The water soluble polysaccharide produced by the green variant of tetrasporic Gigartina skottsbergii was found to be composed of D-galactose and sulfate groups in a molar ratio of 1.0:0.65. (1)H and (13)C NMR spectroscopy studies of the desulfated polysaccharide showed a major backbone structure of alternating 3-linked β-D-galactopyranosyl and 4-linked α-D-galactopyranosyl units, and minor signals ascribed to 3-O-methyl-substitution on the latter unit. Ethylation analysis of the polysaccharide indicated that the sulfate groups are mainly located at position O-2 of 4-linked α-D-galactopyranosyl residue and partially located at positions O-6 of the same unit and at position O-2 of 3-linked β-D-galactopyranosyl residue, and confirmed the presence of 3-O-methyl-galactose in minor amounts (4.4%). The sulfated d-galactan presents a similar structure to λ carrageenan but with much lower sulfation at position O-6 of the α-residue and at position O-2 of β-residue. The antioxidant capacity of the sulfated d-galactan was evaluated by the peroxyl radicals (ORAC method), hydroxyl radicals, chelating activity, and ABTS(+) assays. Kinetic results obtained in these assays were compared with those obtained for the commercial λ carrageenan. The antioxidant activity toward peroxyl radicals was higher for commercial λ carrageenan, this agrees with its higher content of sulfate group. The kinetics of the reaction of both polysaccharides with hydroxyl and ABTS(+) radicals showed a complex mechanism, but the antioxidant activity was higher for the polysaccharide from the green variant of tetrasporic Gigartina skottsbergii.
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