(61c) Study of Reaction Between Hydroxyl Radical and β-O-4 Lignin for Understanding of Termites Lignin Deconstruction Mechanism

The presence of lignin in the plant cell wall hinders the access of enzymes to cellulose during the enzymatic hydrolysis process, resulting in low yield of sugar that can be used for biofuel production. Breaking down the lignin using different pretreatment techniques can enhance the access of enzymes to cellulose. Yet presently used pretreatment techniques are energy and resource consuming. In general, pretreatment accounts 20% of the total costs of industrial bioethanol production, and thus there is a need to develop new technology for optimizing the pretreatment process and minimizing the cost of biofuel production. Termites can consume 74-99% of cellulose and 65-87% of hemicellulose in their digestive tract within 24 hours. The observation of 99% sugar recovery within the termite digestive tract has motivated researchers to study the mechanism of how the termite modifies the recalcitrant lignin. Understanding lignin deconstruction mechanism in termite may facilitate the development of pretreatment methods that mimic the termite lignin modification process.

Hydroxyl radical (OH^•) mediated reactions are common in biological processes. The digestive tract of termite includes the foregut, midgut and hindgut. The chemical environment within the midgut is highly oxidative and favorable for the production of OH^•. To date there have been no fundamental and systematic studies regarding the reaction of OH^• with lignin and the subsequent activation of specific linkages for lignin degradation. The main objective of this study is to elucidate potential mechanisms for OH^• reaction with lignin, to add in the understanding of lignin modification mechanisms that may be used by termites. Two different reactions were studied: In the first step, we studied the OH^• addition reaction mechanism to the carbon sites in the aromatic ring. In the second step, we studied the dissociation reactions of the β-O-4 lignin linkages. Density functional theory and G4 calculations were employed to reveal the OH^• and lignin reaction chemistry. The bond dissociation energy of the β-O-4 lignin linkages before and after the OH^• addition to the carbon site of the aromatic ring indicate that OH^• addition significantly decreases the bond dissociation energy of lignin isomers compared to original lignin. The results obtained from this study may be useful in designing and optimizing of OH^• based lignocellulosic biomass pretreatment mechanism.