Lignin (sometimes "lignen") is a chemical compound that is most commonly derived from wood and is an integral part of the cell walls of plants, especially in tracheids, xylem fibres and sclereids. It is the second most abundant organic compound on earth after cellulose. Lignin makes up about one-quarter to one-third of the dry mass of wood.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose and pectin components. It confers mechanical strength to the cell wall and therefore the entire plant. It is particularly abundant in compression wood, but curiously scarce in tension wood.
Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water. Lignin makes it possible to form vessels which conduct water efficiently.
Lignin is difficult to degrade and is therefore an efficient physical barrier against pathogens which would invade plant tissues. For example an infection by a fungus causes the plant to deposit more lignin near the infection site.
Highly lignified wood is durable and therefore a good raw material for many applications. It is also an excellent fuel, since lignin yields more energy when burned than cellulose. However, lignin is detrimental to paper manufacture and must be removed from pulp before paper can be manufactured. This is costly both in terms of energy and environment.
In the sulfite and sulfate (also called kraft) chemical pulping processes, lignin is removed from wood pulp as sulphates. These materials have several uses:
• Dispersants in high performance cement applications, water treatment formulations and textile dyes
• Additives in specialty oil field applications and agricultural chemicals
• Raw materials for several chemicals, such as vanillin, DMSO, ethanol, torula yeast, xylitol sugar and humic acid
• Environmentally friendly dust supression agent for roads
The first investigations into commercial use of lignin were done by Marathon Corporation in Rothschild, Wisconsin (USA), starting in 1927. The first class of products which showed promise were leather tanning agents. The lignin chemical business of Marathon is now known as LignoTech USA, Inc., and is owned by the Norwegian company, Borregaard.
Lignin is a large macromolecule with molecular mass in excess of 10,000 amu. It is hydrophobic and aromatic in nature. The molecule consists of various types of substructures which repeat in random manner.
Structure of a small piece of lignin polymer
Lignin biosynthesis begins with the synthesis of monolignols. The starting material is the amino acid phenylalanine. The first reactions in the biosynthesis are shared with the phenylpropanoid pathway, and monolignols are considered to be a part of this group of compounds. There are three types of monolignols: coniferyl alcohol, sinapyl alcohol and paracoumaryl alcohol. Different plants use different monolignols. For example, Norway spruce lignin is almost entirely coniferyl alcohol while paracoumaryl alcohol is found almost exclusively in grasses.
Structures of the three commonly occurring monolignols
Monolignols are synthetised in the cytosol as glucosides. The glucose is added to the monolignol to make them water soluble and to reduce their toxicity. The glucosides are transported through the cell membrane to the apoplast. The glucose is then removed and the monolignols are polymerised into lignin.
Polymerisation of coniferyl alcohol to lignin. The reaction has two alternative routes catalysed by two different oxidative enzymes, peroxidases or oxidases.
The polymerisation step 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. The oxidative enzyme catalyses the formation of monolignol radicals. These radicals then undergo chemical coupling to form the lignin polymer. The details of this final step are being debated, since it is not known how the abundance of various possible bond types between monolignols in controlled. Some theories favour pure chemical coupling, while other state that dirigent proteins control this step.
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