Lignin Structure : Recent Developments

Studies of lignin structure and reactivity have been re-energized by the emergence of modern technologies, new analytical methods, and by exciting recent findings. The structural aspects which were considered complex but well understood some years back have seen revisions that add to the complexity in some ways but are clarified by the logical chemistry in others. One of the most significant recent findings has been the elucidation of dibenzodioxocins as significant components of natural lignins. Now found in lignins from all classes of plants which have a guaiacyl lignin component, they are new structures derived from 5—5-coupling of lignin oligomers followed by coupling with a new lignin monomer; other lignification reactions then follow to fully incorporate these structures into the polymer. Another new structural unit, aryl-isochromans, expose a logical new pathway following β—1-coupling. These new structures and others have been revealed by modern analytical and instrumental methods, particularly NMR. The now common use of 2D and 3D NMR, using pulsed field gradients, inverse detection, and modern digital instruments allows ever more sensitive detection and, more importantly, diagnostic interpretation. Because it is now possible to reveal a given unit in the complex polymer in a diagnostic way, structures are being suggested directly from spectra of the polymer (although confirmation and authentication using model compounds remains crucial). Studying lignins from non-woody plants has also provided significant new knowledge. Acylated (e.g. acetylated, p-coumaroylated, p-hydroxybenzoylated) lignins are now better understood thanks to NMR methods, as well as new and established degradative methods (e.g. the DFRC method, and thioacidolysis) combined with mass-spectrometry. Acylated units can be a significant component of lignins; for example, lignins in the bast fibers of kenaf are more than 50% acetylated. Cell wall cross-linking in grasses by ferulates has also recently been clarified. Mutant and transgenic plants in which steps in the monolignol biosynthetic pathway are downor up-regulated are also providing a rich source of insight into the processes of lignification. Massive compositional shifts, significantly beyond traditional bounds, are found in a variety of mutants/transgenics. In some cases, units that are minor in normal lignins become major components. This facilitates their identification and may elucidate their radical-coupling reactions. Pathways interacting with lignification become identified, and the very definition of lignin itself becomes clouded or simplified depending on ones perspective. This paper explores recent advances in understanding lignin structure, and provides applications that show the breadth and plasticity of the lignification process. Introduction Lignification is the polymerization process in plant cell walls that takes phenolic monomers, produces radicals, and couples them with other monomer radicals (only during initiation reactions), or more typically cross-couples them with the growing lignin polymer/oligomer, to build up a phenylpropanoid polymer (Harkin, 1967; Freudenberg and Neish, 1968; Harkin, 1973; Brunow, 1998). As addressed below, there is current controversy over just what constitutes a lignin monomer or a lignin polymer. In the purest sense, lignins arise from radical coupling reactions of three primary precursors, the monolignols p-coumaryl, coniferyl and sinapyl alcohols, Figure 1. Since this process produces a polydisperse polymer with no extended sequences of regularly repeating units, its composition is generally characterized by the relative abundance of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units (derived from each of the 3 primary monolignols) and by the distribution of interunit linkages in the polymer (e.g. βaryl ether or β—O—4, phenylcoumaran or β —5, resinol or β−β, biphenyl or 5—5, diphenyl ether or 4—O—5). There are also hydroxycinnamyl alcohol endgroups from the few initial dimerization reactions, structures such as non-cyclic α-aryl ethers (which arise from addition of a phenol to an intermediate β-aryl ether quinone methide) and various other structures.