Hydroxycinnamates in lignification |
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Authors: | John Ralph |
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Institution: | 1. Departments of Biochemistry and Biological Systems Engineering, University of Wisconsin-Madison, Madison, WI, 53706, USA 2. The DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI, 53706, USA 3. Department of Biochemistry (Enzyme Institute), 1710 University Avenue, Madison, WI, 53706-4087, USA
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Abstract: | Hydroxycinnamates incorporate into lignins by various mechanisms. The polysaccharide esters of ferulate, in particular, and the range of dehydrodiferulates and higher oligomers in grasses, participate in free-radical (cross-)coupling reactions during lignification to become integrally bound into the lignin polymer, resulting in extensive cross-linking between lignins and polysaccharides. Monolignol-hydroxycinnamate (primarily monolignol-p-coumarate) conjugates are primary building blocks for lignins, again in grasses (but analogously with monolignol acetates and p-hydroxybenzoates in other plants); radical coupling reactions of the monolignol moiety of the conjugate result in lignins with pendant p-coumarate units acylating a variety of lignin structures. Recent evidence suggests that even the hydroxycinnamic acids themselves can be monomers in lignification in wild-type and transgenic plants, undergoing radical cross-coupling reactions to incorporate into the polymer with interesting consequences. The compatibility of ferulate, in particular, with lignification suggests that plants able to utilize monolignol-ferulate conjugates in their primary monomer supply will be particularly well suited for subsequent chemical delignification, potentially improving processes for biomass conversion to biofuels, and for chemical pulping. |
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