LIGNIN: STRUCTURE, FUNCTIONS, EXTRACTION, DEGRADATION, USES - BIOLOGY - 2025

The lignin (from Latin term lignum, meaning wood or timber) is a polymer itself vascular plants dimensional, amorphous and complex structure. In plants it serves as a "cement" that gives strength and resistance to plant stems, trunks and other structures.

It is located mainly in the cell wall and protects it against mechanical forces and pathogens, being also found in a small proportion inside the cell. Chemically it has a wide variety of active centers that allow them to interact with other compounds. Within these common functional groups we have phenolic, aliphatic, methoxy hydroxyls, among others.

Possible model of lignin. Source: real name: Karol Głąbpl.wiki: Karol007commons: Karol007e-mail: kamikaze007 (at) tlen.pl

Because lignin is a highly complex and diverse three-dimensional network, the structure of the molecule has not been elucidated with certainty. However, it is known to be a polymer formed from coniferyl alcohol and other phenylpropanoid compounds derived from the aromatic amino acids phenylalanine and tyrosine.

The polymerization of the monomers that constitute it varies depending on the species, and does not do it in a repetitive and predictable way like other abundant polymers of vegetables (starch or cellulose).

So far, only hypothetical models of the lignin molecule are available, and for its study in the laboratory they usually use synthetic variants.

The way of extraction of lignin is complex, since it is bound with other components of the wall and is very heterogeneous.

Discovery

The first person to report the presence of lignin was Swiss-born scientist AP de Candolle, who described its fundamental chemical and physical properties and coined the term "lignin."

Main characteristics and structure

Lignin is the second most abundant organic molecule in plants after cellulose, the majority component of plant cell walls. Every year the plants produce 20 × 10 ⁹ tons of lignin. However, despite its abundance, its study has been quite limited.

A significant proportion of all lignin (approximately 75%) is located in the cell wall, after the cellulose structure culminates (spatially speaking). The placement of lignin is called lignification and this coincides with the events of cell death.

It is an optically inactive polymer, insoluble in acid solutions but soluble in strong bases, such as sodium hydroxide and similar chemical compounds.

Difficulties in the extraction and characterization of lignin

Various authors argue that there are a series of technical difficulties related to the extraction of lignin, a fact that complicates the study of its structure.

In addition to technical difficulties, the molecule is covalently bound to cellulose and the rest of the polysaccharides that make up the cell wall. For example, in wood and other lignified structures (such as stems), lignin is strongly associated with cellulose and hemicellulose.

Lastly, the polymer is extremely variable between plants. For these reasons mentioned, it is common for synthetic lignin to be used for the study of the molecule in laboratories.

Most used extraction methods

The vast majority of lignin extraction methods modify its structure, preventing its study. Of all the existing methodologies, the most important seems to be kraft. During the procedure, the lignin is separated from the carbohydrates with a basic solution of sodium hydroxide and sodium sulfide in 3: 1 proportions.

Thus, the insulation product is a dark brown powder due to the presence of phenolic compounds, whose average density is 1.3 to 1.4 g / cm ³.

Monomers derived from phenylpropanoids

Despite these methodological conflicts, it is known that the lignin polymer consists mainly of three phenylpropanoid derivatives: coniferilic, coumaric and synapillic alcohols. These compounds are synthesized starting from the aromatic amino acids called phenylalanine and tyrosine.

The total composition of the lignin framework is almost totally dominated by the mentioned compounds, since incipient concentrations of proteins have been found.

The proportion of these three phenylpropanoid units is variable and depends on the plant species studied. It is also possible to find variations in the proportions of monomers within the organs of the same individual or in different layers of the cell wall.

Three-dimensional structure of lignin

The high ratio of carbon-carbon and carbon-oxygen-carbon bonds generates a highly branched three-dimensional structure.

Unlike other polymers that we find in abundance in vegetables (such as starch or cellulose), lignin monomers do not polymerize in a repetitive and predictable way.

Although the binding of these building blocks appears to be driven by stochastic forces, recent research has found that a protein appears to mediate polymerization and forms a large repeating unit.

Features

Although lignin is not a ubiquitous component of all plants, it fulfills very important functions related to protection and growth.

First of all, it is responsible for protecting the hydrophilic components (cellulose and hemicellulose) that do not have the typical stability and rigidity of lignin.

As it is found exclusively on the outside, it serves as a protective sheath against distortion and compression, leaving the cellulose to be responsible for the tensile strength.

When the wall components get wet, they lose mechanical strength. For this reason, the presence of lignin with the waterproof component is necessary. It has been shown that the experimental reduction of the percentage of lignin in the wood is related to the reduction of the mechanical properties of the same.

The protection of lignin also extends to possible biological agents and microorganisms. This polymer prevents the penetration of enzymes that could degrade vital cellular components.

It also plays a fundamental role in modulating the transport of liquid to all the structures of the plant.

Synthesis

The formation of lignin begins with a deamination reaction of the amino acids phenylalanine or tyrosine. The chemical identity of the amino acid is not very relevant, since the processing of both leads to the same compound: 4-hydroxycinnamate.

This compound is subjected to a series of chemical reactions of hydroxylation, transfer of methyl groups and reduction of the carboxyl group until an alcohol is obtained.

When the three precursors of lignin mentioned in the previous section have been formed, it is presumed that they are oxidized to free radicals, in order to create active centers to promote the polymerization process.

Regardless of the force that promotes the union, the monomers to each other through covalent bonds and create a complex network.

Degradation

Chemical degradation

Due to the chemical characteristics of the molecule, lignin is soluble in solutions of aqueous bases and hot bisulfite.

Fungal-mediated enzymatic degradation

The degradation of lignin mediated by the presence of fungi has been extensively studied by biotechnology for the bleaching and treatment of the remains produced after the manufacture of paper, among other uses.

The fungi that are capable of degrading lignin are called white rot fungi, which are in contrast to brown rot fungi that attack cellulose molecules and the like. These fungi are a heterogeneous group and their most prominent representative is the Phanarochaete chrysosporium species.

Through oxidation reactions - indirect and random - the bonds that hold the monomers together are gradually broken.

The action of fungi that attack lignin leaves behind a great variety of phenolic compounds, acids and aromatic alcohols. Some residues can mineralize, while others produce humic substances.

The enzymes that carry out this degradation process must be extracellular, since lignin is not bound by hydrolyzable bonds.

Lignin in digestion

For herbivores, lignin is a fibrous component of plants that is not digestible. That is, it is not attacked by the typical enzymes of digestion or by the microorganisms that live in the colon.

In terms of nutrition, it does not contribute anything to the body that consumes it. In fact, it can decrease the percentage of digestibility of other nutrients.

Applications

According to some authors, although agricultural residues can be obtained in almost inexhaustible quantities, so far there is no important application for the polymer in question.

Although lignin has been studied since the late 19th century, complications related to its processing have made it difficult to handle. However, other sources suggest that lignin can be exploited and propose several potential uses, based on the stiffness and strength properties we have discussed.

Currently, a series of wood preservatives based on lignin combined with a series of compounds are being developed to protect it from damage caused by biotic and abiotic agents.

It could also be an ideal substance for building insulators, both thermal and acoustic.

The advantage of incorporating lignin into the industry is its low cost and its possible use as a replacement for raw material developed from fossil fuels or other petrochemical resources. Thus, lignin is a polymer with great potential that seeks to be exploited.

References

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