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Lignin is a class of complex
organic polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inco ...
s that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
s, especially in
wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. ...

wood
and
bark Bark may refer to: * Bark (botany), an outer layer of a woody plant * Bark (sound), a vocalization of some animals Places * Bark, Germany * Bark, Warmian-Masurian Voivodeship, Poland Arts, entertainment, and media * ''Bark'' (Jefferson Airp ...
, 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 Precipitation is the process of conversion of a chemical substance into a solid from a solution by converting the substance into an insoluble form or a Supersaturated solution, super-saturated solution. When the reaction occurs in a liquid solu ...
from solution using acid. He named the substance “lignine”, which is derived from the Latin word ''
lignumLignum is Latin for wood Wood is a porous and fibrous structural tissue found in the Plant stem, stems and roots of trees and other woody plants. It is an organic materiala natural composite material, composite of cellulose fibers that are st ...
'', meaning wood. It is one of the most abundant
organic polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inco ...
s on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, exceeded only by
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
. Lignin constitutes 30% of non-
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

fossil
organic
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
and 20 to 35% of the dry mass of wood. Lignin is present in
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

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 ''Calliarthron'' is a genus containing two species of thalloid intertidal alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that ...
'', where it supports joints between
calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origi ...
segments.


Structure

Lignin is a highly
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally ...
polymer derived from a handful of precursor lignols that crosslink in diverse ways. The lignols that
crosslink In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * Chin ...
are of three main types, all derived from phenylpropane:
coniferyl alcohol Coniferyl alcohol is a colourless crystalline A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed ...

coniferyl alcohol
(4-hydroxy-3-methoxyphenylpropane) (G, its radical is sometimes called guaiacyl),
sinapyl alcohol Sinapyl alcohol is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

sinapyl alcohol
(3,5-dimethoxy-4-hydroxyphenylpropane) (S, its radical is sometimes called syringyl), and
paracoumaryl alcohol Paracoumaryl alcohol is a phytochemical Phytochemicals are chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemi ...

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 mass The molecular mass (''m'') is the mass of a given molecule: it is measured in dalton Dalton may refer to: Science * Dalton (crater), a lunar crater * Dalton (program), chemistry software * Dalton (unit) (Da), the atomic mass unit Entertainmen ...
es exceed 10,000 u. It is
hydrophobic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...
as it is rich in
aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...

aromatic
subunits. The
degree of polymerisationThe degree of polymerization, or DP, is the number of monomeric units in a macromolecule macromolecule A macromolecule is a very large molecule, such as a protein. They are composed of thousands of covalent bond, covalently bonded atoms. Many m ...
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 A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
between
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
,
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

hemicellulose
, and
pectin Pectin (from grc, πηκτικός ', "congealed, curdled") is a structural acidic heteropolysaccharide contained in the primary and middle lamella The middle lamella is a layer that cements together the primary cell wall A cell wall is a ...

pectin
components, especially in vascular and support tissues:
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
tracheid A tracheid is a long and tapered lignified cell in the xylem Xylem is one of the two types of transport in s, the other being . The basic function of xylem is to transport from roots to stems and leaves, but it also transports . The word ' ...
s,
vessel element 300px, SEM image (top) and Transmission Light Microscope image (bottom) of vessel elements in Oak A vessel element or vessel member (also called trachea or xylem vessel) is one of the cell types found in xylem, the water conducting tissue of p ...
s and
sclereid Sclereids are a reduced form of sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary ...
cells. Lignin plays a crucial part in conducting water and aqueous nutrients in
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
stems. The
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
components of plant
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
s are highly
hydrophilic A hydrophile is a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In phys ...
and thus
permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwee ...

permeable
to water, whereas lignin is more
hydrophobic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence ...
. 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 plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large group of plants ( 300,000 ...
s, but not in
bryophyte Bryophytes are an informal group consisting of three divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new n ...

bryophyte
s, supporting the idea that the original function of lignin was restricted to water transport. It is
covalent A covalent bond is a chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and take ...

covalent
ly linked to
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

hemicellulose
and therefore
cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * Chin ...
s different plant
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallograp ...
s, conferring mechanical strength to the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
and by extension the plant as a whole. Its most commonly noted function is the support through strengthening of wood (mainly composed of
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Ame ...

xylem
cells and lignified
sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary walls and usually remain alive after they bec ...
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 Newsprint is a low-cost, non-archival paper Paper is a thin sheet material Material is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, ...
, 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
lignosulfonatesLignosulfonates, or sulfonated lignin are water-soluble anionic polyelectrolyte polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM. ...
, for which many applications have been proposed. They are used as
dispersantA dispersant or a dispersing agent is a substance, typically a surfactant Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfac ...
s,
humectant A humectant is a hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal ...
s, emulsion stabilizers, and sequestrants (
water treatment Water treatment is any process that improves the quality Quality may refer to: Concepts *Quality (business), the ''non-inferiority'' or ''superiority'' of something *Quality (philosophy), an attribute or a property *Quality (physics), in respo ...

water treatment
). Lignosulfonate was also the first family of water reducers or
superplasticizer Superplasticizers (SP's), also known as high range water reducers, are additives used in making high strength concrete File:Pantheon cupola.jpg, Interior of the Pantheon dome, seen from beneath. The concrete for the coffered dome was laid on mo ...
s to be added in the 1930s as admixture to fresh
concrete Concrete is a composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material Material is a substance Substance may refer to: * Substance (Jainism), a ter ...

concrete
in order to decrease the water-to-cement (''w/c'') ratio, the main parameter controlling the concrete
porosity Porosity or void fraction is a measure of the void Void may refer to: Science, engineering, and technology * Void (astronomy) Cosmic voids are vast spaces between filaments (the largest-scale structures in the universe The universe ( ...
, and thus its
mechanical strength Strength of materials, also called mechanics of materials, deals with the behavior of solid objects subject to stresses and strains. The theory began with the consideration of the behavior of one and two dimensional members of structures, whose s ...
, its diffusivity and its
hydraulic conductivity Hydraulics (from Greek language, Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the ...
, 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 The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of into , which consists of almost pure fibers, the main component of . The kraft process entails treatment of wood chips with a hot mixture of water, ...

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 In industrial chemistry The chemical industry comprises the companies that produce industrial chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that che ...
for higher value uses: LignoBoost (Sweden) and LignoForce (Canada). Higher quality lignin presents the potential to become a renewable source of
aromatic In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...

aromatic
compounds for the chemical industry, with an addressable market of more than $130bn. Given that it is the most prevalent biopolymer after
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

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 Biosynthesis is a multi-step, enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and ...

biosynthesis
begins in the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
with the synthesis of
glycosylated Glycosylation (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylated
monolignols from the
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organ ...

phenylalanine
. These first are shared with the
phenylpropanoid The phenylpropanoids are a diverse family of organic compounds that are synthesized by plants from the amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to ...
pathway. The attached
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
renders them water-soluble and less
toxic Toxicity is the degree to which a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

toxic
. Once transported through the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
to the
apoplast 300px, The apoplastic and symplastic pathways Inside a plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert ligh ...
, the glucose is removed, and the polymerisation commences. Much about its
anabolism Anabolism () is the set of metabolic pathway In biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be d ...
is not understood even after more than a century of study. The
polymerisation In polymer chemistry, polymerization (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United State ...

polymerisation
step, that is a radical-radical coupling, is
catalysed that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalysed
by
oxidative enzyme An oxidative enzyme is an enzyme that Catalysis, catalyses an oxidation reaction. Two most common types of oxidative enzymes are peroxidases, which use hydrogen peroxide, and oxidases, which use molecular oxygen. They increase the rate at which Ade ...
s. Both
peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within org ...
and
laccase Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidation (mild reducing agent) are added to powdered potassium permanganate Potassium per ...
enzymes are present in the
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
cell walls Cell most often refers to: * Cell (biology) The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often refer ...
, 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 A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

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 In enzymology, a manganese peroxidase () is an enzyme that catalysis, catalyzes the chemical reaction :2 Mn(II) + 2 H+ + H2O2 \rightleftharpoons 2 Mn(III) + 2 H2O The 3 substrate (biochemistry), substrates of this enzyme are Mn(II), hydrogen ion, ...

manganese peroxidase
s, versatile peroxidases, and dye-decolourizing peroxidases as well as copper-based
laccase Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidation (mild reducing agent) are added to powdered potassium permanganate Potassium per ...
s. 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
lignocelluloseLignocellulose refers to plant dry matter ( biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth Earth is the third planet from the Sun and the only astronomical object known to harbor l ...
, but others lack this ability. Most fungal lignin degradation involves secreted
peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within org ...
s. Many fungal
laccase Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidation (mild reducing agent) are added to powdered potassium permanganate Potassium per ...
s 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 Biodegradation is the breakdown of organic matter Organic matter, organic material, or natural organic matter refers to the large source of Carbon compounds, carbon-based compounds found within natural and engineered, terrestrial, and aquatic ...
of lignin, including bacterial community composition, mineral associations, and redox state.


Pyrolysis

Pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition Thermal decomposition, or thermolysis, is a chemical decompositionChemical decomposition, or chemical breakdown, is the process or effect of simplifying a single chemica ...

Pyrolysis
of lignin during the
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
of wood or
charcoal Charcoal is a lightweight black carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 perc ...

charcoal
production yields a range of products, of which the most characteristic ones are
methoxy A methoxy group is the functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo ...
-substituted
phenols In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prope ...
. Of those, the most important are
guaiacol Guaiacol () is a naturally-occurring organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's abilit ...

guaiacol
and
syringol Syringol is a naturally occurring aromatic forms of benzene (top) combine to produce an average structure (bottom) In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compound ...

syringol
and their derivatives. Their presence can be used to trace a
smoke Smoke is a collection of airborne and es emitted when a material undergoes or , together with the quantity of air that is or otherwise mixed into the mass. It is commonly an unwanted of fires (including s, s, s, s, and s), but may als ...

smoke
source to a wood fire. In
cooking Cooking, cookery, or culinary arts is the art, science, and craft of using heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy ...

cooking
, lignin in the form of
hardwood Hardwood is wood Wood is a porous and fibrous structural tissue found in the stems and roots of tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and ...

hardwood
is an important source of these two compounds, which impart the characteristic aroma and taste to smoked foods such as
barbecue Barbecue or barbeque (informally BBQ in the UK; Barbie in Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian contine ...

barbecue
. The main flavor compounds of
smoked ham Ham is pork Pork is the culinary name for the meat of the Pig, domestic pig (''Sus scrofa domesticus''). It is the most commonly consumed meat worldwide, with evidence of pig animal husbandry, husbandry dating back to 5000 BC. Pork is ...
are
guaiacol Guaiacol () is a naturally-occurring organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's abilit ...

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 Hydroxymethylfurfural (HMF), also 5-(hydroxymethyl)furfural, is an organic compound formed by the Dehydration reaction, dehydration of reducing sugars. It is a white low-melting solid (although commercial samples are often yellow) which is highly ...
). or NREL A solution of hydrochloric acid and
phloroglucinol Phloroglucinol is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, ...

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

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References


External links


Tecnaro website
{{Authority control Phenylpropanoids Papermaking Plants Polymers