Lignin
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Lignin (sometimes "lignen") is a chemical compound (complex, highly cross-linked aromatic polymer) that is most commonly derived from wood and is an integral part of the cell walls of plants, especially in tracheids, xylem fibres and sclereids.
The term lignin was introduced in 1819 and is derived from the latin word lignum.[1] It is one of most abundant organic compounds on earth after cellulose and chitin. Lignin makes up about one-quarter to one-third of the dry mass of wood. It has several unusual properties for being a biopolymer, such as having a network structure and lacking a defined primary structure.
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[edit] Biological function
Lignin fills the spaces in the cell wall between cellulose, hemicellulose and pectin components. It is covalently linked to hemicellulose. It also forms covalent bonds to polysaccharides and thereby crosslinks different plant polysaccharides.
It confers mechanical strength to the cell wall (stabilizing the mature cell wall) and therefore the entire plant. 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. Lignin is present in all vascular plants, but not in bryophytes, supporting the idea that the main function of lignin is related 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. It is suggested that enzymes that employ free radicals for depolymerization reactions are employed.[2] Well understood lignolytic enzymes are manganese 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. It is also suggested that lignin may help boost plant's defense against pathogens and pests. For example an infection by a fungus causes the plant to deposit more lignin near the infection site[citation needed].
[edit] 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 is detrimental to paper manufacture and must be removed from pulp before paper can be manufactured. This is costly both in terms of energy and environment.
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 friendly 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.
[edit] Structure and biosynthesis
Lignin is a large, cross-linked macromolecule with molecular mass in excess of 10,000 u. It is relatively hydrophobic and aromatic in nature. The molecular weight in nature is difficult to measure, since it is fragmented during preparation. The molecule consists of various types of substructures which appear to repeat in random manner.
Lignin biosynthesis begins with the synthesis of monolignols. The starting material is the amino acid phenylalanine. The first reactions in the biosynthesis are shared with the phenylpropanoid pathway, and monolignols are considered to be a part of this group of compounds. There are three main monolignols: coniferyl alcohol, sinapyl alcohol and paracoumaryl alcohol. (There are a number of other monolignols present in special plants or in low concentration.) Different plants use different monolignols. For example, gymnosperms as Norway spruce have a lignin that consist almost entirely coniferyl alcohol. Dicotyledonic lignin is a mixture of conifyl alcohol and sinapyl alcohol (normally more of the latter), and monocotylednic lignin is a mixure of all three monolignols. Some monocotyledons have mostly coniferyl alcohol (as many grasses), while other have mainly sinapyl alcohols, as some palms. Monolignols are synthesized in the cytosol as glucosides. The glucose is added to the monolignol to make them water soluble and to reduce their toxicity. The glucosides are transported through the cell membrane to the apoplast. The glucose is then removed and the monolignols are polymerized into lignin.
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.[3]
[edit] Pyrolysis
Pyrolysis of lignin, eg. during combustion, 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.
[edit] Ligninase
Ligninase is the lignin enzyme, a haemoprotein from phanerochaete chrysosporium with a variety of lignin-degrading reactions, all dependent on hydrogen peroxide; incorporates molecular oxygen into the products
The registry number is EC 1.14.99
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[edit] References
- ^ Sjöström, E. (1993). Wood Chemistry: Fundamentals and Applications. Academic Press.
- ^ Carlile, Michael J.; Sarah C. Watkinson (1994). The fungi. Academic Press. ISBN 0-12-159959-0.
- ^ Davin, L.B.; Lewis, N.G. (2005). "Lignin primary structures and dirigent sites". Current Opinion in Biotechnology 16: 407-415.
[edit] External links
- Biosynthesis pathway of lignin
- The Lignin Institute A promotional site by a trade association of lignin manufacturers and users.
