Abstract: Sulfated modification of Laminaria japonica polysaccharides (LJP) was performed using a concentrated sulfuric acid method to obtain sulfated LJP (S-LJP). Periodate oxidation, Smith degradation, methylation, Congo red assay, Fourier infrared transform (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) were applied to determine the structural characteristics of LJP and S-LJP. The antioxidant activities of LJP and S-LJP were evaluated by determining their scavenging capacity toward 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, O2–·, and ·OH. Structural analyses revealed that sulfation markedly altered the types and proportions of glycosidic linkages in LJP. S-LJP was predominantly composed of (1→3) linkages, along with multiple other linkage types including (1→2), (1→4), and (1→6). The backbone consisted mainly of →3)-Glc-(1→ and →3)-Gal-(1→ residues, with minor amounts of →2)-Glc-(1→ and →4)-Glc-(1→, forming a branched α/β-type heteropolysaccharide chain. No triple-helix conformation was observed. MALDI-TOF MS indicated that its major ion peak corresponded to a molecular mass of approximately 6 844 Da. The FTIR spectrum exhibited characteristic absorption bands at 1 227, 1 046, and 844 cm–1¹, confirming successful sulfation. Sulfation significantly enhanced the radical scavenging activity of LJP, with half maximal inhibitory concentration (IC50) for DPPH radical, O2–·, and ·OH scavenging activities decreasing from 0.35, 2.82, and 2.05 to 0.19, 2.00, and 0.86 mg/mL, respectively (P < 0.05). These findings suggest that sulfation improves the biological activity of LJP by modulating its advanced structural features, providing theoretical support for the potential application of seaweed polysaccharides in functional foods.

Key words: sulfation; Laminaria japonica polysaccharides; structural characterization; biological activity