Lignin is a complex biopolymer containing a group of aromatic alcohols namely p-coumarylalcohol, coniferyl alcohol, and sinapyl alcohol. This polymeric molecule is made by cross-linking phenolic precursors. The structure of this compound is very complex as monolignols are connected through several linkages in a three-dimensional network.
This organic substance is produced by certain plants. The secondary cell walls of plants are constructed by this biomolecule as a binder for cellulosic and hemicellulosic fibers and provide stiffness to the cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. This important compound was invented by the Swiss botanist Augustin Pyramus de Candolle in 1813. The word ‘lignin’ comes from the Latin word lignum meaning wood.
Lignin structure
It is a highly heterogeneous biopolymer consisting of aliphatic and aromatic structures. It has a three-dimensional amorphous polymeric structure as well as a higher molecular weight. This organic molecule contains phenyl propane units. These units are formed from hydroxyl- and methoxy-substituted phenylpropane units. These units related are guaiacol (G) units from the trans-coniferyl alcohol, syringe (S) units that represent trans-sinapoyl alcohol, and p-hydroxyphenyl (H) units. These units are responsible for the precursor trans-p-coumaric alcohol.
This biopolymer is classified into two main groups namely guaiacyl lignins and guaiacyl-syringyl lignins. Guaiacyl class lignin is found in softwoods and consists of coniferyl alcohol units. Guaiacyl-syringyl type lignin is found in hardwoods and consists of monomeric units that are obtained from both of coniferyl and sinapyl alcohols.
Composition of lignin
This biopolymer is an important component of the plant cell walls. It is composed of coniferyl, p-coumaryl, and sinapyl alcohols in varying ratios. There exist many monomers such as hydroxycinnamic acids and aldehydes, as well as coniferyl and sinapyl acetates or curates that take part in the polymerization of these molecules.
Lignin solvents
There have various solvents namely dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), 2-methoxyethanol, diethylene glycol monobutyl ether (butyl carbitol), and 1-methoxy-2-propanol.
Functions
This polymeric molecule provides strength and stiffness to the plant cell wall as well as helps them withstand the compressive forces of gravity. This molecule creates a waterproof state of the plant cell wall and facilitates the upward transport of water in xylem tissues. It helps the plant cell wall against the damaging effects of UV radiation. It also helps the transportation of water molecules into the cell wall.
Properties of lignin
The molecular formula of this compound is C81H92O28
It contains yellow, light to dark brown color.
The glass transition temperature of this compound is 97-162°C
The ash content of this organic molecule is 2-6wt%
The density of this polymer is 1,350-1,500 kg/m3 at 20°C.
It has both hydrophobic and hydrophilic groups
Applications
It is used for the production of fuel.
It is used for the synthesis of nanoparticles, polycarboxylic acid, supercapacitor electrodes, photocatalysts, photovoltaics, etc.
It is used in industries for the production of automotive brakes, dispersants, wood panel products, polyurethane foams, and epoxy resins for printed circuit boards.
This compound can be used after modification of chemical reaction and directly constitute the nutrient for fertilizers.
It is used as a coating for the active ingredient.
This biopolymer can be applied as a chelating agent for trace element release.
This compound is used as an antioxidant in pharmaceutical products and as protection against oxidative stress.
This compound has antimutagenic, antiviral, anti-diarrheal, antimicrobial activity and antitumor activity.
It has the ability to create immunomodulatory effects.
It shows anti-UV and good biocompatible properties.
Lignin has no solubility in water; its hydrophilicity can be improved by grafting ionizable chemical functionalities.
Frequently asked questions (FAQ’s)
What is lignin?
It is a heterogeneous biopolymer consisting of aliphatic and aromatic structures. It has a three-dimensional amorphous polymeric structure as well as a higher molecular weight. This organic molecule contains phenyl propane units. These units are formed from hydroxyl- and methoxy-substituted phenylpropane units.
What is the structure of lignin?
This organic molecule contains phenyl propane units. These units are formed from hydroxyl- and methoxy-substituted phenylpropane units. These units related are guaiacol (G) units from the trans-coniferyl alcohol, syringe (S) units that represent trans-sinapoyl alcohol, and p-hydroxyphenyl (H) units. These units are responsible for the precursor trans-p-coumaric alcohol.
What are the compositions of lignin?
It is composed of coniferyl, p-coumaryl, and sinapyl alcohols in varying ratios. There exist many monomers such as hydroxycinnamic acids and aldehydes, as well as coniferyl and sinapyl acetates or coumarates take part the polymerization of lignin molecules.
Why lignin cannot be digested by humans?
Animals cannot digest lignin because it is composed of complex units that no animal can use.
What are the medicinal uses of lignin?
It has antimutagenic, antiviral, anti-diarrheal, antimicrobial activity and antitumor activity. As a result, a lot of pharmaceutical products are produced by this component.
What are the industrial uses of lignin?
It is used in various industries for the production of automotive brakes, dispersants, wood panel products, polyurethane foams, and epoxy resins for printed circuit boards.
References
Annunziata, M. G. (2019). What is lignin made of? New components discovered!. Plant Physiology, 180(3), 1255-1255.
Lourenco, A., Rencoret, J., Chemetova, C., Gominho, J., Gutierrez, A., Del Rio, J. C., & Pereira, H. (2016). Lignin composition and structure differs between xylem, phloem, and phellem in Quercus suber L. Frontiers in plant science, 7, 1612.
