Figure 1.

Structure of carboxylated lignins CD and FD obtained from horseradish peroxidase-catalyzed oxidative coupling of caffeic and ferulic acid, respectively. Carbons designations (α, β, 4, 5 and others), unique to the lignin field, are as shown. X can be either –H or –OH. Oxidative coupling through radical intermediates give rises to oligomers with different types of inter-monomeric linkages. The most common linkages formed include β-O-4 and β-5. Other less common linkages include β-β, 5-5 and 5-O-4, for which oligomerization tends to arrest chain growth.⁵,⁶ The length of the chain greatly depends on the conditions of oligomerization.