Ulla Westermark

Nanostructural assembly of cellulose, hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid.

Physical, chemical, and biological properties of wood depend largely on the properties of cellulose, noncellulosic polysaccharides, and lignin, and their assembly mode in the cell wall. Information on the assembly mode in the main part of the ginkgo tracheid wall (middle layer of secondary wall, S2) was drawn from the combined results obtained by physical and chemical analyses of the mechanically isolated S2 and by observation under scanning electron microscopy. A schematic model was tentatively proposed as a basic assembly mode of cell wall polymers in the softwood tracheid as follows: a bundle of cellulose microfibrils (CMFs) consisting of about 430 cellulose chains is surrounded by bead-like tubular hemicellulose-lignin modules (HLM), which keep the CMF bundles equidistant from each other. The length of one tubular module along the CMF bundle is about 16 ± 2 nm, and the thickness at its side is about 3–4 nm. In S2, hemicelluloses are distributed in a longitudinal direction along the CMF bundle and in tangential and radial directions perpendicular to the CMF bundle so that they are aligned in the lamellae of tangential and radial directions with regard to the cell wall. One HLM contains about 7000 C6-C3 units of lignin, and 4000 hexose and 2000 pentose units of hemicellulose.

Calcium promoted phenolic coupling by superoxide radical —a possible lignification reaction in wood

SummaryThe reaction of enzymatically generated superoxide radical with coniferyl alcohol in aqueous solution at pH 8.0 was studied. In the absence of calcium (II) no reaction occurred, but with a calcium (II) concentration of about 50 mM or more, coniferyl alcohol was oxidized by the superoxide radical. Dehydrodiconiferyl alcohol and pinoresinol were identified in the reaction mixture. From this result it is concluded that in the presence of calcium (II) the superoxide radical can achieve a one electron oxidation of coniferyl alcohol.It is suggested that a superoxide-generating enzyme outside the cell wall is responsible for the lignification. This suggestion is based on knowledge of the pore structure of wood cell walls, the high calcium concentration in non-lignified cells, the course of lignification and the existence of superoxide-producing enzyme systems in plant cell walls.

The content of protein and pectin in the lignified middle lamella/primary wall from spruce fibers

Samples enriched in material from the lignified middle lamella/primary wall (compound middle lamella) of spruce wood have been analysed with respect to their content of protein and pectin. Protein was determined by nitrogen analysis and the biuret reaction. The analyses showed the presence of nitrogen, indicating that the glycoprotein component of the primary wall is also retained in the lignified tissue. A gas Chromatographie procedure has been developed for the determination of the pectin components galacturonic acid and rhamnose in highly lignified tissues. The low contents of galacturonic acid and rhamnose found in the analyses show that very little pectin remains in the lignified middle lamella/primary wall. This suggests that the high amount of pectin originally present in the unlignified middle lamella/primary wall is mostly degraded or removed during the maturation of the cell wall. The possibility that the pectin can act äs a Ca*-regulator during the biosynthesis of lignin is discussed.

The distribution of acidic and esterified pectin in cambium, developing xylem and mature xylem of Pinus sylvestris

Homogalacturonans with low and high degree of methyl esterification have been immuno-localised in the cambium, differentiating xylem and mature xylem of Pinus sylvestris, by monoclonal antibodies JIM5 and JIM7. In the unlignified cambial tissue the antibodies revealed a similar distribution for acidic and esterified pectin in the compound middle lamella, ray cell walls and pit membranes. In the lignified xylem tissue, pectin was also found in the compound middle lamella, although dominantly in the methylesterified form. Lignification seemed to coincide with a decrease in the presence of acidic pectin in the compound middle lamella. Both antibodies indicated labelling in pit membranes and ray cell walls in partially and fully lignified wood fibres.

Removal of Lignin-rich Surface Material from Unbleached Kraft Fibres

The composition and structure of the surface material of kraft fibres can affect properties of the pulp such as the reactivity in, for example, bleaching processes. In order to study the nature of the surface material, conditions for a gentle removal of fibre surface material from unbleached kraft fibres have been evaluated. An unbleached softwood kraft pulp with a lignin content of 6.2% (kappa number 32) was treated mechanically using three different types of laboratory equipment : a PFI-mill, a high intensity mixer, and a disintegrator. The number of revolutions and the pulp concentration were varied and the removal of surface material was followed by chemical and microscopical analyses. These analyses revealed that there is lignin-rich material present on the fibre surface which can be removed by a mild mechanical treatment. The lignin content of the surface material abstracted was between 2.5 and 4.5 times higher than that of the fibres. The primary fines (obtained by fractionation of unbeaten pulp, 20μm), also originating from the fibre surface, had a lignin content almost 5 times higher than that recorded in the fibres. For analytical studies of the surface material, a mechanical treatment of the kraft fibres in a disintegrator at a pulp concentration of 4.5% and at 20.000-200.000 revolutions is an efficient and suitable treatment for gradual removal of material from the fibre surface.

Ultrastructural Changes in the Compound Middle Lamella of Pinus thunbergii During Lignification and Lignin Removal

Summary The structure of the middle lamella in Pinus thunbergii has been studied by the rapid-freeze deep-etching (RFDE) technique in combination with transmission electron microscopy (TEM). The ultrastructure of the compound middle lamella was studied in the early phases of the development of woody tissue in the cambial and differentiating xylem, before the heavy incrustation with lignin had occurred. Lignified middle lamella in the xylem was studied both directly and after delignification. It was found that the structure of the unlignified middle lamella in the cambium/developing xylem consists of a fine irregular network probably containing pectin and hemicellulose. As a result of lignin incrustation, the middle lamella becomes increasingly dense and the surface structure of the fully lignified middle lamella appeared to be compact and partly covered with globular structures. After delignification of the lignified middle lamella a thin network with a different structure was revealed. This network probably mainly consists of hemicellulose. No microfibrils of the type that occurs in the primary and secondary walls were found in the middle lamella.

2D-NMR (HSQC) difference spectra between specifically C-13-enriched and unenriched protolignin of Ginkgo biloba obtained in the solution state of whole cell wall material

Abstract In the structural analysis of lignins by 13C-NMR, signal overlap limits definitive assignment and accurate intensity measurement. Selective labeling by 13C-enrichment of a specific carbon in lignin enhances its signal intensity in the spectrum. Further enhancement of the specifically labeled carbons can be realized via difference spectra created from the enriched and unenriched samples. Difference 2D 13C-1H correlation (HSQC) NMR spectra, derived from the spectra of specifically 13C-enriched lignin model polymers (so-called dehydrogenation polymers) and their unenriched counterparts, take advantage of the enhanced dispersion afforded by both 13C and 1H chemical shifts, diminishing the difficulties arising from the signal-overlap problem and aiding in definitive signal assignments. In this research, protolignin in xylem cell walls was specifically 13C-enriched at all of the individual phenylpropanoid side-chain carbons by feeding 13C-enriched coniferins to growing stems of Ginkgo biloba. The whole xylem fractions containing 13C-enriched and unenriched protolignins were dissolved in a mixture of N-methylimidazole and DMSO, and then acetylated. Solution state 2D-NMR (HSQC) spectra of the acetylated whole cell wall were acquired. Difference spectra between the walls containing 13C-enriched and unenriched lignins afforded simplified 2D spectra in which well-separated signals were assigned exclusively to the specifically enriched carbons. This novel NMR technique provides a useful tool for elucidation of entire protolignin in the cell wall of ginkgo xylem.

Proposed supramolecular structure of lignin in softwood tracheid compound middle lamella regions

Abstract The structure of lignin in the compound middle lamella (CML) of softwood tracheids differs from that in the secondary wall (SW) in regard to the content of condensed structures (5-5′-biphenyl, dibenzodioxocin and 4-O-5′-diphenyl ether). In an early stage of cell wall formation, random coarse networks composed of thin cellulose microfibrils (CMFs), hemicelluloses, and pectin are formed in the CML, then globular p-hydroxyphenyl/guaiacyl lignin (HG-lignin) is deposited quickly into the network. The globular lignin is assumed to be a micellar aggregate of oligolignols folded at the β-O-4 bond with their phenolic ends on the outer part of the aggregate. When 3D clusters of the globules are deposited on the preformed network of polysaccharides, further growth of the oligolignols by endwise addition of new monolignols is spatially limited, so frequent condensation occurs between growing aromatic ends of adjacent HG-oligolignols within the globule and between the wide contact boundaries of the 3D clustered globules to produce a highly condensed supramolecule in CML. In SW, the folded G-oligolignols are deposited slowly in the narrow tubular space surrounding thick CMFs coated with hemicelluloses. Condensation occurs mostly between adjacent growing ends of the oligolignols within the tubular aggregates. Spatial regulation of condensation of folded polylignols is one of the factors producing a different supramolecular structure for CML lignin than for SW lignin.

Distribution of Coniferyl Alcohol and Coniferaldehyde Groups in the Cell Wall of Spruce Fibres

The morphological distribution of coniferyl alcohol and coniferaldehyde groups in the cell wall of spruce wood fibrers was determined using staining techniques combined with UV-VIS microscopic analysis. Phloroglucinol-HCI was used as a specific stain for coniferaldehyde groups. The distribution of coniferaldehyde groups was measured on 1 μm sections at 550 nm. Coniferyl alcohol groups were transformed into p-dimethylamino-anilide of styryl-glyoxylonitrile and measured at 475nm. The analysis showed that there is an enrichment of coniferyl alcohol and coniferaldehyde groups in the P/S I region of the fibre wall, and coniferyl alcohol groups in the S 3 layer. The high concentration of coniferyl alcohol and coniferaldehyde groups at PIS I coincides with a preferential sulphonation of this region at mild sulphite treatment in chemimechanical pulping.

Bromination of different morphological parts of spruce wood (Picea abies)

SummaryIt has been found that the lignin in a middle lamella fraction isolated by a sieving technique has the same reactivity to bromine as the whole wood lignin. A plataue value of 1 mole bromine/mole C9-unit was obtained for both samples. The lignin in compression wood had a considerably lower reactivity to bromine than the normal wood lignin. A plataue value of 0.68 mole bromine/mol C9-unit was obtained with such lignin. The discrepancy between this and earlier results from the bromination of middle lamella and the implication of the results for lignin determination by the SEM- and TEM-EDXA technique are discussed.