Fig. 1.

Major backbone units and representative linkages in lignin molecule. a The building blocks of lignin consist of three primary types of monolignols, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The alcohols form the corresponding phenylpropanoid units like p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) in lignin polymer, respectively. b Backbone units are conjugated via different chemical bonds (e.g., β-O-4, β-β, 5–5, and β-5) resulting in high resistance to lignin depolymerization