Structure of small lignin fragments retained in water-soluble polysaccharides extracted from birch MWL isolation residue

To analyze the structural features of lignin in the vicinity of lignin-carbohydrate linkages, water-soluble lignin-carbohydrate complex (LCC) with low lignin content was prepared from residual birch wood meal after the extraction of milled wood lignin (MWL). The molecular weight distribution of lignin in this LCC appeared together with carbohydrate in the relatively high molecular weight region of the gel permeation chromatogram. This result was consistent with our previous results obtained for the same fraction of Japanese cedar (sugi); however, after treatment with polysaccharide-degrading enzyme, the molecular weight distribution of carbohydrate and that of lignin shifted significantly to the lower region. These results demonstrated that molecular size of this LCC is determined by carbohydrates while lignin is present as a minor fragment in this fraction. The syringyl/guaiacyl (S/V) ratio of this LCC was higher than other lignin fractions. Ozonation analysis implied that this LCC has a relatively high number of 4 structures. It is likely that lignin that exists near lignin-carbohydrate linkages has more endwise-type features than other lignin fractions.