Lignin is a phenolic polymer. It is the second most abundant bio-polymer on Earth (after cellulose), and plays an important role in providing structural support to plants. Its hydrophobicity also facilitates water transport through the vascular tissue. Finally, the chemical complexity and apparent lack of regularity in its structure make lignin extremely suitable as a physical barrier against insects and fungi.
Like lignans, lignin is synthesized primarily from three monolignol precursors: p-coumaryl alcohol (1.68), coniferyl alcohol (1.69), and sinapyl alcohol (1.70). Additional compounds are incorporated into the lignin, but typically in small quantities. Some of these compounds include: conifer-aldehyde (1.75; Pillonel et al., 1991; Halpin et al., 1994; Ralph et al., 2001), sinapaldehyde (1.76; Pillonel et al., 1991), dihydroconiferyl alcohol (1.77; Ralph et al., 1997), 5-hydroxyconiferyl alcohol (1.78; Lapierre et al., 1988; Ralph et al., 2001; Marita et al., 2003), tyramine ferulate (1.79; Ralph et al., 1998), p-hydroxy-3 -methoxybenzaldehyde (1.80; Kim et al., 2003), p-hydroxybenzoate (1.81; Landucci et al., 1992), p-coumarate (1.13; Lu and Ralph, 1999) and acetate (Ralph, 1996).
The latter three compounds are esterified to the y-carbon of the monolignols. These compounds are found in higher quantities in certain mutants or genetically engineered plants in which the expression of specific lignin biosynthetic genes has been altered (Sederoff et al., 1999; Boerjan et al., 2003; Ralph et al., 2004). The presence of these compounds in lignin has prompted a broader definition of lignin, based more on the function than on a narrowly defined chemical composition (Brunow et al., 1999).
HO "t" OCH3
HO "t" OCH3
Lignin is formed through a radical-mediated polymerization process, but lignin is not optically active (Ralph et al., 1999), and the structure of lignin is believed to be under chemical control, rather than under the control of dirigent proteins or enzymes (Hatfield and Vermerris, 2001). The lignin polymer enlarges as additional monolignol radicals react with reactive sites on the polymer.
After polymerization the different lignin subunits are referred to as p-hyrdoxyphenyl (H), guaiacyl (G), and syringyl (S) residues, depending on whether they originated from p-coumaryl alcohol, coniferyl alcohol, or sinapyl alcohol, respectively.
Different kinds of interunit linkages can be formed depending on the position of the delocalized radical electron at the time two radicals are coupled. The most common interunit linkage in lignin is the P-0-4 linkage (1.82). Other coupling modes include: 5-0-4' (1.83), p-1 (1.84), 5-5' (1.85), P-P' (1.86), P-5 (1.87), and the dibenzodioxocin linkage (1.88; Brunow et al., 1998). The interunit linkages involving the P-carbon are favored. The 5-5'
and the 5-0-4' linkages are present in only small amounts, and tend to originate from preformed oligomers, rather than from the addition of new monolignol radicals to the growing ligninpolymer (Ralph et al., 2004). In plants that accumulate substantial amounts of 5-hydoxyconiferyl alcohol (1.78) as a result of reduced activity of the enzyme caffeic acid 0-methyltransferase (see Chapter 3), the benzodioxane linkage (1.89) has been identified. This is a novel linkage between two subunits involving a P-0-4' and an a-0-5' bond (Ralph et al., 2001; Marita et al., 2003). The substituents on the phenol ring in structures (1.82) through (1.89) are indicated with an R. In the case of H-residues both substituents are hydrogens, in the case of G-residues C3 contains a methoxyl group, and in the case of syringyl residues both C3 and C5 contain methoxyl groups.
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