Sulfated Xyloglucuronofucans from Pocockiella variegata: Chemical Structure

Brown seaweeds (Phaeophyceae) produce three main types of polysaccharides: sulfated fucans, alginic acid and laminarans. The former frequently contains other monosaccharides besides L-fucose, being D-xylose, D-galactose and D-glucuronic acid the most common ones. D-mannose and D-glucose are also found as minor components. Fucans have been extensively studied due to their diverse biological activities, since they present anticoagulant, antitumor, and antiviral activities. The structure/biological activities relationships are not clearly established in fucans due to many difficulties related with the determination of its fine structure. The present work is devoted to the structural analysis of the polysaccharides produced by the brown seaweed Pocockiella variegata (Dictyotales). The seaweed was submitted to an extraction procedure (100°C, 12h) resulting a crude fraction (PVQ) that was fractioned by stepwise elution on DEAE-Sephacel using aqueous sodium chloride of increasing concentrations (0.1-4.0M). Two main fractions: Q1-4 and Q2-6 (16% and 23% of sulfate, respectively) were obtained. The ¹³C NMR spectra of both fractions showed absorptions at 15.3 ppm corresponding to methyl groups (C-6) of fucose units, 63.0 ppm (C-5 of xylose C-4 substituted), 65.1 ppm (C-5 of xylose) and 174.3 ppm (C-6 of uronic acid). After carboxireduction and GC-MS analysis of Q1-4 fraction the presence of glucuronic acid was confirmed (16%). This fraction was also purified by gel filtration chromatography (Sepharose-2B) and the methylation analysis of the main fraction (Q14d) demonstrated that fucosyl units are mainly 2-, 2-3-, 3-4- and 4-substituted with sulfate groups mainly located at C-3 and C-4. The elevated amount of non-reducing units detected in this analysis evidences a high degree of branching. The glucuronic acid units, in the native polysaccharide, are principally 4-substituted. The presence of xylosyl units 1®4-linked was confirmed by methylation analysis and it is in agreement with ¹³C NMR data. The chemical and spectroscopic results showed that P. variegata produces a highly branched xyloglucuronofucan with a heterogeneous sulfation pattern.