Structure of Lignin
Lignin is primarily the structural material that adds strength and rigidity to cell walls of woody plants. Lignin is resistant to most forms of biological attack compared to cellulose and other structural polysaccharides, and plants with a higher lignin content have been reported to be more resistant to direct sunlight and frost [14].
Lignin is a crosslinked polymeric material, and representative molecular masses of isolated lignin are in the range 1000-20,000 g/ mol, but the degree of polymerization in nature is difficult to measure since lignin is invariably fragmented during extraction and consists of several types of substructures, which repeat in an apparently haphazard manner. The macromolecular structure of lignin is based on phenylpropanoid monomer units that differ in the degree of oxygen substitution on the phenyl ring.
There are three main monolignol units in lignin structure as shown in Figure 10.1:
1. p-Coumaryl alcohol (4-hydroxyl phenyl, H), has a single hydroxy or methoxy group attached to the aromatic ring.
2. Coniferyl alcohol (guaiacyl, G), has two hydroxy or methoxy groups attached to the aromatic ring.
3. Sinapyl alcohol (syringyl, S) has three such groups attached to the aromatic ring.
During the biosynthesis of lignin, polymerization of phenylpro — panoid monomers is initiated by oxidases or peroxidases. While the precise mechanism is unclear, it is postulated that radical-radical combination of free radicals produced by enzymatic dehydrogenation is the key reaction, either under enzymatic control [15] or in a random combinatorial manner [16]. The two paths of free radical
Figure 10.1 Monolignol monomer units: (a) p-Coumaryl alcohol (4-hydroxyl phenyl, H), (b) coniferyl alcohol (guaiacyl, G), and (c) sinapyl alcohol (syringyl, S).
formation from coniferyl alcohol in the polymerization to lignin are shown as an example in Figure 10.2.
In the free radical polymerization steps both C-C and C-O bond formation between monomer units are known. Lignin structure is presented in Section 4.4.3, and a partial structure of lignin is shown in Figure 4.8 in Chapter 4. The most common functionality accounting for about half the bonds between monomers in lignin from most sources is a carbon-oxygen link between a p-hydroxy moiety and the b-end of the propenyl group (b-O-4) [17]. The rigidity of the lignocellulosic structure depends on the degree of crosslinking possible in lignin and this is dependent on the degree of substitution of the monomer units. This is evident by the fact that coniferyl alcohol (guaiacyl, G) structure is dominant in soft woods, while hardwood lignins normally contain a mixture of sinapyl alcohol (S) and G structures with S in the majority, while p-coumaryl alcohol (4-hydroxyl phenyl, H) structures predominate in lignins found in grasses [18].