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ARS Home » Midwest Area » Madison, Wisconsin » U.S. Dairy Forage Research Center » Research » Publications at this Location » Publication #195599
Title: NON-ENZYMATIC REDUCTION OF QUINONE METHIDES DURING OXIDATIVE COUPLING OF MONOLIGNOLS: IMPLICATIONS FOR THE ORIGIN OF BENZYL STRUCTURES IN LIGNINS

Author
item HOLMGREN, ANDERS - KTH, STOCKHOLM, SWEDEN
item BRUNOW, GOSTA - UNIV. HELSINKI, FINLAND
item HENRIKSSON, GUNNAR - KTH, STOCKHOLM, SWEDEN
item ZHANG, LIMING - KTH, STOCKHOLM, SWEDEN
item Ralph, John

Submitted to: Organic and Biomolecular Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2006
Publication Date: 8/7/2006
Citation: Holmgren, A., Brunow, G., Henriksson, G., Zhang, L., Ralph, J. 2006. Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins. Organic and Biomolecular Chemistry. 4(18):3456-3461.

Interpretive Summary: Lignins are the polymer glues that hold fibers together in the plant cell wall. For the most part, we know they are synthesized from three monomers, the so-called monolignols. However, there are other structures in lignins that do not appear to derive directly from these three monomers. Fundamental delineation of how such structures arise reveals new details of the process (the biochemistry and chemistry) of lignin formation. It also reveals how flexible the system is in being able to incorporate components other than the traditional monomers. It is therefore possible that plants or humans can alter plant lignins by incorporating more of these components, to obtain fibers with different properties that might, for example, be more readily degraded by microbes or more readily broken by chemicals used in pulping (to make paper). Here we examine a possible mechanism by which certain troubling structures known to be in lignins might come to be produced. Indeed, we could get a non-enzymatic system to produce similar structures in lignins, but not the exact structures of interest. This means that the origin of such structures remains unresolved. Our fundamental studies on lignins are ultimately aimed at improving the utilization of valuable plant cell wall resources in a variety of processes ranging from ruminant digestion in animals, through energy production, and to industrial chemical pulping.

Technical Abstract: Lignin is believed to be synthesized by oxidative coupling of 4-hydroxyphenylpropanoids. In native lignin there are some types of reduced structures that cannot be explained by oxidative coupling. In the present work we showed via biomimetic model experiments that nicotinamide adenine dinucleotide (NADH), in an uncatalyzed process, reduced a beta-aryl ether quinone methide to its benzyl derivative. A number of other biologically significant reductants, including the enzyme cellobiose dehydrogenase, failed to produce the reduced structures. Synthetic dehydrogenation polymers of coniferyl alcohol synthesized (under oxidative conditions) in the presence of the reductant NADH produced the same kind of reduced structures as in the model experiment, demonstrating that oxidative and reductive processes can occur in the same environment, and that reduction of the in situ-generated quinone methides was sufficiently competitive with water addition. In situ reduction of beta-beta-quinone methides was not achieved in this study. The origin of racemic benzyl structures in lignins therefore remains unknown, but the potential for simple chemical reduction is demonstrated here.