Lignin or lignen is a complex
chemical compoundmost commonly derived from woodand an integral part of the secondary cell walls of plants.cite encyclopedia |last=Lebo |first=Stuart E. Jr. |author= |authorlink= |coauthors=Gargulak, Jerry D. and McNally, Timothy J. |editor= |encyclopedia=Kirk‑Othmer Encyclopedia of Chemical Technology |title=Lignin |url= http://www.mrw.interscience.wiley.com/emrw/9780471238966/kirk/article/lignlin.a01/current/pdf |accessdate=2007-10-14 |year=2001 |publisher= John Wiley & Sons, Inc |location= |doi=10.1002/0471238961.12090714120914.a01.pub2 ] The term was introduced in 1819 by de Candolle and is derived from the Latin word "lignum",cite book|author= E. Sjöström|title= Wood Chemistry: Fundamentals and Applications|publisher= Academic Press|year= 1993] meaning wood. It is one of the most abundant organic polymers on Earth, superseded only by cellulose, employing 30% of non- fossilorganic carboncite journal| author = W. Boerjan, J. Ralph, M. Baucher| month = Jun| year = 2003| title = Lignin bios| journal = Ann. Rev. Plant Biol.| volume = 54| pages = 519–549| doi = 10.1146/annurev.arplant.54.031902.134938] and constituting from a quarter to a third of the dry mass of wood. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure.
Lignin fills the spaces in the
cell wallbetween cellulose, hemicellulose, and pectincomponents, especially in tracheids, sclereids and xylem. It is covalently linked to hemicelluloseand thereby crosslinks different plant polysaccharides, conferring mechanical strength to the cell walland by extension the plant as a whole. [cite journal|author=M. Chabannes, "et al."|year=2001|title="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|journal=Plant J.|pages=271–282|volume=28|doi=10.1046/j.1365-313X.2001.01159.x|unused_data=|volume-28] It is particularly abundant in compression wood but scarce in tension wood.
Lignin plays a crucial part in conducting water in
plantstems. The polysaccharidecomponents of plant cell walls are highly hydrophilicand 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.cite book|author=K.V. Sarkanen & C.H. Ludwig (eds)|year=1971|title=Lignins: Occurrence, Formation, Structure, and Reactions|location=New York|publisher=Wiley Intersci.] 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 animal
enzymes, but some fungiand bacteriaare able to biodegrade the polymer. The details of the biodegradation are not well understood. The pathway depends on the type of wood decay - in fungi either brown rot, soft rotor white rot. The enzymes involved may employ free radicals for depolymerization reactions. [cite book|last= Carlile|first= Michael J.|coauthors= Sarah C. Watkinson|title= The Fungi|publisher= Academic Press|year= 1994|isbn= 0-12-159959-0] 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 overall plant biomass, which helps defend against pathogensand pests.
Lignin peroxidase (also "ligninase",
EC number1.14.99) is a hemoproteinfrom the white-rot fungus " Phanerochaete chrysosporium" with a variety of lignin-degrading reactions, all dependent on hydrogen peroxideto 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, laccaseand cellobiose dehydrogenase.
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 humusas 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 capacityin the soil and expanding the capacity of moisture retention between floodand droughtconditions.
woodis 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. Mechanical, or high yield pulp used to make newsprintcontains most of the lignin originally present in the wood. This lignin is responsible for newsprint yellowing with age. Lignin must be removed from the pulp before high quality bleached paper can be manufactured from it.
In sulfite pulping, lignin is removed from wood pulp as
sulfonates. These lignosulfonateshave several uses: [cite web|url=http://www.lignin.org/whatis.html|title=Uses of lignin from sulfite pulping|accessdate=2007-09-10]
Dispersants in high performance cementapplications, water treatmentformulations and textile dyes
* Additives in specialty
oil fieldapplications and agriculturalchemicals
* Raw materials for several chemicals, such as
vanillin, DMSO, ethanol, torula yeast, xylitolsugar and humic acid
Environmentally sustainabledust suppression agent for roads
The first investigations into commercial use of lignin were reported by
Marathon Corporationin Rothschild, Wisconsin( USA), starting in 1927. The first class of products which showed promise were leather tanningagents. The lignin chemical business of Marathon was operated for many years as Marathon Chemicals. It is now known as LignoTech USA, Inc., and is owned by the Norwegian company, Borregaard, itself a subsidiary of the Norwegian conglomerate Orkla AS.
Lignin removed via the
kraft process(sulfate pulping) is usually burned for its fuel value, providing more than enough energy to run the mill and its associated processes.
More recently, lignin extracted from shrubby willow has been successfully used to produce expanded polyurethane foam. [ [http://www.genesis.co.nz/Press+Releases/2007/GREEN+PLASTIC+PRODUCED+FROM+BIOJOULE+MATERIAL.html Green plastic produced from biojoule material] BioJoule Technologies Press Release, 12 July 2007.]
Lignin is a cross-linked
racemic macromoleculewith molecular masses in excess of 10,000 u. It is relatively hydrophobicand aromaticin nature. The degree of polymerisationin natureis difficult to measure, since it is fragmented during extraction and the moleculeconsists of various types of substructures which appear to repeat in a haphazard manner. Different types of lignin have been described depending on the means of isolation. [cite web |url=http://www.lignin.org/01augdialogue.html |title=Lignin and its Properties: Glossary of Lignin Nomenclature |accessdate=2007-10-14 |last= |first= |coauthors= |month=July | year=2001 |work=Dialogue/Newsletters Volume 9, Number 1 |publisher=Lignin Institute]
There are three
monolignol monomers, methoxylated to various degrees: "p"-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol[cite book|author= K. Freudenberg & A.C. Nash (eds)|year=1968|title=Constitution and Biosynthesis of Lignin|location=Berlin|publisher=Springer-Verlag] (Figure 3). These lignols 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.Fact|date=July 2007 All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants. [cite journal|author=J. Ralph, "et al."|year=2001|title=Elucidation of new structures in lignins of CAD- and COMT-deficient plants by NMR|journal=Phytochem.| volume=57 |pages=993–1003|doi=10.1016/S0031-9422(01)00109-1]
biosynthesis(Figure 4) begins in the cytosolwith the synthesis of glycosylatedmonolignols from the amino acid phenylalanine. These first reactions are shared with the phenylpropanoid pathway. The attached glucoserenders them water soluble and less toxic. Once transported through the cell membraneto the apoplast, the glucose is removed and the polymerisation commences.Fact|date=July 2007 Much about its anabolismis not understood even after more than a century of study.
polymerisationstep, that is a radical-radical coupling, is catalysed by oxidative enzymes. Both peroxidaseand laccaseenzymes 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. [cite journal |author=Davin, L.B. |coauthors=Lewis, N.G. |year=2005 |title=Lignin primary structures and dirigent sites |journal=Current Opinion in Biotechnology |volume=16 |pages=407–415 |doi=10.1016/j.copbio.2005.06.011] The alternative theory that involves an unspecified biological control is however not widely accepted.
Pyrolysisof lignin during the combustionof wood or charcoalproduction yields a range of products, of which the most characteristic ones are methoxy phenols. Of those, the most important are guaiacoland syringoland their derivatives; their presence can be used to trace a smokesource to a wood fire. In cooking, lignin in the form of hardwoodis an important source of these two chemicals which impart the characteristic aroma and taste to smoked foods.
* [http://www.genome.ad.jp/kegg/pathway/map/map00940.html Biosynthesis pathway of lignin]
* [http://www.lignin.org The Lignin Institute] A promotional site by a trade association of lignin manufacturers and users.
* [http://technology.newscientist.com/article/dn14360-chemical-breakthrough-turns-sawdust-into-biofuel.html?DCMP=ILC-hmts&nsref=news1_head_dn14360 Chemical breakthrough turns sawdust into biofuel] - breakthrough efficiency achieved in converting lignin into liquid hydrocarbons, "New Scientist", 18 July 2008
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