Fractionation and characterization of lignin-carbohydrate complexes (LCCs) of Eucalyptus globulus in residues left after MWL isolation. Part II: Analyses of xylan-lignin fraction (X-L)

The residual wood meal left after milled wood lignin (MWL) isolation [milled wood residue (MWR)] of 5-year-old Eucalyptus globulus was fractionated to afford a xylan-lignin fraction (X-L) in 2.9% yield (based on MWR) by the method reported previously. X-L was further fractionated with the lignin solvent 1,4-dioxane/water (9:1, v/v) to give a soluble fraction (XL-F1; 24.0%) and an insoluble fraction (XL-F1-residue; 74.6%; both yields based on X-L). XL-F1-residue was further extracted with the good xylan solvent dimethyl sulfoxide and the soluble fraction was termed XL-F2 (43.0%; based on the XL-F1-residue). XL-F1 was mainly composed of lignin with a small amount of xylan and it is similar to purified MWL, whereas XL-F2 was mainly composed of xylan with some amount of lignin and it is similar to a fraction that was prepared by the extraction of crude MWL with acetic acid [lignin-carbohydrate complex (LCC)-AcOH]. The two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectra of XL-F1 and XL-F2 were interpreted that the former has alpha-ether-type lignin-carbohydrate (LC) linkages and the latter might have LC linkages of the phenyl glycoside type, which are different from those in LCC-AcOH.