Fractionation of Alkali-Solubilized Hemicelluloses from Delignified Populus gansuensis: Structure and Properties

The dewaxed cell walls of Populus gansuensis were delignified with NaClO2 and then sequentially extracted with 0.25, 0.5, and 1.0 M KOH under a solid to liquid ratio of 1: 25 (g mL−1) at 25 °C for 10 h. The successive treatments together resulted in the dissolution of 83.7% of original hemicelluloses. The solubilized hemicellulosic fractions were further fractionated into six hemicellulosic subfractions by an iodine-complex precipitation technique. Their chemical and physical characteristics were determined by HPAEC, GPC, FT-IR, and 1H and 13C NMR spectroscopy. Neutral sugar composition and molecular weight analysis showed that, for each extract, the hemicellulosic subfractions that precipitated with aqueous potassium iodide−iodine had lower overall uronic acid/xylose (Uro/Xyl) ratios and higher molecular weights (Mw) than those remaining in the solution. FT-IR, 1H, and 13C NMR spectroscopy analysis indicated that the alkali-soluble hemicelluloses of Populus gansuensis had a structure composed of the (1 → 4)-linked β-d-xylopyranosyl backbone with 4-O-methyl-α-d-glucuronic acid attached to O-2 of the xylose residues.