Hydroxycinnamic acids such as ferulic acid and p-coumaric acid acylated polysaccharides forming ferulate or p-coumarate esters in grass cell walls. Radical coupling of ferulate catalyzed by peroxidase produces dimers, trimers resulting in cross-linking of polysaccharides. Meanwhile cross-couplings of ferulate and lignin monomers or oligomers through free radical coupling reactions produce so-called lignin-hydroxycinnamate-carbohydrate complexes. Such cross-linkings have a significant role in limiting cell wall degradability of grasses. It has long been our endeavor to understand the nature and scope of hydroxycinnamte-polysaccharide-lignin interactions to provide a basis for genetic modification of plants aimed at improved utilization of plant cell walls. Two distinct mechanisms have been proposed to be responsible for ferulate or p-coumarate being incorporated to lignin: a). Nucleophilic addition of the ferulate/p-coumarate phenol to quinone methide intermediates, produced during lignification via the coupling of a monolignol with a growing lignin oligomer, results in lignin a-aryl ethers; b). Cross-coupling of the ferulate phenolic radical and a monolignol radical could produce a variety of crossed dimers that becomes part of the lignin macromolecule through the normal lignification pathway. In this study, series of model compounds relevant to the potential chemical structures involved in incorporation of ferulate or p-coumarate into lignins were synthesized, and lignin samples were isolated from corn stem, sugarcane bagasse and characterized by NMR. New evidence for ferulate incorporation into lignin through radical coupling reactions as well as quinone methide addition mechanisms has been found suggesting that p-coumarate etherified to lignin via Nucleophilic addition pathway whereas there are two pathways for ferulate incorporation into lignin via  in grass plants.

Incorporation of hydroxycinnamates into lignin in grass cell walls: NMR evidence for pathways

247th National Meeting (2014)

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