Structural characterization of hemicelluloses fractionated by graded ethanol precipitation from Pinus yunnanensis

Fractionation of hemicelluloses from delignified Pinus yunnanensis was carried out with KOH/H(3)BO(3) solution followed by graded precipitation in 15%, 60%, and 90% (v/v) ethanol solutions, respectively. Chemical compositions, physicochemical properties, and structures of the precipitated hemicellulosic fractions were elucidated by a combination of sugar analysis, GPC, FT-IR, and (1)H, (13)C and 2D HSQC NMR spectroscopy. Sugar analysis showed that the hemicellulosic fraction precipitated by 15% ethanol solution (H(1)) had a predominance of xylose (58.52%), while mannose was the major sugar component in the hemicellulosic fractions precipitated by 60% (H(2)) and 90% (H(3)) ethanol solutions. GPC results revealed that the hemicelluloses precipitated by low concentration of ethanol solutions had higher weight-average molecular mass (50,090-79,840 g/mol) than those obtained in the high concentration of ethanol solutions (16,500 g/mol). The fraction precipitated by 60% ethanol solution (H(2)) was composed of D-galactose, D-glucose and D-mannose in a ratio of approximately 1:1:3.5. Structural determination indicated that the hemicellulosic fraction (H(2)) had a main structure of (1→4)-linked β-glucomannans backbone with (1→6)-linked α-D-galactose as a side chain attached to C-6 of mannose units. In addition, this fraction also contained minor amounts of xylans.