**2.3 Lignin**

Lignin is different than the hemicellulose and cellulose because it comprise of aromatic heterogeneous polymer phenyl-propane units. The long phenyl-propane units are attached togather by ether bonds. The main function of lignin is to strongly drag hemicellulose and cellulose in complex structure of lignin like a glue to seal each gap in plant biomass. Therefore, this lignin make it harder for the degradation. The concentration of lignin is depending on the size, growth, nature and properties of plants biomass [24]. It is soluble in varios solvents, dioxane, pyridine, dimethyl sulfoxide, acetone, and in high concentration of acidic solution at high temperature. Notably, the lignin is easily soluble in alkaline solutions at both low and high temperature [25]. In a general thermal temperature heating of range 150–300°C degrade hemicellulose, 315–400°C degrade cellulose and 250–500°C degrade lignin from plant biomass. In lignin structure, three aromatic phenolic components coniferyl (G), p-coumaryl (H), and sinapyl (S) are the major monomers (**Figure 3**).

## **Figure 3.**

*Three main components in lignin structure.*

Generally phenolic hydroxyl, methoxyl, and terminal aldehyde groups are present in the lignin. Softwoods lignin comprise of guaiacyl units, while in hardwoods the lignin contain guaiacyl-syringyl, whereas *p*-coumaryl alcohol units are found in Graminaceous lignin. In lignin synthesis, alcoholic hydroxyl and carbonyl groups are

**Figure 4.** *Basic phenylpropane linkages in lignin structure.*

*Composition and Role of Lignin in Biochemicals DOI: http://dx.doi.org/10.5772/intechopen.106527*

assimilated into the structure of lignin during enzymatic dehydrogenation. Lignin structure has some mutual functional groups and common linkages between the aromatic phenolic components. Few of them are, β-O-4, α-O-4, dibenzodioxocin, β-5, 5–5, 4-O-5, β-β, and β-1 linkages. The density of the lignin structure is from the monomers ratio, compositions pattern and due to a number of linkages options between lignin monomers. Some of the common bonds arrange in carbon-carbon bonds (β-1, β- β, 5–5′) carbon-oxygen bonds (α-O-4, β-O-4, and 4-O-5), carbonoxygen and carbon-carbon bonds (β-5′/ α-O-4, β- β´/α-O-γ). One the prominent 60% of total is ether β-O-4 linkage present in the polymer (**Figure 4**) [26].
