Submitted to: Biomacromolecules
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/6/2012
Publication Date: 8/13/2012
Citation: Elumalai, S., Tobimatsu, Y., Grabber, J.H., Pan, X., Ralph, J. 2012. Epigallocatechin gallate incorporation into lignin enhances the alkaline delignification and enzymatic saccharification of cell walls. Biomacromolecules. 5(1):1-14. Interpretive Summary: Plant cell walls are the world’s most abundant source of carbohydrates for fermentation into biofuels. Prior to their fermentation in biofuels, however, these carbohydrates must first be liberated from lignin by harsh and costly chemical pretreatments. Therefore, we are testing ways to modify lignin formation in plants so that it is easier to remove by chemical pretreatments. In this study, we artificially lignified cell walls from corn with normal precursors (i.e. monolignols) plus epigallocatechin gallate, a natural antioxidant found in many plants that is not normally a component of lignin. We found that epigallocatechin gallate readily formed polymers with normal monolignols and it rendered lignin easier to remove with relatively mild alkaline pretreatments. Incorporation of epigallocatechin gallate into lignin also improved the production of fermentable sugars from cell walls following alkaline pretreatment. These results provide compelling evidence that epigallocatechin gallate would be a promising plant genetic engineering target for improving the production of biofuels from biomass crops.
Technical Abstract: Epigallocatechin gallate (EGCG) was evaluated as a potential lignin bioengineering target for rendering biomass more amenable to processing for biofuel production. In vitro peroxidase-catalyzed polymerization experiments revealed that both gallate and pyrogalloyl (B-ring) moieties in EGCG underwent radical cross-coupling with monolignols mainly by ß-O-4 type cross-coupling, producing benzodioxane units following rearomatization reactions. Biomimetic lignification of maize cell walls with a 3:1 molar ratio of monolignols and EGCG permitted extensive alkaline delignification of cell walls (72 to 92%) that far exceeded that for lignified controls (44 to 62%). Improved delignification may be attributed to cleavage of ester intra-unit linkages within EGCG and internal trapping of quinone-methide intermediates to prevent benzyl ether cross-linking of lignin to structural polysaccharides. Alkali-insoluble residues from EGCG lignified walls yielded up to 34% more glucose and total sugars than lignified controls during enzymatic saccharification. Overall, our results suggest that apoplastic deposition of EGCG for incorporation into lignin would be a promising plant genetic engineering target for improving the delignification and saccharification of biomass crops.