Ligniinit

Ligniinit are a group of complex phenolic polymers that form a major component of plant secondary cell walls, especially in wood. They create a heterogeneous, cross‑linked network that embeds cellulose and hemicellulose, contributing to the mechanical strength and hydrophobic character of plant tissue. Ligniinit are produced by oxidative coupling of monolignols—primarily coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol—driven by enzymes such as peroxidases and laccases in the cell wall. The resulting polymer is diverse in composition and structure, containing guaiacyl (G), syringyl (S), and hydroxyphenyl (H) units. The relative amounts of these units vary by species, tissue, and developmental stage, influencing properties like rigidity and degradability. Cross-links between lignin chains and with polysaccharides strengthen the matrix but also hinder enzymatic breakdown.

Role and occurrence in plants: ligniinit are widely distributed in vascular plants and are particularly abundant in wood. Hardwood species tend to have higher S content, while softwoods are richer in G units. Lignification typically occurs during the formation of the secondary cell wall, contributing to water transport efficiency, pathogen defense, and overall structural integrity.

Industrial relevance and processing: ligniinit are relatively resistant to chemical and enzymatic degradation, a major source of recalcitrance in converting lignocellulosic biomass to fuels and chemicals. In pulp and paper production, lignin is removed or altered during pulping; in some processes, it is recovered as a separate product. Lignin is also explored as a renewable feedstock for phenolic chemicals, polymers, carbon fibers, and other materials. Analytical methods such as thioacidolysis and NMR are used to characterize lignin composition and linkages, supporting efforts to tailor lignin structure for improved processing and valorization.