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Contemporary View of Lignin Substructures

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Lignification: Random vs Template Directed. In terms of energy content, lignins are thought to be the most abundant of all biopolymers. They are composed of p-hydroxyphenylpropanoid units interconnected through 8-O-4, 8-5, 8-8, 8-1, 5-5, and 4-O-5 linkages. Corresponding substructures in the polymer include alkyl aryl ethers, phenylcoumarans, resinols, tetrahydrofuran-spiro-cyclohexadienones, biphenyls, dibenzodioxocins, and diaryl ethers (see Fig. A). The primary precursors themselves—the three monolignols p-coumaryl, coniferyl, and sinapyl alcohols—differ only according to their aromatic methoxy substitution patterns. These monolignols are oxidized enzymatically through single-electron transfer to generate the respective phenoxy radicals. The actual coupling of a monolignol radical with the growing end of a lignin chain, however, may not fall under direct enzymatic control. Accordingly, many investigators have assumed that lignin primary structures must be random or combinatorial as far as sequences of interunit linkages are concerned. More recently, this theory has been reinforced by reports that certain kinds of non-native monolignols can be incorporated into macromolecular lignin structures. Lignins and lignin derivatives exhibit two fundamental characteristics that traditionally have been viewed as evidence in favor of randomness in their configurations: They are both noncrystalline and optically inactive.1 Nevertheless, a number of observations are thought by some to point in the opposite direction. The individual molecular components in (nonpolyionic) lignin preparations tend to associate very strongly with one another in a well-defined way. These processes are thought to be governed by vital structural motifs derived from corresponding features disposed nonrandomly along the native biopolymer chain. Moreover, dimeric pinoresinol moieties are linked predominantly to the macromolecular lignin chain through at least one of their aromatic C-5 positions.We do not know whether such features can be explained through combinatorial mechanisms under simple chemical control or if higher-level control mechanisms are required. One hypothesis proposes a way to replicate specific sequences of interunit linkages through a direct template polymerization mechanism. According to this model, an antiparallel double-stranded lignin template, maintained in a dynamic state at the leading edge of each lignifying domain, determines the configuration of the daughter chain being assembled on the proximal strand’s exposed face. Furthermore, replication fidelity could be controlled by strong nonbonded orbital interactions between matching pairs of aromatic rings in the parent and the growing daughter chains. The overall process seems to be consistent with the lack of both crystallinity and optical activity in macromolecular lignin domains.Finally, required sequence information may be encoded in polypeptide chains that embody arrays of adjacent lignol-binding sites analogous to those found in dirigent positioning proteins.2 Cited References: 1. J. Ralph et al. 2004. Lignins: Natural Polymers from Oxidative Coupling of 4-Hydroxyphenylpropanoids, Phytochemistry Rev. 3, 29–60. 2. S. Sarkanen. 1998. Template Polymerization in Lignin Biosynthesis, pp. 194–208 in Lignin and Lignan Biosynthesis 697, ed. N. G. Lewis and S. Sarkanen, American Chemical Society, Washington, D.C. Theory proposed by G. Brunow and coworkers in 1998 (reproduced with permission).

Credit or Source: Biological and Environmental Research Information System, Oak Ridge National Laboratory. Sponsored by the U.S. Department of Energy Biological and Environmental Research Program.

Citation(s):

U.S. DOE. 2006. Breaking the Biological Barriers to Cellulosic Ethanol: A Joint Research Agenda, DOE/SC/EE-0095, U.S. Department of Energy Office of Science and Office of Energy Efficiency and Renewable Energy. (p. 94) (website)

Prepared by the Biological and Environmental Research Information System, Oak Ridge National Laboratory, genomicscience.energy.gov/ and genomics.energy.gov/.