ABSTRACT
Lignin is a complex phenolic heteropolymer that imparts strength, rigidity, and hydrophobicity to plant secondary cell wall. Found in all vascular plants, particularly within the woody tissues, lignin makes up a substantial fraction of the total organic carbon in the biosphere and is exceeded in abundance only by cellulose.
The evolution of lignin and other hydrophobic polymers some 400 million years ago was one of several critical developments in the colonization of terrestrial environments by vascular plants, allowing them to support increasingly large aerial structures, to supply these structures with water and to protect them against desiccation
Lignin is the generic term for a large group of aromatic polymers resulting from the oxidative combinatorial coupling of 4-hydroxyphenylpropanoids (monolignols). It is the only naturally synthesized polymer with an aromatic backbone. Monolignol structures differ in the number of methoxy groups attached to the aromatic ring: p-coumaryl alcohol has no methoxy, coniferyl alcohol has one and sinapyl alcohol has two. The units in the lignin polymer are linked by a variety of chemical bonds that have different chemical properties.
Lignin polymerization occurs via oxidative radicalization of phenols, followed by combinatorial radical coupling (without enzymatic activity). In the first step, the monolignol phenol is oxidized, that is, dehydrogenated. The resulting phenolic radical is relatively stable due to delocalization of the unpaired electron in the conjugated system. Subsequently, two monomer radicals may couple to form a dimer. This radical-radical coupling occurs in a chemical-combinatorial fashion; thus, the ratio of each of the possible coupling products depends largely on the chemical nature of each of the monomers and the conditions in the cell wall.
