Chemical characterization of wood constituents, especially lignin, in fractions separated from middle lamella and secondary wall of Norway spruce (Picea abies)

Abstract

Spruce wood (Picea abies) was disintegrated into extremely fine powder and the resulting particles were resolved according to their density by centrifugation into the following main fractions: M (middle lamella fraction) containing 49.1% lignin and S (secondary wall fraction) containing 23.1% lignin. These main fractions were further subjected to ball milling at -15°C and extraction (dioxane-water) after which the polysaccharides were hydrolyzed by enzymic treatment and lignin was extracted. Finally, the material representing the main fractions M and S was divided into two subfractions (soluble (s) and insoluble (i)) using 1,2-dichloroethane-ethanol (2:1) as solvent. The lignin in the subfractions (primarily the s fractions) was characterized by spectroscopic methods (1H NMR, 13C NMR, IR, and UV). Further information was obtained by methoxyl analyses and chromatography of the fragments obtained after methylation and permanganate and nitrobenzene oxidations of the main fractions. The molecular weight distribution of the isolated lignin was also determined (by GPC). Finally, the main fractions were hydrolyzed and the resulting monosaccharides were determined as alditol acetates by GLC. Because of uncertainties associated with the experimental techniques no strictly quantitative conclusions could be drawn from the results obtained and there were only few indications of structural differences in the lignins according to their morphological location. The rather small differences obtained allow the following tantative conclusions: Compared with secondary wall lignin, middle lamella lignin contained somewhat more open α-O-4 linkages as well as structures with β-β linkages but less structures containing β-O-4 linkages. No conclusive evidence was found for the differences in the content of either β-1 linkages or phenolic and aliphatic hydroxyl groups. However, somewhat fewer methoxyl groups and phenylcoumaran structures and more carbonyl groups (at least at α-position associated with free phenolic structures) were present in the middle lamella than in the secondary wall lignin and the data also indicated a slightly higher degree of condensation of the former lignin. Considerable differences were found in the carbohydrate composition: the middle lamella fraction contained more rhamnose, arabinose, and xylose moieties than the secondary wall fraction whereas the opposite was true for the content of mannose and glucose.