title: Radical formation and coupling of hydroxycinnamic acids containing 1,2-dihydroxy substituents.
abstract: Hydroxycinnamic acids involved in the deposition and cross-linking of plant cell-wall polymers do not usually contain 1,2-dihydroxy substituents, despite the presence of both 3,4-dihydroxycinnamic acid and 4,5-dihydroxy-3-methoxycinnamic acid as intermediates in the biogenesis of lignin. Since the O-methyl transferases, enzymes catalysing methylation, are targets for the genetic manipulation of lignin biosynthesis, the potential incorporation of these 1,2-dihydroxated substrates becomes increasingly significant. Using EPR spectroscopy, it was observed that 1,2-dihydroxy substituents did not have an inhibitory effect on radical formation. Increasing the extent of hydroxylation and methoxylation, resulted in an increased ease of substrate oxidation. Despite formation of the parent radicals, coupling did not proceed, under conditions that generally result in phenylpropanoid polymerisation. It is postulated that intermolecular radical-coupling reactions are inhibited due to rapid conversion to the o-quinone. In contrast, when methoxylated at C3, as in 4,5-dihydroxy-3-methoxycinnamic acid, radical coupling proceeds with the major product resulting from 8-O-3 radical coupling and formation of a substituted 2,3-dihydro-1,4-dioxin ring.
authors.affiliation: Rowett Research Institute, Greenburn Road, Bucksburn, AB21 9SB Aberdeen, UK. wrr@rri.sari.ac.uk
authors.forename: Wendy R
authors.initials: WR
authors.lastname: Russell
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Burkitt
authors.forename: Lorraine
authors.initials: L
authors.lastname: Scobbie
authors.forename: Andrew
authors.initials: A
authors.lastname: Chesson
mesh_terms: D003373:Coumaric Acids / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000138:chemical synthesis*; D006861:Hydrogen Peroxide / Q000737:chemistry; D006900:Hydroxylation; D015394:Molecular Structure; D009195:Peroxidase / Q000378:metabolism; D015032:Zinc / Q000737:chemistry
chemical_list: D003373:Coumaric Acids; D005609:Free Radicals; D006861:Hydrogen Peroxide; D009195:Peroxidase; D015032:Zinc