Galactomannan with novel structure produced by the coral endophytic fungus Aspergillus ochraceus

•Exopolysaccharide AW1 was isolated from marine fungus Aspergillus ochraceus.•AW1 contains a mannan core of (1 ⟶ 2)-linked α-mannopyranose residues.•The mannopyranose residues in the backbone were substituted at C-6.•Side substituents are single mannopyranose and galactofuranose oligosaccharides with different DP.•AW1 is a galactomannan distinguishing from other exopolysaccharides.

The homogeneous extracellular polysaccharide, AW1, was obtained from the fermented broth of the fungus Aspergillus ochraceus derived from coral Dichotella gemmacea. AW1 was a galactomannan with a molar ratio of mannose and galactose of 2.16:1.00 and a molecular weight of about 29.0 kDa. The structure of AW1 was investigated by chemical and spectroscopic methods, including methylation analysis, one- and two-dimensional nuclear magnetic resonance (1D, 2D NMR) and electrospray mass spectrometry with collision-induced dissociation (ES-CID MS/MS) spectroscopic analyses. The results showed that the backbone of AW1 consisted of (1 ⟶ 2)-linked α-d-mannopyranose residues. The mannopyranose residues in the backbone were substituted at C-6 by the (1 ⟶ )-linked α-d-mannopyranose units and (1 ⟶ 5)-linked β-d-galactofuranose oligosaccharides with different degrees of polymerization. The investigation demonstrated that AW1 was a novel galactomannan with different structural characteristics from other fungal galactomannans, and could be a potential resource of the (1 ⟶ 5)-linked β-d-galactofuranose oligosaccharides.