title: Radical coupling reactions in lignin synthesis: a density functional theory study.
abstract: 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.
authors.affiliation: UT/ORNL Center for Molecular Biophysics, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831-6309, United States.
authors.forename: Amandeep K
authors.initials: AK
authors.lastname: Sangha
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Parks
authors.forename: Robert F
authors.initials: RF
authors.lastname: Standaert
authors.forename: Angela
authors.initials: A
authors.lastname: Ziebell
authors.forename: Mark
authors.initials: M
authors.lastname: Davis
authors.forename: Jeremy C
authors.initials: JC
authors.lastname: Smith
mesh_terms: D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D008031:Lignin / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D011789:Quantum Theory
chemical_list: D005609:Free Radicals; D008031:Lignin