Oxidative Coupling during Lignin Polymerization Is Determined by Unpaired Electron Delocalization within Parent Phenylpropanoid Radicals

The high degree of selectivity observed in the incorporation of phenylpropanoids into lignin may be a consequence of the influence exerted by methoxyl substituents on the ambident radicals generated during biosynthesis. Since unpaired electron distribution may be regarded as an important factor in determining positional selectivity during oxidative coupling, electron spin resonance spectroscopy and Austin Model 1 molecular computation were used to study the effects of methoxyl substitution on unpaired electron distribution in lignin precursor radicals. The data obtained were used to predict the effect of substitution on coupling and were compared with the linkage types observed in complementary dehydrogenation polymerization studies employing each of the lignin precursors. We report that methoxyl substitution increases unpaired electron density on the phenolic oxygen of the precursor phenylpropanoid radicals and that this subsequently determines the nature of the bond formation during polymerization.