Lignin and its structure

Lignin is a complex class of organic polymers that form the key structural material in the support tissues of most plants.Lignin is of particular importance in the formation of cell walls,especially in wood and bark,because they are rigid and less prone to decay. Chemically, lignin is a polymer made by crosslinking phenolic precursors.

Structure 

Lignin is a collection of highly heterogeneous polymers derived from a small number of precursor lignin alcohols.The heterogeneity stems from the variety and degree of cross-linking between these lignin alcohols.Cross-linked lignin alcohols fall into three main types, all derived from phenylpropane:coniferyl alcohol (4-hydroxy-3-methoxyphenylpropane) (G, whose group is sometimes called guaiacyl), mustard Alcohol (3,5-dimethoxy-4-hydroxyphenylpropane) (S,whose group is sometimes called syringyl) and p-coumaryl alcohol (4-hydroxyphenylpropane) (H,whose group is sometimes known as 4-hydroxyphenyl).The relative amount of the precursor "monomer" (lignol) varies depending on the plant source.Lignins are generally classified according to their syringyl/guaiacoyl ratio. Lignin from gymnosperms (softwoods,grasses) is derived from coniferyl alcohol,which yields guaiacol upon pyrolysis.In angiosperms (hardwoods),some coniferyl alcohol is converted to sinapyl alcohol.Thus,lignin in angiosperms has both guaiac-based and syringyl-based components.The molecular weight of lignin exceeds 10,000 u.It is hydrophobic because it is rich in aromatic subunits.The degree of polymerization is difficult to measure because the material is heterogeneous.Different types of lignin have been described according to the isolation methods.Many grasses are G-dominated,while some palms are S-dominated.All lignins contain small amounts of incomplete or modified lignin monomers,other monomers are prominent in nonwoody plants.

Biological function

Lignin fills the cell wall spaces between cellulose,hemicellulose and pectin components,especially in vascular and supporting tissues: xylem tracheids,vascular elements and sclerocytes.Lignin plays a vital role in channeling water and aqueous nutrients in plant stems. The polysaccharide constituents of plant cell walls are highly hydrophilic and therefore permeable to water,whereas lignin is more hydrophobic.The cross-linking of polysaccharides by lignin is an obstacle to water uptake by cell walls.Thus, lignin enables the plant's vascular tissue to efficiently conduct water.Lignin is present in all vascular plants but not in bryophytes,supporting the idea that lignin's original function was limited to water transport.It is covalently linked to hemicellulose, thus cross-linking different plant polysaccharides,imparting mechanical strength to cell walls,and by extending the plant as a whole.Its most common function is to provide support by strengthening wood in vascular plants,consisting mainly of xylem cells and lignified sclerenchyma fibers.Finally, lignin also confers disease resistance by accumulating at pathogen penetration sites,making plant cells less susceptible to cell wall degradation.

Economic significance

The global commercial production of lignin is a consequence of papermaking.In 1988, more than 220 million tons of paper were produced worldwide.Most of this paper is delignified; lignin makes up about 1/3 of the mass of lignocellulose.Lignin is an impediment to papermaking because it is colored,turns yellow in air, and its presence weakens.Once separated from the cellulose,it is burned as fuel.Only a small fraction is used in a wide range of low-volume applications where form rather than quality is important.The mechanical or high-yield pulp used to make newsprint still contains most of the lignin that was originally present in the wood.This lignin is what causes newsprint to yellow over time.High-quality paper requires the removal of lignin from the pulp.These delignification processes are a core technology in the paper industry and a source of major environmental problems.In sulfite pulping, lignin is removed from wood pulp as lignosulfonate, for which many applications have been proposed.They are used as dispersants, humectants,emulsion stabilizers and sequestrants (water treatment).Lignosulfonates were also the first family of water reducers or superplasticizers added to fresh concrete as admixtures in the 1930s to reduce the water-cement ratio,which is the main parameter controlling the porosity of concrete,Hence its mechanical strength,diffusivity and hydraulic conductivity, all parameters necessary for its durability.It has applications in environmentally sustainable road dust suppressants.Furthermore,lignin can be used together with cellulose to create biodegradable plastics as an alternative to hydrocarbon plastics if lignin extraction is achieved through a more environmentally friendly process than common plastic manufacturing.The lignin removed by the kraft process is usually burned for its fuel value, providing energy to the paper mill.Two commercial processes exist for removing lignin from black liquor for higher value uses:LignoBoost (Sweden) and LignoForce (Canada).Higher-quality lignin has the potential to become a renewable source of aromatic compounds in the chemical industry, with a potential market of more than $130 billion.Given that it is the most prevalent biopolymer after cellulose,lignin has been studied as a feedstock for biofuel production and could be an important plant extract for the development of novel biofuels.