The Formation and Structure of Lignin

Besides cellulolytic enzyme lignin, the so-called Bjorkman lignin, alternatively referred to as milled wood lignin (MWL) is the best preparation known so far, and it has been widely used for structural studies. When wood meal is ground in a ball mill either dry or in the presence of nonswelling solvents, e.g., toluene, the cell structure of the wood is destroyed and a portion of lignin (usually not more than 50%) can be obtained from the suspension by extraction with a dioxane-water mixture. MWL preparations always contain some carbohydrate material. 

Lignins are polymers of phenylpropane units. Many aspects in the chemistry of lignin still remain unclear, for example, the specific structural features of lignins located in various morphological regions of the woody xylem. Nevertheless, the principal structural elements in lignins have been largely 

Methods based on classical organic chemistry led to the conclusion, already by 1940, that lignin is built up of phenylpropane units. Examples of typical reactions used in these studies are illustrated in Fig. 4-1. However, the concept of a phenylpropanoid structure failed to win unanimous acceptance, and as late as 30 years ago, some scientists were not convinced that lignin in its native state was an aromatic substance. Finally, the problem was solved by Lange in 1954, who applied UV microscopy at various wavelengths directly on thin wood sections, obtaining spectra typical of aromatic compounds. 

The role of coniferyl alcohol as the immediate precursor of softwood lignin has been demonstrated by using 14C labeling. Administration of labeled coniferyl alcohol as /3-glucoside (coniferin) to seedlings of spruce results in the exclusive formation of radioactive lignin. 

The enzymic dehydrogenation reaction is initiated by an electron transfer which results in the formation of resonance-stabilized phenoxy radicals (Fig. 4-4). The combination of these radicals produces a variety of dimers and oligomers, termed lignols (Fig. 4-5). 

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Reactions that involve the formation and elimination of multiple-bond functional groups may significantly influence the color of residual lignin in bleached and unbleached pulps. Thus, ethylenic and carbonyl groups conjugated with phenolic or quinonoid structures may contribute to color as components of chromophore or leucochromophore systems. 

Chiang VL, Funaoka M (1988) The formation and quantity of diphenylmethane-type structures in residual lignin during kraft deligmfication of Douglas-fir Holzforschung 42 385-391... 

We now wish to report our results on isolating lignin-derived dimers by alkaline hydrogenation and to propose mechanisms for their formation. In addition, we will present some information on the (NMR) spectra of these and related compounds this technique has been useful for analyzing the structure of lignin degradation products and related synthetic materials. 

The formation of lignin proceeds by the uncontrolled coupling of the radical species, which leads to an irregular structure (Figure 2). The structure of lignin can be represented by the statistic ratio of each monomer, the frequency of each coupling mode, and the steric hindrance of substituents. This reaction mechanism has been successfully simulated [22],... 

Fig. 4-1. Examples of classical methods indicating a phenylpropanoid structure of lignin. (A) Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) (1) in a yield of 8% and minor amounts of isohemipinic (4,5-dimethoxyisophtalic acid) (2) and dehydrodiveratric (3) acids. The formation of isohemipinic acid supports the occurrence of condensed structures (e.g., /3-5 or y-5). (B) Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde) (4) (about 25% of lignin). Oxidation of hardwoods and grasses results, respectively, in syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) (5) and p-hydroxybenzaldehyde (6). (C) Hydrogenolysis yields pro-pylcyclohexane derivatives (7). (D) Ethanolysis yields so-called Hibbert ketones (8,9,10, and 11).

Neutral and Alkaline Sulfite Pulping In neutral sulfite pulping the most important reactions of lignin are restricted to phenolic lignin units only. The first stage always proceeds via the formation of a quinone methide with simultaneous cleavage of an a-hydroxyl or an a-ether group (Fig. 7-12). At least in noncyclic structures, the quinone methide is readily attacked by a... 

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