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Radical coupling reactions in lignin synthesis: A density functional theory study

journal contribution
posted on 2012-04-26, 00:00 authored by A K Sangha, J M Parks, R F Standaert, Angela ZiebellAngela Ziebell, M Davis, J C Smith
Lignin is a complex, heterogeneous polymer in plant cell walls that provides mechanical strength to the plant stem and confers resistance to degrading microbes, enzymes, and chemicals. Lignin synthesis initiates through oxidative radical-radical coupling of monolignols, the most common of which are p-coumaryl, coniferyl, and sinapyl alcohols. Here, we use density functional theory to characterize radical-radical coupling reactions involved in monolignol dimerization. We compute reaction enthalpies for the initial self- and cross-coupling reactions of these monolignol radicals to form dimeric intermediates via six major linkages observed in natural lignin. The 8-O-4, 8-8, and 8-5 coupling are computed to be the most favorable, whereas the 5-O-4, 5-5, and 8-1 linkages are less favorable. Overall, p-coumaryl self- and cross-coupling reactions are calculated to be the most favorable. For cross-coupling reactions, in which each radical can couple via either of the two sites involved in dimer formation, the more reactive of the two radicals is found to undergo coupling at its site with the highest spin density. © 2012 American Chemical Society.

History

Journal

Journal of Physical Chemistry B

Volume

116

Issue

16

Pagination

4760 - 4768

ISSN

1520-6106

eISSN

1520-5207