Further studies on the composition and structure of a fucoidan preparation from the brown alga Saccharina latissima

The polysaccharide composition of a fucoidan preparation isolated from the brown alga Saccharina latissima (formerly Laminaria saccharina) was reinvestigated. The preparation was fractionated by anion-exchange chromatography, and the fractions obtained were analyzed by chemical methods combined with NMR spectroscopy. Several 2D procedures, including HSQC, HMQC-TOCSY, and HMQC-NOESY, were used to obtain reliable structural information from the complex spectra, and the signal assignments were additionally confirmed by comparison with the literature spectra of the related polysaccharides and synthetic oligosaccharides. In accordance with the previous data, the main polysaccharide component was shown to be a fucan sulfate containing a backbone of 3-linked α-l-fucopyranose residues sulfated at C-4 and/or at C-2 and branched at C-2 by single sulfated α-l-fucopyranose residues. In addition, three other types of sulfated polysaccharide molecules were detected in the total fucoidan preparation: (i) a fucogalactan having a backbone of 6-linked β-d-galactopyranose residues branched mainly at C-4 and containing both terminal galactose and fucose residues; (ii) a fucoglucuronomannan having a backbone of alternating 4-linked β-d-glucopyranosyluronic acid and 2-linked α-d-mannopyranose residues with α-l-fucopyranose residues as single branches at C-3 of α-d-Manp; and (iii) a fucoglucuronan having a backbone of 3-linked β-d-glucopyranosyluronic acid residues with α-l-fucopyranose residues as single branches at C-4. Hence, even a single algal species may contain, at least in minor amounts, several sulfated polysaccharides differing in molecular structure. Partial resolution of these polysaccharides has been accomplished, but unambiguous evidence on their presence as separate entities was not obtained.

Graphical abstractIn accordance with the previous data, the main polysaccharide component of a total fucoidan isolated from Saccharina latissima was shown to be a fucan sulfate having a backbone of 3-linked α-l-Fucp sulfated at C-4 and/or at C-2 and branched at C-2 by single sulfated α-l-Fucp. In addition, three other types of sulfated polysaccharide molecules were detected: (i) a fucogalactan having a backbone of 6-linked β-d-Galp branched mainly at C-4 and containing both terminal Galp and Fucp; (ii) a fucoglucuronomannan having a backbone of alternating 4-linked β-d-GlcpA and 2-linked α-d-Manp with α-l-Fucp as single branches at C-3 of α-d-Manp; and (iii) a fucoglucuronan having a backbone of 3-linked β-d-GlcpA with α-l-Fucp as single branches at C-4. Polysaccharide structures were elucidated by combination of methylation analysis and NMR spectroscopy.