Ulvans, sulfated β-d-galactans and xyloarabinogalactan: A comparative approach to extraction, structural characterization and antioxidant activity assessment

Antioxidants can be extracted from various seaweeds as natural ingredients for use in foods, cosmetics, and pharmaceuticals. The composition and structure of polysaccharides extracted from four green seaweeds: Ulva lactuca (UL), Halimeda opuntia (HO), Caulerpa racemosa (CR) and Chaetomorpha antennina (CA) were correlated with their antioxidant activities. Crude extracts were characterized by NMR (1H and 13C), HP-SEC, HPAEC-PAD and FTIR spectroscopy. Extracted polysaccharides were assayed spectrophotometrically for their total sugar, sulfate, phenolic and protein contents. The antioxidant capacities of each fractionated polysaccharide sample were evaluated by five methods, i.e., ABTS (2,2-azino-bis-[3-ethylbenzothiazoline-6-sulfonic acid]), DPPH (2,2-diphenyl-1-picrylhydrazyl radical), ferric-reducing power in FRAP (Ferric-reducing Antioxidant Power), superoxide dismutase (SOD), and the hydroxyl radical (OH) scavenging capacity assay. All polysaccharides had molecular weights (MW) under 1000 kDa, with HO having the lowest MW (<100 kDa). Polysaccharide extracts of UL (i.e., l-iduronic acid, 1,4-linked α-l-rhamnose, β-d-glucuronic acid) and CA were promising sources of antioxidants. The highest antioxidant capacity was measured in extracts from CA-2B (52.1 ± 0.9 μg/mL) using the ABTS assay, while CR-1B exhibited the highest antioxidant activity in both the FRAP (303.7 ± 8.2 μm/mL) and OH (28.4 ± 0.8%) assays. Taken together, useful insights were gained into the potential health benefits of selected green seaweed polysaccharides (i.e., ulvans, sulfated or/and pyruvylated β-d-galactans and sulfated xyloarabinogalactan) for applications in the pharmaceutical, food and biotechnology industries.