Lignin is composed of phenylpropane (C9) units that are linked together by carbon-to- carbon as well as carbon-to-oxygen (ether) bonds. Our present knowledge of the lignin
Figure 1 Phenyl propanoid units of lignin: R, R2 = H, OCH3; R3 = H, CH3, CH2; (f) = possible linkage to other phenyl propanoid units.
structure is based on the assumption that it is formed from p-hydroxycinnamyl alcohols by oxidative coupling [2,3] oxidized by hydrogen peroxide and peroxidase to a phenoxy radical (R). The unpaired electron in R is delocalized and reacts at three different sites of the radical, indicated by four resonance structures, leading to dilignols.