Purification, structural characterization and antioxidant property of an extracellular polysaccharide from Aspergillus terreus

•Exopolysaccharide YSS was prepared from marine fungus Aspergillus terreus.•YSS is a novel branched galactomannan with molecular weight of 18.6 kDa.•The backbone of YSS is composed of specific mannopyranose residues.•Branches consist of galactofuranose residues and disaccharide units of mannose.•YSS possessed a good antioxidant activity in vitro.

The marine fungus Aspergillus terreus produces an extracellular polysaccharide, YSS, when grown in potato dextrose-agar medium. YSS was isolated from the fermented liquids using ethanol precipitation, anion-exchange and size-exclusion chromatography. YSS was mainly composed of mannose and galactose in a molar ratio of 7.68:1.00, its average molecular weight was estimated to be about 18.6 kDa. On the basis of chemical and spectroscopic analyses, including one- and two-dimensional nuclear magnetic resonance (1D and 2D NMR) spectroscopy, structure of YSS may be represented, at an average, as a backbone of mannan with two types of branches. The mannan backbone is mainly composed of (1→2)-linked α-mannopyranose with small amounts of (1→6)-linked α-mannopyranose residues. The branches consist of terminal β-galactofuranose residues, and disaccharide units of (1→6)-linked α-mannopyranose. The branches are linked to C-6 of (1→2)-linked α-mannopyranose residues of backbone. The antioxidant activity of YSS was evaluated with the scavenging abilities on 1,1-diphenyl-2-picrylhydrazyl (DPPH), superoxide and hydroxyl radicals in vitro, and the results indicated that YSS had good antioxidant activity, especially scavenging ability on DPPH radicals. The investigation demonstrated that YSS is a novel branched galactomannan with antioxidant activity, and differs from previously described extracellular polysaccharides.