Lignines

Lignines are complex, high-molecular-weight phenolic polymers found predominantly in the secondary cell walls of vascular plants. They are formed by oxidative radical coupling of three primary monolignols—p-coumaryl, coniferyl, and sinapyl alcohols—resulting in a heterogeneous, irregular network of carbon–carbon and ether linkages. The composition and relative abundance of these monomers vary by species and tissue type, with gymnosperms rich in guaiacyl units, angiosperms containing both guaiacyl and syringyl units, and many grasses incorporating hydroxycinnamates.

Functionally, lignines confer mechanical strength, rigidity, and hydrophobicity to cell walls, facilitating upright growth and efficient water transport through xylem vessels. They also act as a barrier against pathogens and pests by limiting microbial access to carbohydrates. Lignin biosynthesis proceeds via the phenylpropanoid pathway, and polymerization is catalyzed extracellularly by peroxidases and laccases.

Industrial and biotechnological interest in lignines stems from its abundance and recalcitrance. While lignin is a major impediment in pulping and lignocellulosic biofuel production, it is also a potential renewable feedstock for chemicals, adhesives, and carbon materials. Microbial degradation is principally carried out by white-rot fungi through lignin-modifying enzymes (lignin peroxidase, manganese peroxidase, laccase), with bacteria playing a lesser role. Analytical methods for lignin characterization include Klason lignin determination, thioacidolysis, and NMR spectroscopy. Research continues on methods to valorize lignin and to modify its biosynthesis for improved biomass processing.