Lignin-Derived and Associated Polyphenolic Compounds in Woody Plants: Structural Elucidation, Synthetic Pathways, Characterization Techniques, and Industrial Bioactivities

Abstract

Lignin is the second most abundant natural biopolymer on Earth, functioning as a primary structural matrix in the secondary cell walls of woody plants. Composed of complex, cross-linked, hydroxylated, and methoxylated phenylpropanoid units, lignin and its associated polyphenols possess unique structural flexibility, UV-shielding, radical scavenging, and antimicrobial properties. Historically managed as a low-value byproduct of the pulp and paper industry, recent biorefinery innovations have repositioned lignin as a crucial sustainable precursor for high-value biomaterials, green composite fillers, and targeted biomedical carriers. This review provides a rigorous, critical synthesis of lignin chemistry, detailing monolignol biosyntheses (p-coumaryl, coniferyl, and sinapyl alcohols), radical coupling polymerization mechanisms, and taxonomic variations across gymnosperm softwoods, angiosperm hardwoods, and woody bamboos. Modern structural characterization methodologies including 2D 1H-13C HSQC NMR, 31P NMR, Py-GC/MS, SEC/GPC, and FTICR-MS are comprehensively evaluated alongside commercial extraction protocols (kraft, organosolv, lignosulfonate, and soda processes). Furthermore, quantitative performance metrics across antioxidant (DPPH IC50), UV-blocking (SPF values), antimicrobial (MIC), and drug-delivery (encapsulation efficiency) domains are critically synthesized. Technical limitations regarding structural heterogeneity, condensation reactions, sulfur contamination, commercial scalability, life cycle assessments (LCA), and regulatory hurdles for biomedical translation are thoroughly examined. This work establishes an integrated framework bridging taxonomic lignin chemistry with industrial valorization and sustainable bioeconomy strategy.