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

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.