Ligniini

Ligniini, known in English as lignin, is a heterogeneous, aromatic biopolymer that reinforces the secondary cell walls of vascular plants. It is one of the most abundant natural polymers, typically comprising about 20–30% of dry wood. Its presence provides mechanical strength, hydrophobicity, and resistance to microbial decay. The relative composition varies among plants, with hardwood lignins generally richer in syringyl units and softwood lignins in guaiacyl units.

Lignin is formed by radical coupling of monolignols derived from the phenylpropanoid pathway, mainly p-coumaryl, coniferyl, and sinapyl alcohols, yielding H, G, and S units. The resulting network is highly cross-linked, with linkages such as beta-O-4, beta-5, and beta-beta, creating a three-dimensional matrix that intertwines with cellulose and hemicellulose in the wall.

Functions include providing structural support, reducing porosity, and aiding water transport and defense against pathogens. Lignification occurs mainly in the secondary cell walls of xylem cells and fibers, influencing wood density and mechanical properties.

Industrially, lignin is a major byproduct of pulping and biorefining. Kraft and sulfite processes yield lignin in various forms (kraft lignin, lignosulfonates), while organosolv methods can produce relatively clean lignin suitable for valorization. Lignin is investigated as a source of phenolic compounds, carbon materials, and bio-based chemicals, although its complex structure poses processing challenges.

Ecologically, lignin is moderately recalcitrant to degradation; white-rot fungi and some bacteria can oxidize it using enzymes such as lignin peroxidases, manganese peroxidases, and laccases, contributing to soil formation and the global carbon cycle. Technical lignins differ in structure and reactivity based on their extraction method.