**2. Lignin structure and composition in biomass**

### **2.1 What is lignin?**

Lignin is a complex polymer present in vascular plants, particularly in woody tissues. It is the second most abundant natural polymer, representing 15–30% dry weight of lignocellulosic materials (e.g., wood and bark). Its natural production on earth ranges between 5 and 36 � <sup>10</sup><sup>8</sup> tons annually [1]. Lignin is mainly deposited in the secondary cell walls of the supporting and water-conducting tissues, but it is also present in the middle lamella between cells and cell corners. Besides xylem cells, lignification can also occur in sclereids cells (phloem cells), roots, and seeds [2].

### **2.2 What is lignin made of?**

Lignin is a heterogeneous aromatic polymer constituted by three phenylpropane precursors: *p*-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol that present a hydroxyl group linked to C4 and substitutions with one or two methoxyl groups in C3 and C5. If the aromatic ring is unmethoxylated, then it is called *p*-hydroxyphenyl (H), and if it has one methoxyl group in C3, it is called guaiacyl, or with two methoxyl groups linked with C3 and C5, it is a syringyl unit (S, **Figure 4**) [3]. Therefore, lignin is a highly complex phenolic polymer whose content and composition vary depending on many factors, for example, species, specimen age, plant part, environmental growth conditions, and harvesting time [2–7]. In hardwoods, lignin typically represents between 20 and 25% of the wood, with a monomeric composition mainly of syringyl and guaiacyl but few *p*-hydroxyphenyl units. In comparison, softwoods are richer in lignin (25–35%), where guaiacyl units prevail, fewer *p*-hydroxyphenyl units are found, and almost no syringyl units are present [3]. In herbaceous species, the lignin content ranges from 15 to 24% [8] and has a monomeric composition where no specific lignin units predominate [3]. It was found that other monomers are also included in the lignin polymer, such as tricin flavones, hydroxycinnamaldehydes, hydroxystilbenes, and xenobiotics [9–15]. The monomers are linked by ether bonds and carbon-to-carbon bonds in different proportions. The ether bonds prevail (e.g., βdOd4<sup>0</sup> and αdOd4<sup>0</sup> ), representing between 35 and 60% in softwoods and from 50 to 80% in hardwoods [16]. Generally, CdO linkages have a bond-dissociation

#### **Figure 4.**

*Lignin monomeric composition distributed by softwood, hardwood, and herbaceous. The new lignin building block: 1—coumaric acid, 2—ferulic acid, 3—sinapic acid, if R*]*H; 4—resveratrol (hydroxstilbene).*

enthalpy (BDE) of 42–83 kcal/mol (they are easier to break); while CdC linkages (e.g. βdβ<sup>0</sup> , βd5<sup>0</sup> ) have BDE values between 54 and 118 kcal/mol, so they are harder to break down, presenting more considerable challenges for lignin deconstruction [17]. Slightly different values were compiled by Liu et al. [18] and are presented in **Table 1**.


#### **Table 1.**

*General characteristics of lignin from softwood, hardwood, and herbaceous species. Adapted from Liu et al. [18].*
