ligninei

Ligninei, commonly known in English as lignin, are complex polyphenolic polymers that form a major part of the secondary cell walls in vascular plants. They fill the spaces between cellulose, hemicellulose, and other polymers, contributing to the rigidity, hydrophobicity, and overall strength of plant tissues. Lignin is particularly abundant in wood and bark, where it helps confer resistance to mechanical and microbial attack.

Chemically, lignin is an irregular, three-dimensional network derived from phenylpropanoid units. The primary building blocks are hydroxycinnamyl alcohols, mainly p-coumaryl, coniferyl, and sinapyl alcohols, which generate p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units after polymerization. The resulting polymer exhibits diverse linkages (such as C–O–C and C–C bonds) and shows substantial variability among species, tissues, and developmental stages. This structural diversity makes lignin highly resistant to degradation.

Biosynthesis occurs in the plant’s phenylpropanoid pathway, starting from phenylalanine (or tyrosine). Key enzymes include phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate–CoA ligase (4CL), which channel carbon into hydroxycinnamyl alcohols. Polymerization in the cell wall is driven by oxidative enzymes such as peroxidases and laccases, forming the cross-linked lignin network that integrates with cellulose and hemicellulose.

Industrial relevance centers on lignin’s abundance and recalcitrance. It complicates biomass processing for biofuels and pulping, but valorization strategies aim to convert lignin into value-added chemicals, carbon fibers, or precursors for polymers. Variants include native lignin, kraft lignin, and lignosulfonates, which differ in structure and reactivity.