Softwood lignin structure

This research was undertaken to study aqueous alkaline reactions of monomeric structures, similar to polymeric wood constituents, from which color would be formed. Since hardwood pulps were of the greatest immediate interest, the reaction of syringyl alcohol, representing the hardwood lignin structure, in aqueous alkaline solution at room temperature has been studied extensively up to the present time, but the reactions of vanillyl alcohol and a-methylvanillyl alcohol, representing the softwood lignin structure, have also been studied to some extent under the same reaction conditions. 

Fig. 17 Schematic representation of a softwood lignin structure, with the monolignols, the building blocks of lignin, depicted in the inset...

Smit, R., Suckling, I. D., and Ede, R. M., A new method for the quantification of condensed and uncondensed softwood lignin structures. Proceedings of 9th International Symposium on Wood and Pulping Chemistry. Montreal, Canada, June 1997, Vol. 1, L4-1-L4-6. 

Figure 2.5 A representative molecular structure of a softwood lignin.

Ishikawa, H., W. J. Schubert, and F. F. Nord The Enzymatic Degradation of Softwood Lignin by White Rot Fungi. The Degradation by Polyporus versicolor and Forties fomentarius of Aromatic Compounds Structurally Related to Softwood Lignin. Arch. Biochem. Biophys. 100, 131-149 (1963). 

Phenolic and etherified softwood lignin model compounds were also successfully crosslinked at the meta positions. Special care was taken to assign the structure of the dimer (22) (Fig. 4C). With NMR-SPI (selective population inversion) techniques it was unevocably proved that the methylene linkage was situated at the 6-positions (13). 

The similar spectrum of products isolated under similar conditions of reaction from hard and softwoods indicates a basic similarity in the structural features of the releasable fragment of each species. The liberation, as the major product, of either dihydroconiferyl alcohol or dihydro-coniferyl alcohol plus dihydrosinapyl alcohol (depending on the wood species) indicates that a fairly accessible portion of the lignin substance is of a polyphenylpropanoid structure. The other, more highly fragmented products are most likely secondary reaction products derived from the initially liberated phenylpropanoid compounds. As such they should not be considered as unique units of lignin structure. 

Isolation and characterization of lignin structures Hydrolysis and separation. Mild hydrolysis has previously been used for the structural analysis of both softwood and hardwood lignins (10 -14). About 20% of the lignin was liberated from spruce wood and 40% of the lignin from beech wood by percolation of finely ground wood meal with water at 100 °C for several weeks (72). Such a mild hydrolytic treatment can be expected to lead to the rupture of only... 

Fig. 4-9. Prominent structures of softwood lignin (Adler, 1977) comprising 16 phenylpropane units.

In normal softwood lignin, usually referred to as guaiacyl lignin, the structural elements are derived principally from coniferyl alcohol, (1) (more than 95% of the total number of structural units), with the remainder consisting mainly of p-coumaryl alcohol-type units (Fig. 1.1, unit 2) and trace amounts of sinapyl alcohol-derived units (Fig. 1.1, unit 13). 

Goldschmid O (1971) Ultraviolet spectra In Sarkanen KV, Ludwig CH (eds) Lignins Occurrence, formation, structure and reactions Wiley-Interscience, New York, 160-263 Hardell H-L, Leary GJ, Stoll M, Westermark U (1980) Variation in lignin structure in defined morphological parts of spruce Sven Papperstidn 83 44-49 Hatton JV (1975) Kappa number-permanganate number relationships for softwoods and hardwoods Tappi 58(10) 150-151... 

Although, the ultraviolet spectroscopy of softwood lignin and guaiacyl model compounds has been thoroughly studied, the same cannot be said about hardwood lignins. It has been shown that the absorptivity of hardwood milled wood lignins at 280 nm decreases linearly with the methoxyl/carbon ratio (Sarkanen et al. 1967), but the effects of side chain structures on the ultraviolet absorption properties of hardwood lignins have yet to be determined. 

In modified softwood lignins, NE quantitatively converts not only noncondensed guaiacyl nuclei (type Ga) having side chain structures a, b, c, d, and e (Fig. 6.6.4) into guaiacol and catechol, but also guaiacyl nuclei Ga and Gb in DPM moieties having these side chain structures into the same products. To... 

Funaoka M, Abe I (1983b) Structures of softwood lignins dissolved by successive acidolysis Mokuzai Gakkaishi 29 781-788... 

Fig. 4 Proposed structure of softwood lignin showing the functional groups thought to be...

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