galactofucan, which consists of a backbone of alternating (Gal)n and (Fuc)n and sulfated randomly on Gal and mainly on C-2 in Fuc. In addition, ST-1 was degraded in 1 M sulfuric acid. The solution was centrifuged, and the supernatant was concentrated and precipitated in ethanol to obtain the precipitate (ST-1-P). ST-1-P was then separated using gel chromatography and anion exchange chromatography to obtain the oligomers. ESI-MS spectra of oligomers indicated that ST-1 mostly contained sulfated glucuronomannan and fucoglucuronan. ESI-MS with collision-induced dissociation tandem mass spectrometry (ESI-CID-MS/MS) suggested that glucuronomannan contained alternating 2-linked Man and 4-linked GlcA, while fucoglucuronan contained 4-linked glucuronan with branched Fuc at C-3. Finally, the neuroprotective activities of ST, ST-1, ST-2 and MIX (a mixture of ST-1 and ST-2) were determined. ST showed the most neuroprotective activity, which indicated that ST might be a good candidate for curing neurodegenerative diseases.