Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 5/19/2008
Publication Date: N/A
Interpretive Summary: [This article is a review of the past 30 years of research by our group (and others) on quinone methides, important intermediates associated with lignin.] Lignins are large complex molecules that are prominent in plant cell walls, perhaps accounting for as much as 20% of terrestrial biomass. They hold together the fibers, and are involved in plant defense and water transport throughout the plant. Lignins are made from coupling reactions of the small-molecule lignin building blocks, with each other or, more importantly, with the growing, large complex molecule. Each coupling reaction produces an intermediate structure, a quinone methide, that then reacts in various ways to produce the final structure in lignin. Therefore, quinone methide reactivity and structure are key factors in the important process of lignification. Lignification, and the reactions of the intermediary quinone methides in particular, are increasingly seen as crucial to understanding the key role of lignin in limiting the efficient conversion of lignocellulosic biomass in a variety of natural and industrial processes. These include: digestibility by ruminant animals, the isolation of cellulose for pulp and paper production, and the conversion of lignocellulosic feedstocks to biofuels. This review describes many aspects of the process of quinone methide formation, the fate of such quinone methides, their syntheses (outside the plant) for research purposes, their structure and reactivity, and novel quinone methides that arise during lignification in stressed or “perturbed” plants (including transgenics). It also covers new avenues of how quinone methides might lead to reduced structures in lignins, and a new quinone methide pathway that might explain the currently more poorly understood aspects of lignification. However, the latter was shown to probably not be a pathway in the plant, but was included to stimulate further discussion among researchers.
Technical Abstract: Quinone methides play an important role in lignification. They are produced directly, as intermediates, when lignin monomers, be they hydroxycinnamyl alcohols, hydroxycinnamaldehydes, or hydroxycinnamates, couple or cross-couple at their 8-positions. A variety of post-coupling quinone methide rearomatization reactions leads to a wide array of structures in the complex lignin polymer.