Fractionation and Physicochemical Characterization of Sulfated Polysaccharides from the Vietnamese Brown Alga Dictyota spinulosa: Antioxidant Properties

Brown seaweeds contain structurally diverse sulfated polysaccharides, but those from Dictyota spinulosa remain poorly characterized. This study characterized fractions from Vietnamese D. spinulosa and compared their antioxidant properties. Cetyltrimethylammonium bromide precipitation followed by sequential NaCl treatment yielded four fractions (M1–M4). Fucose and galactose were the predominant detected monosaccharides. Sulfate contents ranged from 18.79% to 29.41%, uronic acid contents from 4.30% to 12.10%, and apparent weight-average molecular weights from 47.6 to 62.7 kDa. M2 had a recovery of 26.0% and the highest carbohydrate content (40.63%), sulfate content (29.41%), and galactose-to-fucose molar ratio (1.40). It was therefore selected for nuclear magnetic resonance analysis. The spectra supported a heterogeneous, galactose-rich sulfated fucose-containing fraction but did not resolve individual linkage positions or sulfation sites. M3 had the lowest mean DPPH and ABTS IC50 values (2.12 and 1.31 mg/mL, respectively). M4 had the highest mean ferric reducing antioxidant power (4.00 mg ascorbic acid equivalents/g). These descriptive comparisons indicate compositional heterogeneity and assay-dependent antioxidant responses that did not consistently follow total sulfate content. The contribution of individual physicochemical characteristics remains unresolved.