lignin

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in

wood

and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.

History

Lignin was first mentioned in 1813 by the Swiss botanist A. 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 be precipitated from solution using acid. He named the substance “lignine”, which is derived from the Latin word ''lignum'', meaning wood. It is one of the most abundant organic polymers on

Earth

, exceeded only by

cellulose

. Lignin constitutes 30% of non-

fossil

organic

carbon

and 20 to 35% of the dry mass of wood.

Lignin is present in

red algae

, which suggest that the common ancestor of plants and red algae also synthesised lignin. This finding also suggests that the original function of lignin was structural as it plays this role in the red alga ''Calliarthron'', where it supports joints between calcified segments.

Structure

Lignin is a highly heterogeneous polymer derived from a handful of precursor lignols that crosslink in diverse ways. The lignols that crosslink are of three main types, all derived from phenylpropane:

coniferyl alcohol

(4-hydroxy-3-methoxyphenylpropane) (G, its radical is sometimes called guaiacyl),

sinapyl alcohol

(3,5-dimethoxy-4-hydroxyphenylpropane) (S, its radical is sometimes called syringyl), and

paracoumaryl alcohol

(4-hydroxyphenylpropane) (H, its radical is sometimes called 4-hydroxyphenyl).

The relative amounts of the precursor "monomers" vary according to the plant source. Broadly speaking:
*hardwoods are rich in coniferyl and sinapyl units
*softwoods are rich in coniferyl units
*grasses are rich in coniferyl and sinapyl units

Lignin's molecular masses exceed 10,000 u. It is hydrophobic as it is rich in

aromatic

subunits. The degree of polymerisation is difficult to measure, since the material is heterogeneous. Different types of lignin have been described depending on the means of isolation.

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.

Biological function

Lignin fills the spaces in the cell wall between

cellulose

,

hemicellulose

, and

pectin

components, especially in vascular and support tissues:

xylem

tracheids, vessel elements and sclereid cells.

Lignin plays a crucial part in conducting water and aqueous nutrients 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

bryophyte

s, supporting the idea that the original function of lignin was restricted to water transport.

It is

covalent

ly linked to

hemicellulose

and therefore cross-links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole. Its most commonly noted function is the support through strengthening of wood (mainly composed of

xylem

cells and lignified sclerenchyma fibres) in vascular plants.

Finally, lignin also confers disease resistance by accumulating at the site of pathogen infiltration, making the plant cell less accessible to cell wall degradation.

Economic significance

Global commercial production of lignin is a consequence of papermaking. In 1988, more than 220 million tons of paper were produced worldwide. 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.

Mechanical, or high-yield pulp, which is used to make newsprint, still contains most of the lignin originally present in the wood. This lignin is responsible for newsprint's yellowing with age. 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.

In sulfite pulping, lignin is removed from wood pulp as lignosulfonates, for which many applications have been proposed. They are used as dispersants, humectants, emulsion stabilizers, and sequestrants (

water treatment

). Lignosulfonate was also the first family of water reducers or superplasticizers to be added in the 1930s as admixture to fresh

concrete

in order to decrease the water-to-cement (''w/c'') ratio, the main parameter controlling the concrete porosity, and thus its mechanical strength, its diffusivity and its hydraulic conductivity, all parameters essential for its durability. It has application in environmentally sustainable dust suppression agent for roads. Also, can be used in making of 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.

Lignin removed by the

kraft process

is usually burned for its fuel value, providing energy to power the mill. Two commercial processes exist to remove lignin from black liquor for higher value uses: LignoBoost (Sweden) and LignoForce (Canada). Higher quality lignin presents the potential to become a renewable source of

aromatic

compounds for the chemical industry, with an addressable market of more than $130bn.

Given that it is the most prevalent biopolymer after

cellulose

, 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.

Biosynthesis

Lignin

biosynthesis

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

. An alternative theory invokes an unspecified biological control.

Biodegradation

In contrast to other bio-polymers (e.g. proteins, DNA, and even cellulose), lignin is resistant to degradation and acid- and base-catalyzed hydrolysis. However the extent to which lignin does or does not degrade varies with species and plant tissue type. For example, syringyl (S) lignol is more susceptible to degradation by fungal decay as it has fewer aryl-aryl bonds and a lower redox potential than guaiacyl units. 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.

Some ligninolytic enzymes include heme peroxidases such as lignin peroxidases,

manganese peroxidase

s, versatile peroxidases, and dye-decolourizing peroxidases as well as copper-based laccases. 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 their ''in 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.

Lignin degradation by fungi

Well-studied ligninolytic enzymes are found in '' Phanerochaete chrysosporium'' and other white rot fungi. Some white rot fungi, such as '' C. subvermispora'', can degrade the lignin in lignocellulose, but others lack this ability. Most fungal lignin degradation involves secreted peroxidases. Many fungal laccases 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 of lignin peroxidase and other heme peroxidases.

Lignin degradation by bacteria

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 that degrade lignocellulose to metabolize abundant polysaccharides such as cellulose, hemicellulose, and pectin. Yet, bacterial degradation can be quite extensive, 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.

In the environment, lignin can be degraded either biotically via bacteria or abiotically via photochemical alteration, and oftentimes the latter assists in the former. In addition to the presence or absence of light, several of environmental factors affect the biodegradability of lignin, including bacterial community composition, mineral associations, and redox state.

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-substituted 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 compounds, which impart the characteristic aroma and taste to smoked foods such as

barbecue

. The main flavor compounds of smoked ham are

guaiacol

, 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.

Chemical analysis

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 its Ultraviolet spectroscopy. The carbohydrate composition may be also analyzed from the Klason liquors, although there may be sugar breakdown products (furfural and 5-hydroxymethylfurfural). or NREL

A solution of hydrochloric acid and

phloroglucinol

is used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence of coniferaldehyde groups in the lignin.

Thioglycolysis is an analytical technique for lignin quantitation. Lignin structure can also be studied by computational simulation.

Thermochemolysis (chemical break down of a substance under vacuum and at high temperature) with tetramethylammonium hydroxide (TMAH) or Copper(II) oxide, cupric oxide 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). 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. 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 many Wood-decay fungus, white-rot and some Wood-decay fungus, soft rot fungi.

Lignin and its models have been well examined by ¹H and ¹³C NMR spectroscopy. Owing to the structural complexity of lignins, the spectra are poorly resolved and quantitation is challenging.

Further reading

*

References

External links

Tecnaro website

{{Authority control

Phenylpropanoids
Papermaking
Plants
Polymers