Structural characterization of the polysaccharide moiety of an aqueous glycopeptide from mannatide

A homogeneous glycopeptide with a molecular weight of 1.61 × 105 Da, termed as MT2-A, was isolated from crude mannatide by DEAE-52 cellulose column and Sephacryl S-300 gel column. The polysaccharide moiety of MT2-A was mainly composed of mannose and trace amount of glucose. Besides, MT2-A contained 16 kinds of amino acids and the total amino acid content was 4.41%. The detailed chemical structure of MT2-A was elucidated using methylation analysis, Fourier transform infrared spectroscopy (FT-IR), partial acid hydrolysis, selective acetolysis and 1D/2D nuclear magnetic resonance (NMR) spectroscopy. Based on the experimental results, it was concluded that the polysaccharide moiety of MT2-A had a backbone of (1 → 6)-α-D-mannopyranose residues, which highly branched at O-2 position of (1 → 2,6)-α-D-mannopyranose residues. The side chains were mainly composed of (1→)-α-D-mannopyranose, (1 → 2)-α-D-mannopyranose and (1 → 3)-α-D-mannopyranose residues. The length of the side chains could not be longer than four mannose residues.