## 2 Structural Complexity and Composition of Native Lignin  
L Lignin, a vital structural component of vascular plant cell walls, provides rigidity, hydrophobicity, and microbial resistance through its complex biopolymer structure [15]. It is formed via the oxidative polymerization of three monolignol precursors: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which give rise to the three primary lignin units p-hydroxyphenyl (H), guiaacyl (G), and syringyl (S) [16]. Lignin, found in lignocellulosic biomass, commonly referred to as native lignin, is a highly complex polymer composed of interlinked units derived from methylated hydroxycinnamyl alcohols, collectively known as monolignols. The three principal monolignols are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, each incorporated into native lignin in species-dependent proportions. The relative abundance of these monolignols varies across plant types. In softwoods, lignin is predominantly composed of coniferyl alcohol units (\(\approx\)95%), with minor contributions from p-coumaryl (\(\approx\)4%) and sinapyl alcohol units (\(\approx\)1%). In contrast, hardwood lignin typically exhibits a more balanced composition of coniferyl and sinapyl alcohol units, with only trace amounts of p-coumaryl alcohol (Figure 2). The distribution of these units varies significantly across plant species, tissue types, and cell wall layers, with softwoods predominantly containing G units, hardwoods a mix of G and S units, and grasses all three units in varying proportions [17]. This structural diversity is further amplified by the presence of various interunit linkages, with \(\beta\)-O-4 ether bonds comprising 50-60% of the total bonds, alongside carbon-carbon bonds such as \(\beta\)-5 (phenylcoumaran), \(\beta\)-\(\beta\) (resinol), and 5-5 (biphenyl) [18]. The random coupling of these linkages results in a highly complex, three-dimensional structure that distinguishes lignin from other biopolymers like cellulose and hemicellulose [19]. The structural complexity of lignin, characterized by its diverse functional groups hydroxyl, metboxyl, carbonyl, and carboxyl, poses both opportunities and challenges for its industrial utilization [20]. While this complexity is essential for lignin's biological functions, it creates significant barriers for its conversion into high-value products such as biofuels, aromatic chemicals, and biopolymers [21]. The recalcitrance of lignin, driven by its irregular network and steric hindrance, complicates depolymerization processes, particularly the cleavage of \(\beta\)-O-4 ether bonds and more stable carbon-carbon linkages [22]. Additionally, the heterogeneity of lignin, which varies not only between species but also within individual molecules, further complicates its processing. For example, softwood lignin, rich in G units and carbon-carbon linkages, is more resistant to degradation than hardwood lignin, which contains higher proportions of S units and \(\beta\)-O-4 linkages [23]. Lignin's renewable nature and unique properties make it a promising feedstock for carbon fibers, adhesives, and composite materials. However, its molecular weight, functional group composition, and solubility significantly influence the performance of lignin-derived materials, necessitating precise extraction and processing methods [24].