## 2 Structure of Lignin for Valuable Aromatic Compounds  
Lignin is one of the main components consisting of a plant biomass. Lignin has a unique structure of closely associated cellulose and hemicellulose, which resists the bio-degradation of plant cell walls to protect plants against environmental stress [19; 20; 21; 22]. Lignin is a heterogeneous polymer containing various aromatic structures consisting of phenylpropane subunits, linked with carbon-carbon (C-C) or ether (C-O-C) into a complex three-dimensional structure [20; 21; 22]. The complex structure of lignin is initially biosynthesized from an aromatic amino acid phenylalanine or tyrosine. Then, the aromatic amino acid is converted into \(p\)-coumaryl alcohol via the phenylpropanol pathway following deamination, ring hydroxylation, methylation, and carboxyl reduction. Following that, \(p\)-coumaryl alcohol (H-unit) is elaborated to coniferyl and sinapyl alcohols (G- and S-units).  
These phytochemical substances of the three alcohol types are called monolignols or lignols, which are the main building blocks of lignin structures [23; 24; 25] (Figure 1).  
Interestingly, the biochemical synthesis of lignin is a unique polymerization through chemoenzymatic reactions, in which oxidases and peroxididases generate reactive quinone methide intermediates as lignin precursors from hydroxylated monolignols, \(p\)-hydroxyphenyl, guaiacyl, and a syringyl unit, and then these compounds are spontaneously polymerized [23; 24; 25]. Lignin polymers have been assumed as non-optically active chemicals since their monomers do not contain chiral structures. Nevertheless, the subunit structures of phenylpropanol have two potential asymmetric carbons at the \(\alpha\)- and \(\beta\)-positions of the side-chain moiety [23; 26]. These two chiral centers generate structural variety of lignin in the monomer condensation process. In the polymerization reaction, benzylic hydroxyl groups in the \(\beta\)-aryl ether structural units in monolignols are lost, forming benzylic cation [26]. One of them is formed at the \(\beta\)-position by a \(\beta\)-O-4, \(\beta\)-\(\beta\), \(\beta\)-5, or \(\beta\)-1 coupling reaction between new C\({}_{\alpha}\)-C\({}_{\beta}\) cross-linkage bonds from monolignols or the phenolic end of the growing polymer [23; 26; 27]. This polymerization of the precursors randomly occurs to form lignin complex structures. This reduces the enzyme accessibility and solubility for biodegradation, compared with the uniform structures of other cellulose and hemicellulose polymers in plant cell walls.  
Although lignin, cellulose, and hemicellulose contents vary across plant species, plant cell walls usually contain around 20-35% lignin by weight [22; 25]. Comparing lignin subunits in soft and hard woods, soft wood plants contain higher guaiacyl content and lower \(p\)-hydroxyphenyl unit content in their lignin structures [28]. Hardwood lignin primarily consists of guaiacyl and syringyl units in various proportions, together with minor levels of \(p\)-hydroxyphenyl units [28]. These different subunits of lignin contribute to differences in their physical properties.  
Figure 1: Schematic representation of biosynthesis for three main monolignol subunit structures for lignin using two aromatic amino acid precursors. Moss green, green, and Prussian blue indicate the biosynthetic pathways for the H-, G-, and S-unit, respectively.