Lignin dimers: Structures, distribution, and potential geochemical applications

Abstract

An extensive suite of thirty lignin-derived phenolic dimers and fourteen additional monomers has been identified among the CuO reaction products of twenty-four different vascular plant tissues. The various lignin dimers are characterized by five different types of linkages between phenolic units, including direct 5,5'-ring-ring bonding, as well as β,1-diketone, α,1-monoketone, α,5-monoketone, and α,2-methyl sidechain-ring couplings. The new lignin-derived monomeric CuO reaction products include vanillyl and syringyl glyoxalic acids and vanillyl phenols with formyl and carboxyl functional groups attached at various ring positions. The distribution of all these novel compounds in twenty-four different vascular plant tissues indicates important differences in the structure and chemical composition of the lignin macromolecule among these sources. The abundances of these compounds in a selected set of sedimentary samples suggest that the lignin dimers and novel lignin monomers can characterize the ultrastructure, sources, and diagenetic state of sedimentary lignin in ways not possible from the routinely utilized lignin monomers alone.