Complex sulfated galactans from hot water extracts of red seaweed Asparagopsis taxiformis comprise carrageenan and agaran structures

Hot water extraction from the red seaweed Asparagopsis taxiformis yielded three extracts which showed sulfated galactans with a D:L-galactose ratio non consistent with carrageenan or agaran backbones. The major extract was fractionated by cetrimide precipitation and redissolution with increasing sodium chloride concentrations due to their low solubility. Seven fractions were obtained, and studied by methylation analysis, desulfationmethylation, and NMR spectroscopy of the partially hydrolyzed and the native samples. Fractions with the highest yield were those obtained at high concentrations of NaCl. They comprised both agaran and crageenan structures in considerable amounts. The main agaran structures were beta-D-galactose 4-sulfate and beta-D-galactose 2sulfate units linked to alpha-L-galactose 2,3-disulfate residues, and beta-D-galactose linked to alpha-L-galactose 3-sulfate or 6-sulfate, or substituted with single stubs of beta-D-xylose on C3, while the carrageenan structures comprised beta-Dgalactose (2-sulfate) linked to alpha-D-galactose 3-sulfate or 2,3-disulfate, and beta-D-galactose 2,4-disulfate linked to alpha-D-galactose 2,3-disulfate. Between the less sulfated fractions, the one obtained by solubilization in 0.5 M NaCl was mainly constituted by agarans, which included 3,6-anhydro-alpha-L-galactose units. Anticoagulant activity was assayed by general coagulation tests (aPTT and TT), showing a moderate action compared with heparin. This is the first detailed study of the sulfated galactans from the order Bonnemaisoniales.