Lignin is a phenylpropanoid polymer derived essentially from the oxidative radical coupling of three p-hydroxycinnamyl alcohols (so-called “monolignols”) differing in their degree of methoxylation, p-coumaryl, coniferyl, and sinapyl alcohols. Several other phenolic compounds, all deriving from the shikimate-derived monolignol biosynthetic pathway, have been found to behave as lignin monomers in many plants, including the monolignol ester conjugates (with acetate, p-coumarate, ferulate, and p-hydroxybenzoate) or the incompletely methylated monomers caffeyl and 5-hydroxyconiferyl alcohols, among others. However, recent studies have revealed that some phenolic compounds derived from other biosynthetic pathways also act as true lignin monomers participating in coupling and cross-coupling reactions during lignification in several plants. This is the case for the flavone tricin recently established as a monomer in the lignins from grasses and other monocots, the newly discovered hydroxystilbenes (resveratrol, isorhapontigenin and piceatannol) that are implicated as monomers in the lignin of palm fruit endocarps, the respective hydroxystilbene glucosides that have been found in the lignin of spruce bark or the diferuloylputrescine that has been found as a monomer in the lignin of maize kernels. The discovery in lignins of ‘non-conventional’ phenolic precursors arising from other biosynthetic pathways expands the traditional definition of lignin and reveals that any phenolic compound that is delivered to the cell wall may be oxidized and incorporated into the lignin polymer during lignification, subject exclusively to its chemical compatibility. On the other hand, these discoveries reveal that valuable flavonoids and hydroxystilbenes could also be obtained from agricultural resides (cereal straws, palm fruit shells of tree barks) or from low-value lignin side-streams in lignocellulose processing mills, which will open new opportunities for the valorization of these residues that are considered as wastes.

Novel lignin monomers: Another brick in the wall

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.