Polysaccharides from brown algae Sargassum duplicatum: the structure and anticancer activity in vitro

•Fucoidan SdF from S. duplicatum was galactofucan with unusual structure.•The main chain of SdF consisted of alternated 1,4-linked α-L-Fuc and β-D-Gal.•Laminaran SdL from S. duplicatum was 1,3;1,6-β-D-glucan (1,3:1,6=6:1).•Fucoidan SdF had anticancer effect in vitro on colon cancer cells. The laminaran SdL and fucoidan SdF were isolated from brown algae Sargassum duplicatum. SdL was 1,3;1,6-β-d-glucan (1,3:1,6=6:1) with a main chain, represented by 1,3-linked glucose residues, due to NMR spectroscopy data. Single glucose residues could form branches at C6. Unusual structure of fucoidan SdF was studied by chemical and enzymatic methods, NMR spectroscopy of desulfated and deacetylated polysaccharide and mass spectrometry of fucoidan fragments labeled with 18O. Fucoidan was sulfated (31.7%) and acetylated galactofucan (Fuc:Gal∼1:1) with a main chain of 1,4-linked alternating α-l-fucose and β-d-galactose residues. Side chains were represented by extensive (DP≥5) 1,3-linked 2,4-disulfated α-l-fucose residues with branching points at C2. Fucose residues in the main chain were sulfated at C2 and less at C3, while galactose residues were sulfated at C2, C3, and less at C4, C6. The fucoidan SdF was effective against colony formation of colon cancer cells in vitro.