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'Lignin' (sometimes "'lignen'") is a complex chemical compound most commonly derived from wood and an integral part of the cell walls of plants. The term was introduced in 1819 and is derived from the Latin word ''lignum'',^[1] meaning wood. It is the most abundant organic polymer on Earth after cellulose, employing 30% of non-fossil organic carbon Lignin biosynthesis, W. Boerjan, J. Ralph, M. Baucher, , , Ann. Rev. Plant Biol., 2003
and constituting from a quarter to a third of the dry mass of wood. The compound has several unusual properties as a biopolymer, not least its heterogeneity in lacking a defined primary structure.

Contents

Biological function

Ecological function

Economic significance

Structure

Biosynthesis

Pyrolysis

References

External links

Biological function

Lignin fills the spaces in the cell wall between cellulose, hemicellulose and pectin components, especially in tracheids, sclereids and xylem. It is covalently linked to hemicellulose and thereby crosslinks different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole.^[2] It is particularly abundant in compression wood, but curiously scarce in tension wood.
Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes it possible for the plant's vascular tissue to conduct water efficiently.
Lignins: Occurence, Formation, Structure, and Reactions, K.V. Sarkanen & C.H. Ludwig (eds), , , Wiley Intersci., 1971, Lignin is present in all vascular plants, but not in bryophytes, supporting the idea that the original function of lignin was restricted to water transport.
Lignin is indigestible by mammalian and other animal enzymes, but some fungi and bacteria are able to biodegrade the polymer. The details of the reaction scheme of the biodegradation are not fully understood to date. These reactions depend on the type of wood decay - in fungi either brown rot, soft rot or white rot. The enzymes involved may employ free radicals for depolymerization reactions.^[3] Well understood lignolytic enzymes are manganese peroxidase, lignin peroxidase and cellobiose dehydrogenase. Furthermore, because of its cross-linking with the other cell wall components, it minimizes the accessibility of cellulose and hemicellulose to microbial enzymes. Hence, lignin is generally associated with reduced digestibility of the over all plant biomass, which helps defend against pathogens and pests.
Lignin peroxidase (also "ligninase", EC number 1.14.99) is a hemoprotein from the white-rot fungus ''Phanerochaete chrysosporium '' with a variety of lignin-degrading reactions, all dependent on hydrogen peroxide to incorporate molecular oxygen into reaction products. There are also several other microbial enzymes that are believed to be involved in lignin biodegradation, such as manganese peroxidase, laccase and cellobiose dehydrogenase.

Ecological function

Lignin plays a significant role in the carbon cycle, sequestering atmospheric carbon into the living tissues of woody perennial vegetation. Lignin is one of the most slowly decomposing components of dead vegetation, contributing a major fraction of the material that becomes humus as it decomposes. The resulting soil humus generally increases the photosynthetic productivity of plant communities growing on a site as the site transitions from disturbed mineral soil through the stages of ecological succession, by providing increased cation exchange capacity in the soil and expanding the capacity of moisture retention between flood and drought conditions.

Economic significance

Highly lignified wood is durable and therefore a good raw material for many applications. It is also an excellent fuel, since lignin yields more energy when burned than cellulose. However, lignin must be removed from pulp before paper can be manufactured from it. This is costly both in terms of energy and environmentally.
In the sulfite and sulfate (also called kraft) chemical pulping processes, lignin is removed from wood pulp as sulphates. These materials have several uses:

★ Dispersants in high performance cement applications, water treatment formulations and textile dyes

★ Additives in specialty oil field applications and agricultural chemicals

★ Raw materials for several chemicals, such as vanillin, DMSO, ethanol, torula yeast, xylitol sugar and humic acid

★ Environmentally sustainable dust suppression agent for roads
The first investigations into commercial use of lignin were done by Marathon Corporation in Rothschild, Wisconsin (USA), starting in 1927. The first class of products which showed promise were leather tanning agents. The lignin chemical business of Marathon is now known as LignoTech USA, Inc., and is owned by the Norwegian company, Borregaard.

Structure

'Fig. 1': An example of a possible lignin structure

'Fig. 2': A small piece of lignin polymer

'Fig. 3': The three common monolignols

'Fig. 4': Polymerisation of coniferyl alcohol to lignin. The reaction has two alternative routes catalysed by two different oxidative enzymes, peroxidases or oxidases.

Lignin is a large, cross-linked, racemic macromolecule with molecular masses in excess of 10,000u. It is relatively hydrophobic and aromatic in nature. The degree of polymerisation in nature is difficult to measure, since it is fragmented during extraction and the molecule consists of various types of substructures which appear to repeat in a haphazard manner.
There are three monolignol monomers, methoxylated to various degrees: ''p''-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol^[4] (Figure 3). These are incorporated into lignin in the form of the phenylpropanoids ''p''-hydroxyphenyl (H), guaiacyl (G), and syringal (S) respectively. Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of Dicotyledonic angiosperms is more often than not a mixture of G and S (with very little H), and monocotyledonic lignin is a mixure of all three. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.^[5]

Biosynthesis

Lignin biosynthesis (Figure 4) begins in the cytosol with the synthesis of glycosylated monolignols from the amino acid phenylalanine. These first reactions are shared with the phenylpropanoid pathway. The attached glucose renders them water soluble and less toxic. Once transported through the cell membrane to the apoplast, the glucose is removed and the polymerisation commences. Much about its anabolism is not understood even after more than a century of study.
The polymerisation step, that is a radical-radical coupling, is catalysed by oxidative enzymes. Both peroxidase and laccase enzymes are present in the plant cell walls, and it is not known whether one or both of these groups participates in the polymerisation. Low molecular weight oxidants might also be involved. The oxidative enzyme catalyses the formation of monolignol radicals. These radicals are often said to undergo uncatalyzed coupling to form the lignin polymer, but this hypothesis has been recently challenged.^[6]

Pyrolysis

Pyrolysis of lignin during the combustion of wood or charcoal production yields a range of products, of which the most characteristic ones are methoxy phenols. Of those, the most important are guaiacol and syringol and their derivatives; their presence can be used to trace a smoke source to a wood fire. In cooking, lignin in the form of hardwood is an important source of these two chemicals which impart the characteristic aroma and taste to smoked foods.

References

1.
Wood Chemistry: Fundamentals and Applications, E. Sjöström, , , Academic Press, 1993,

2.
''In situ'' analysis of lignins in transgenic tobacco reveals a differential impact of individual transformations on the spatial patterns of lignin deposition at the cellular and subcellular levels, M. Chabannes, ''et al.'', , , Plant J., 2001

3.
The Fungi, , Michael J., Carlile, Academic Press, 1994,

4.
Constitution and Biosynthesis of Lignin, K. Freudenberg & A.C. Nash (eds), , , Springer-Verlag, 1968,

5. Elucidation of new structures in lignins of CAD- and COMT-deficient plants by NMR, J. Ralph, ''et al.'', , , Phytochem., 2001
6. Lignin primary structures and dirigent sites, Davin, L.B., , , Current Opinion in Biotechnology, 2005

External links

★ Biosynthesis pathway of lignin

★ The Lignin Institute A promotional site by a trade association of lignin manufacturers and users.