This article is about the wood polymer. For the phytoestrogen, seeLignan.
Idealized structure of lignin from a softwood
Lignin is a class of complexorganic polymers that form key structural materials in the support tissues of most plants.[1] Lignins are particularly important in the formation ofcell walls, especially inwood andbark, because they lend rigidity and do notrot easily. Chemically, lignins are polymers made by cross-linkingphenolic precursors.[2]
Lignin was first mentioned in 1813 by the Swiss botanistA. P. de Candolle, who described it as a fibrous, tasteless material, insoluble in water and alcohol but soluble in weak alkaline solutions, and which can beprecipitated from solution using acid.[3] He named the substance "lignine", which is derived from the Latin wordlignum,[4] meaning wood. It is one of the most abundantorganic polymers onEarth, exceeded only bycellulose andchitin. Lignin constitutes 30% of terrestrial non-fossil organiccarbon[5] on Earth, and 20 to 35% of the dry mass of wood.[6]
Lignin is present inred algae, which suggest that the common ancestor of plants and red algae may have been pre-adapted to synthesize lignin. This finding also suggests that the original function of lignin may have been structural as it plays this role in the red algaCalliarthron, where it supports joints betweencalcified segments.[7]
The composition of lignin varies from species to species. An example of composition from anaspen[8] sample is 63.4% carbon, 5.9% hydrogen, 0.7% ash (mineral components), and 30% oxygen (by difference),[9] corresponding approximately to the formula (C31H34O11)n.
Lignin is a collection of highlyheterogeneous polymers derived from a handful of precursor lignols. Heterogeneity arises from the diversity and degree of crosslinking between these lignols. Thelignols thatcrosslink are of three main types, all derived from phenylpropane:coniferyl alcohol (3-methoxy-4-hydroxyphenylpropane; its radical, G, is sometimes called guaiacyl),sinapyl alcohol (3,5-dimethoxy-4-hydroxyphenylpropane; its radical, S, is sometimes called syringyl), andparacoumaryl alcohol (4-hydroxyphenylpropane; its radical, H, is sometimes called 4-hydroxyphenyl).[citation needed]
The relative amounts of the precursor "monomers" (lignols or monolignols) vary according to the plant source.[5] Lignins are typically classified according to their syringyl/guaiacyl (S/G) ratio. Lignin fromgymnosperms is derived from theconiferyl alcohol, which gives rise to G upon pyrolysis. Inangiosperms some of the coniferyl alcohol is converted to S. Thus, lignin in angiosperms has both G and S components.[10][11]
Lignin'smolecular masses exceed 10,000u. It ishydrophobic as it is rich inaromatic subunits. Thedegree of polymerisation is difficult to measure, since the material is heterogeneous. Different types of lignin have been described depending on the means of isolation.[12]
Many grasses have mostly G, while some palms have mainly S.[13] All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.[14]
Lignin plays a crucial part in conducting water and aqueous nutrients inplant stems. Thepolysaccharide components of plantcell walls are highlyhydrophilic and thuspermeable to water, whereas lignin is morehydrophobic. 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.[15] Lignin is present in allvascular plants, but not inbryophytes, supporting the idea that the original function of lignin was restricted to water transport.
Finally, lignin also confers disease resistance by accumulating at the site of pathogen infiltration, making the plant cell less accessible to cell wall degradation.[20]
In pulp mills (like this one inBlankenstein, Germany) using thekraft or thesulfite process, lignin is removed from lignocellulose to yield pulp for papermaking.
Global commercial production of lignin is a consequence of papermaking. In 1988, more than 220 million tons of paper were produced worldwide.[21] Much of this paper was delignified; lignin comprises about 1/3 of the mass of lignocellulose, the precursor to paper. Lignin is an impediment to papermaking as it is colored, it yellows in air, and its presence weakens the paper. Once separated from the cellulose, it is burned as fuel. Only a fraction is used in a wide range of low volume applications where the form but not the quality is important.[22]
Mechanical, or high-yieldpulp, which is used to makenewsprint, still contains most of the lignin originally present in the wood. This lignin is responsible for newsprint's yellowing with age.[4] High quality paper requires the removal of lignin from the pulp. These delignification processes are core technologies of the papermaking industry as well as the source of significant environmental concerns.[citation needed]
Insulfite pulping, lignin is removed from wood pulp aslignosulfonates, for which many applications have been proposed.[23] They are used asdispersants,humectants,emulsion stabilizers, and sequestrants (water treatment).[24] Lignosulfonate was also the first family ofwater reducers orsuperplasticizers to be added in the 1930s as admixture to freshconcrete in order to decrease the water-to-cement (w/c) ratio, the main parameter controlling the concreteporosity, and thus itsmechanical strength, itsdiffusivity and itshydraulic conductivity, all parameters essential for its durability. It has application in environmentally sustainable dust suppression agent for roads. Also, lignin can be used in making biodegradable plastic along with cellulose as an alternative to hydrocarbon-made plastics if lignin extraction is achieved through a more environmentally viable process than generic plastic manufacturing.[25]
Lignin removed by thekraft process is usually burned for its fuel value, providing energy to power the paper mill. Two commercial processes exist to remove lignin fromblack liquor for higher value uses: LignoBoost (Sweden) and LignoForce (Canada). Higher quality lignin presents the potential to become a renewable source ofaromatic compounds for the chemical industry, with an addressable market of more than $130bn.[26]
Given that it is the most prevalent biopolymer aftercellulose, lignin has been investigated as a feedstock for biofuel production and can become a crucial plant extract in the development of a new class of biofuels.[27][28]
Thepolymerisation step, that is a radical-radical coupling, iscatalysed byoxidative enzymes. Bothperoxidase andlaccase enzymes are present in theplantcell 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 monolignolradicals. These radicals are often said to undergo uncatalyzed coupling to form the ligninpolymer.[30] An alternative theory invokes an unspecified biological control.[1]
In contrast to other bio-polymers (e.g. proteins, DNA, and even cellulose), lignin resists degradation. It is immune to both acid- and base-catalyzed hydrolysis. The degradability varies with species and plant tissue type. For example, syringyl (S) lignin is more susceptible to degradation by fungal decay as it has fewer aryl-aryl bonds and a lower redox potential than guaiacyl units.[31][32] Because it is cross-linked with the other cell wall components, lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes, leading to a reduced digestibility of biomass.[15]
Some ligninolytic enzymes includeheme peroxidases such aslignin peroxidases,manganese peroxidases,versatile peroxidases, anddye-decolourizing peroxidases as well as copper-basedlaccases. Lignin peroxidases oxidize non-phenolic lignin, whereas manganese peroxidases only oxidize the phenolic structures. Dye-decolorizing peroxidases, or DyPs, exhibit catalytic activity on a wide range of lignin model compounds, but theirin vivo substrate is unknown. In general, laccases oxidize phenolic substrates but some fungal laccases have been shown to oxidize non-phenolic substrates in the presence of synthetic redox mediators.[33][34]
Well-studied ligninolytic enzymes are found inPhanerochaete chrysosporium[35] and otherwhite rot fungi. Some white rot fungi, such asCeriporiopsis subvermispora, can degrade the lignin inlignocellulose, but others lack this ability. Most fungal lignin degradation involves secretedperoxidases. Many fungallaccases are also secreted, which facilitate degradation of phenolic lignin-derived compounds, although several intracellular fungal laccases have also been described. An important aspect of fungal lignin degradation is the activity of accessory enzymes to produce the H2O2 required for the function oflignin peroxidase and otherheme peroxidases.[33]
Bacteria lack most of the enzymes employed by fungi to degrade lignin, and lignin derivatives (aliphatic acids, furans, and solubilized phenolics) inhibit the growth of bacteria.[36] Yet, bacterial degradation can be quite extensive,[37] especially in aquatic systems such as lakes, rivers, and streams, where inputs of terrestrial material (e.g.leaf litter) can enter waterways. The ligninolytic activity of bacteria has not been studied extensively even though it was first described in 1930. Many bacterial DyPs have been characterized. Bacteria do not express any of the plant-type peroxidases (lignin peroxidase, Mn peroxidase, or versatile peroxidases), but three of the four classes of DyP are only found in bacteria. In contrast to fungi, most bacterial enzymes involved in lignin degradation are intracellular, including two classes of DyP and most bacterial laccases.[34]
In the environment, lignin can be degraded either biotically via bacteria or abiotically via photochemical alteration, and oftentimes the latter assists in the former.[38] In addition to the presence or absence of light, several of environmental factors affect thebiodegradability of lignin, including bacterial community composition, mineral associations, and redox state.[39][40]
Pyrolysis of lignin during thecombustion of wood orcharcoal production yields a range of products, of which the most characteristic ones aremethoxy-substitutedphenols. Of those, the most important areguaiacol andsyringol and their derivatives. Their presence can be used to trace asmoke source to a wood fire. Incooking, lignin in the form ofhardwood is an important source of these two compounds, which impart the characteristic aroma and taste tosmoked foods such asbarbecue. The main flavor compounds ofsmoked ham areguaiacol, and its 4-, 5-, and 6-methyl derivatives as well as 2,6-dimethylphenol. These compounds are produced by thermal breakdown of lignin in the wood used in the smokehouse.[42]
The conventional method for lignin quantitation in the pulp industry is the Klason lignin and acid-soluble lignin test, which is standardized procedures. The cellulose is digested thermally in the presence of acid. The residue is termed Klason lignin. Acid-soluble lignin (ASL) is quantified by the intensity of itsUltraviolet spectroscopy. The carbohydrate composition may be also analyzed from the Klason liquors, although there may be sugar breakdown products (furfural and5-hydroxymethylfurfural).[43]
A solution of hydrochloric acid andphloroglucinol is used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence ofconiferaldehyde groups in the lignin.[44]
Thioglycolysis is an analytical technique for ligninquantitation.[45] Lignin structure can also be studied by computational simulation.[46]
Thermochemolysis (chemical break down of a substance under vacuum and at high temperature) withtetramethylammonium hydroxide (TMAH) orcupric oxide[47] has also been used to characterize lignins. The ratio of syringyl lignol (S) to vanillyl lignol (V) and cinnamyl lignol (C) to vanillyl lignol (V) is variable based on plant type and can therefore be used to trace plant sources in aquatic systems (woody vs. non-woody and angiosperm vs. gymnosperm).[48] Ratios of carboxylic acid (Ad) to aldehyde (Al) forms of the lignols (Ad/Al) reveal diagenetic information, with higher ratios indicating a more highly degraded material.[31][32] Increases in the (Ad/Al) value indicate an oxidative cleavage reaction has occurred on the alkyl lignin side chain which has been shown to be a step in the decay of wood by manywhite-rot and somesoft rot fungi.[31][32][49][50][51]
Lignin and its models have been well examined by1H and13C NMR spectroscopy. Owing to the structural complexity of lignins, the spectra are poorly resolved and quantitation is challenging.[52]
^In the referenced article, the species of aspen is not specified, only that it was from Canada.
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