Jim Parkås

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.

Origin of the acetylated structures present in white birch (Betula pendula Roth) milled wood lignin

The occurrence and nature of acetate groups in the milled wood lignin (MWL) isolated from birch (Betula pendula Roth) has been addressed by spectroscopic (2D-NMR) and chemical degradative (derivatization followed by reductive cleavage, DFRC) methods. Considerable amounts of acetate groups were present in the MWL preparation. However, 2D-NMR analysis indicated that the lignin polymer is not extensively acetylated and that the major part of the acetate groups is attached to the xylan moieties present in the MWL preparation. Nevertheless, evidence of the presence of minor acetylation of the γ-carbon of the lignin side chain (<3% of both syringyl and guaiacyl lignin units) was provided by DFRC analysis.

Solid state NMR analysis of β-13C-enriched lignocellulosic material during light-induced yellowing

Summary Photoyellowing of lignocellulosic materials has been studied with a new technique based on solid state 13C-NMR analysis of 13C-enriched DHP in cell wall tissue. The selectively 13C-enriched cell wall-dehydrogenation polymer (CW-DHP) was prepared directly on differentiating xylem from spruce (Picea abies) at pH 6.0 by administering β-13C-enriched coniferin in an enzymatic system consisting of glucose oxidase, β-glucosidase, and the naturally occurring water-insoluble enzymes remaining in the cell wall. The bonding pattern of the formed CW-DHP was found to be: 42% β-β, β-5, and β-1 substructures; 36% β-O-4 derived substructures; and 22% coniferyl alcohol and coniferaldehyde end-groups. The 13C-NMR analysis of unirradiated and irradiated tissue revealed a decrease in the relative amount of coniferaldehyde and/or coniferyl alcohol end-groups during irradiation. Prolonged irradiation also introduced new signals centered at 37, 70, and 102 ppm. The results indicate that the present technique, with the formation of DHP in a naturally lignifying carbohydrate environment, has the potential of being a valuable tool for the study of structural changes of lignin during light-induced yellowing.

Synthesis of Lignin-Related Cinnamaldehydes

Cinnamaldehyde units are present in lignins (Adler 1977) and certain types of cinnamaldehydes are therefore of interest as lignin model compounds. Lignin-related cinnamaldehydes [such as, coniferaldehyde (1), pcoumaraldehyde (2) and sinapaldehyde (3)] are well known lignin hydrolysis products and are also frequently found in plant extractives. The bioactivity of 1–3 has been examined in several recent papers (see e.g., Leem et al.1999; Barber et al. 2000). The synthesis of monomeric (Iliefski et al. 1998) and dimeric (Lundquist and Hedlund 1971; Li et al. 1998) lignin-related cinnamaldehydes by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) oxidation of 1-aryl-1-propenes and 3-aryl-1-propenes has been described. Synthesis of phenolic cinnamaldehydes such as 1–3 by this method involves the protection and deprotection of a phenolic group. We have synthesized 1–3 by DDQ oxidation and in this context we have studied the applicability of different types of protective groups. Different methods for the introduction of the protective groups were also examined. We think that the synthetic methods developed are generally applicable to the synthesis of phenolic lignin-related cinnamaldehydes. Lignin-related cinnamaldehydes and their properties are also of interest in connection with studies of lignins of cinnamyl alcohol dehydrogenase (CAD) deficient plants (Ralph et al. 2001a;Li et al. 2001). In this context model compounds of the α-aryloxycinnamaldehyde type are of interest. The synthesis and properties of two compounds of this type (18 and 21) are described in this paper.

Important Issues and Results from Knut Lundquist's Lignin Research at Chalmers

This is a manuscript originally written by Knut Lundquist and Jim Parkås in a more popular science spirit on topics in lignin chemistry where Knut and Chalmers has contributed and Knuts view on the bearing of this research on lignin chemistry. The manuscript was never published and was translated from Swedish by Ulla Westermark and Jim Parkås with a new title. The following topics are briefly addressed: stereochemistry and its implication on lignin structure and lignin biosynthesis, lignin-carbohydrate bonds, oxidative lignin degrading enzymes, reactions of β-ethers at neutral and acidic conditions.

Chemical Modification of Chemithermomechanical Pulps Part 1: Mechanical, Optical, and Aging Properties of Propionylated Spruce CTMP

Abstract The effect of propionic anhydride on the optical, mechanical, and aging properties of hydrogen-peroxide-bleached spruce (Picea abies) chemithermomechanical pulp (CTMP) has been examined. The aging properties were evaluated using different aging conditions simulating aging behind window-glass, ambient storage (in the dark), and dry heat exposure. The propionylation treatment was carried out on paper sheets. Chemical modification with propionic anhydride strongly reduced light-induced yellowing: Up to 80% of the discoloration (calculated from the post color number) could be hindered, although 50% of the free phenolic hydroxyl groups were still present in the propionylated sample. The stability against storage in the dark under ambient conditions (23°C, 50% relative humidity) or exposure to dry heat (105°C) was, however, not improved to the same extent. Propionylation did not extensively change the optical or dry strength properties of the treated paper sheets, whereas the wet tensile strength was substantially improved.

ACCELERATED LIGHT-INDUCED AGING OF α-, β-, AND γ-13C-ENRICHED CELL WALL-DEHYDROGENATION POLYMERS STUDIED WITH SOLID STATE 13C NMR SPECTROSCOPY

ABSTRACT Light-induced aging of lignocellulosic materials has been studied with a new technique involving selectively α-, β-, and γ-13C-enriched cell wall-dehydrogenation polymers (CW-DHPs) and solid state 13C NMR spectroscopy. The results from cross-polarization magic angle spinning (CP/MAS) 13C NMR experiments of unirradiated and irradiated CW-DHP have revealed mainly a decrease in the amount of end-groups of both coniferaldehyde and coniferyl alcohol type. The results suggest that these end-groups become saturated and that the terminal functionalites, i.e., γ-aldehyde and γ-hydroxymethyl groups, at least to some extent, are retained. The results indicate further that no detectable cleavage of the β-O-4 bonds occurs in the examined lignocellulosic model. In terms of proposed mechanisms of yellowing, there is marginal evidence that up to 2% of the α-labeled sites are converted by irradiation to α-carbonyls (aldehyde or ketones); moreover, we cannot dismiss the possibility that the precursor structures giving rise to these few α-carbonyls are β-O-4 structures. The 13C-enriched CW-DHP was formed directly on spruce (Picea abies) wood tissue (differentiating xylem) by administering selectively 13C-labeled coniferin at pH 6.0 in the presence of glucose oxidase and β-glucosidase, i.e., no phenol-oxidizing enzyme was added and the wood cells’ own enzymes polymerized the precursor.