Lignin

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors. 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 from solution using acid. He named the substance "lignine", which is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose and chitin. Lignin constitutes 30% of terrestrial non-fossil organic carbon on Earth, and 20 to 35% of the dry mass of wood. Lignin is present in 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, where it supports joints between calcified segments. Lignin is a collection of highly heterogeneous polymers derived from a handful of precursor lignols. Heterogeneity arises from the diversity and degree of crosslinking between these lignols. The lignols that crosslink are of three main types, all derived from phenylpropane: coniferyl alcohol (4-hydroxy-3-methoxyphenylpropane) (G, its radical is sometimes called guaiacyl), sinapyl alcohol (3,5-dimethoxy-4-hydroxyphenylpropane) (S, its radical is sometimes called syringyl), and paracoumaryl alcohol (4-hydroxyphenylpropane) (H, its radical is sometimes called 4-hydroxyphenyl). The relative amounts of the precursor "monomers" (lignols or monolignols) vary according to the plant source. Lignins are typically classified according to their syringyl/guaiacyl (S/G) ratio. Lignin from gymnosperms (softwoods, grasses) is derived from the coniferyl alcohol, which gives rise to G upon pyrolysis.