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 phenyl propanoids into lignin may be a consequence of the influence exerted b y methoxyl substituents on the ambident radicals generated during bios ynthesis. Since unpaired electron distribution may be regarded as an i mportant factor in determining positional selectivity during oxidative coupling, electron spin resonance spectroscopy and Austin Model 1 mol ecular computation were used to study the effects of methoxyl substitu tion on unpaired electron distribution in lignin precursor radicals. T he data obtained were used to predict the effect of substitution on co upling and were compared with the linkage types observed in complement ary dehydrogenation polymerization studies employing each of the ligni n precursors. We report that methoxyl substitution increases unpaired electron density on the phenolic oxygen of the precursor phenylpropano id radicals and that this subsequently determines the nature of the bo nd formation during polymerization. (C) 1996 Academic Press, Inc.