In recent years, ionic liquids have been used to dissolve carbohydrates, and lignin residue is hoped to be relatively unchanged [21].
The second method involves the formation of soluble lignin derivatives, namely, lignosulfonate.
1.4.3.2 Functional Groups in Lignin
Lignin contains characteristic methoxyl groups, phenolic hydroxyl groups, and some terminal aldehyde groups in the side chains. Only relatively few of the phenolic hydroxyls are free; most of them are occupied by linkages to neighboring phenylpropane units. The syringyl units in hardwood lignin are extensively etherified. Alcoholic hydroxyl groups and carbonyl groups are introduced into the final lignin polymer during the dehydrogenative polymerization process.
1.4.3.3 Evidences for the Phenylpropane Units as Building Blocks of Lignin
The following methods based on classical organic chemistry, namely, degradation and synthesis, led to the conclusion, already by 1940, that lignin is built up by phenylpropane units [11].
1 Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) and minor amounts of isohemipinic (4,5-dimethoxyisophthalic) acid and dehydrodiveratric acid. The formation of isohemipinic acid supports occurrence of condensed structures (e.g., β-5 or γ-5). See structures 1 to 3 in Figure 1.6.
2 Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde). Oxidation of hardwoods and grasses results respectively in syringaldehyde (3,5 dimethoxy-4-hydroxybenzaldehyde) and p-hydroxybenzaldehyde. See structures 4 to 6 in Figure 1.6.
3 Hydrogenolysis yields propylcyclohexane derivatives. See structure 7 in Figure 1.6.
4 Ethanolysis yields so-called Hibbert ketones. See structures 8 to 11 of Figure 1.6.
Figure 1.6 Various degradation products of lignin.
1.4.3.4 Dehydrogenation Polymer (DHP)
The biosynthesis of lignin from the monomeric phenylpropane units can be generally described as a dehydrogenative polymerization. The principal ideas about such a pathway were elaborated by Freudenberg and co-workers [11]. They were the first to produce in vitro lignin called dehydrogenation polymer (DHP) by treating coniferyl alcohol with a fungal laccase from the mushroom Psalliota campestris or with a horseradish peroxidase by hydrogen peroxide.
The first step of the biochemical pathway for building up lignin macromolecules is the enzymatic dehydrogenation of p-hydroxycinnamyalcohols, yielding mesomeric ring systems with a loosened proton. Figure 1.7 shows the formation of phenoxy radicals from coniferyl alcohol by a one-electron transfer:
Figure 1.7 Enzymatic dehydrogenation of coniferyl alcohol yielding phenoxy radicals.
The origin of the hydrogen peroxide was cleared up by discovering cell-wall-bound enzyme systems able to deliver H2O2 [22, 23].
Only 4-phenoxyradical I to IV are actually involved in lignin biosynthesis. Structure V is sterically hindered or thermodynamically not favored [24].
The polymerization of monomeric precursors by random coupling reactions cannot be studied in vivo, but it is known from numerous in vitro experiments to run without enzymatic control as a spontaneous process. The first step in polymerization is the formation of dimeric structures. Some prominent lignol dimers called dilignols are shown in Figure 1.8.
Figure 1.8 Typical dilignol structures [25].
Further polymerization is called end-wise polymerization involving coupling of monolignols with the phenolic end groups of di- or oligolignols or a coupling of two end group free radicals, yielding a branched polymer via tri-, tetra-, penta-, and oligolignols [11].
Summarizing the formation of lignin, as mentioned by Fengel and Wegener [11], it is evident that these macromolecules are not formed by a genetically prescribed regular mechanism, but by a random coupling of lignols to form a nonlinear polymer. The final constitution of lignin is therefore determined mostly by reactivity and the frequency of the building units involved in its polymerization.
Proportions of different types of linkages connecting the phenylpropane units in lignin are given in Table 1.1.
Table 1.1 Proportions of different types of linkages connecting the phenylpropane units in lignin.
| Percent of the total linkages | |||
| Linkage typeb | Dimer structure | Softwooda | Hardwooda |
| β-O-4 | Arylglycerol-β-aryl ether | 50 | 60d |
| α-O-4 | Noncyclic benzyl aryl ether | 2–8c | 7 |
| β-5 | Phenylcoumaran |
9–12
| |