Molecular properties of lignin-carbohydrate complexes from beech (Fagus crenata) and pine (Pinus densiflora) woods

Abstract

Lignin-carbohydrate complexes (LCC) were isolated from pine (Pinus densiflora) and beech (Fagus crenata) milled-wood lignins. The LCCs were treated with enzyme to obtain precipitates (A-P, B-P) and water soluble fractions. The water soluble fraction from beech LCC was subjected to gel filtration to give LCC fragments (B-E-I). In order to protect the phenolic hydroxyl group, B-E-I and B-P were methylated with diazomethane, resulting in nonphenolic LCC fractions B-E-Ip and B-Pp, respectively. On treatment of B-E-I and B-P with sodium hydroxide, a remarkable amount of xylose and a trace of arabinose were detected as monomeric sugars. However, the same alkaline treatment of B-E-Ip and B-Pp gave only a faint trace of xylose. With DDQ treatment of acetylated B-E-I and B-P, monomeric sugars were released in the same amount as those obtained on alkaline treatment. Methylation of the monomeric sugars gave monomethylated xylose. From these results, it was concluded that xylose residues participate in lignin-carbohydrate linkages, and that lignin is linked to xylose at the 0–2 or 0–3 positions through an alkali-labile benzyl ether bond. The molecular-weight distributions of the lignin moieties, measured by HPLC, indicate that the lignin moieties of beech LCC are 100 times larger but less frequent than those of pine LCC. This nonuniformity of distribution of lignin and carbohydrate moieties in hardwood LCC molecules would result in less hydrophobic interaction in aqueous solutions.