Structural Characterization of Hemicelluloses from Bamboo Culms (neosinocalamus Affinis)

Dewaxed bamboo culms (Neosinocalamus affinis) were sequentially extracted with distilled water and 70% ethanol at 80 oC for 3 h, 0.2 and 0.5 M NaOH, 70% ethanol containing 0.6 M NaOH, and 1.0 and 2.0 M NaOH at 50 oC for 3 h using a solid to liquid ratio of 1:25 (g/mL). The hemicellulosic fractions obtained were characterized by high performance anion exchange chromatography (HPAEC), gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) spectroscopy, H and C nuclear magnetic resonance (NMR), and 2D heteronuclear single quantum coherence (HSQC) NMR spectroscopies. Compared to the water-soluble fraction, the alkali-soluble hemicelluloses primarily consisted of xylose (73.5-92.7%), and had a more linear structure and larger macromolecules, evidenced by Xyl/Ara ratios (11.0-22.9) and higher average molecular weights (Μw = 41200-68770 g/mol). Increasing the alkaline concentrations from 0.2 to 2.0 M NaOH decreased the molecular weights from 68770 to 42790 g/mol, but increased the Xyl/Ara ratios from 11.0 to 22.9 in the hemicellulosic fractions. The fractions isolated with 0.5 and 1.0 M NaOH were found to be composed of a linear (1→4)-β-D-xylopyranosyl main chain substituted by a small amount of L-arabinofuranosyl at C-2 and/or C-3 together with a minor quantity of 4-O-methylglucuronic acid at C-2, representing typical polysaccharide structure in bamboo.