Lignins are the most abundant naturally occurring aromatic polymers on earth. Traditionally they are regarded as complex heteropolymers formed from hydroxycinnamyl alcohols, the monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The resulting lignin therefore contains p-hydroxyphenyl (H), guaiacyl (G), and syringyl (s) units. Very recently, tricin (3',5'-dimethoxy, 5,7,4'-trihydroxy flavone), was surprisingly identified in the lignin from wheat straw, and subsequently all monocots examined, including coconut coir fiber, bamboo, maize, and stems of Arundo donax Linn. To provide solid evidence for the incorporation of tricin into lignins, a series of tricin-monolignol oligomers was synthesized and used to provide data for Nuclear Magnetic Resonance characterization and authentication, and for identifying grass metabolites by Mass Spectrometry. Biomimetic radical coupling reactions, using peroxidase or silver (I) oxide as oxidant demonstrated that tricin was capable of cross-coupling with monolignols to form tricin-(4-O-�)-monolignol cross-coupled dimers.  Preliminary analysis of tricin-containing metabolites from corn suggests that coniferyl alcohol, sinapyl alcohol, coniferyl acetate, coniferyl p-coumarate, and sinapyl p-coumarate cross-couple with tricin in vivo. We also found evidence for a trimer [tricin-(4-O-�)-(sinapyl alcohol)- 4-O-�-(coniferyl alcohol)], suggesting that endwise polymerization is occurring, and that 'lignin' chains starting from tricin are involved in grass lignification. These findings suggest that tricin is compatible with lignification reactions and imply that tricin is a lignin initiation or nucleation compound in grasses.

Tricin: A novel monomer in grass lignins

247th National Meeting (2014)

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