A water-soluble high-molecular neutral polysaccharide (NP) was isolated from the eggs of the sea urchin Tripneustes gratilla. The formation of glucose only upon the treatment of NP by amyloglucosidase and the value of its optical rotation [α]_D +233.5 (c 0.2, water) confirmed its belonging to the family of α-d-glucans. According to the results of NMR spectroscopy and methylation analysis, the chains of NP are built up of non-reducing terminal, 4-linked and 4,6-disubstituted glucose residues at a ratio of 1:8:1. A branched structure with an average linear chain length of about five glucose residues was calculated from the spectrum of iodine complex. Contrary to the previously published structure of branched α-d-glucan from the sea urchin Strongylocentrotus nudus bearing single glucose units as branches, the polysaccharide NP contains oligosaccharide branches at position 6, which was confirmed by NMR data. Hence, NP has a glycogen-like structure with a rather high degree of branching, which markedly exceeds that of usual mammalian or fungal glycogens.

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.