Chapter 5
Lignin-Polysaccharide Interactions in Woody Plants Richard F. Helm
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Department of Wood Science and Forest Products, Fralin Biotechnology Center, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0346
Hemicellulose and lignin are theorized to be linked to one another through covalent bonds, and these bonds are thought to limit rapid chemical and biochemical breakdown of woody biomass. After years of study, the mechanism by which lignin and hemicellulose interact with one another is still not clear. This information is crucial to our understanding of wood biosynthesis and the organization of the cell wall three-dimensional matrix. A n overview of what is known about these interactions is presented, and bonding patterns are suggested to support the chemical and spectroscopic data reported in the literature.
Many of the products derived from forest biomass are prepared by chemical wood processing. Yields of desired materials from many of these chemical processes are much less than the amount of material theoretically available. This is due, to a great extent, to the way in which the woody cell wall is biosynthesized. Cellulose, hemicellulose and lignin interact with one another through non-covalent and covalent bonds. These interactions interfere with efficient processing, and thus, chemical procedures are needed which decompose some of the desired biopolymers. Improving the yield and efficiency of chemical/biochemical processes would help us to more effectively utilize our wood resources, and also decrease the amount of toxic effluents present in waste streams. How are we to accomplish this? First we need to understand the regiochemical organization of the wood cell wall matrix. However, in order to understand the cell wall matrix, we need to determine how the individual polymeric components come together to establish the three-dimensional biopolymer composite. When we fully understand the way in which the woody cell wall is constructed on the chemical level, we can begin ascertaining rational methods for improved chemical processing and/or genetic manipulation. The Formation of Lignin-Polysaccharide Bonds in Woody Plant Cell Walls. The theory that is used, almost without exception, to describe the formation of lignincarbohydrate (LC) bonds in lignified tissues, centers on the quinone methide formed during lignin biosynthesis. The free radical coupling of a monolignol and a lignin oligomer as shown in Figure 1, affords an intermediate quinone methide. It has been
© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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suggested that these reactive intermediates undergo nucleophilic attack to form either a-hydroxy compounds, a-alkyl/aryl ethers, a-esters or a-glycosides (1,2). This theory is under revision. In a series of investigations on the structure of forage cell walls (3,4), it has been shown that p-coumaric acid is attached to corn lignin (and other grass lignins) exclusively at the y-position (Figure 2A). Thus the
Figure 1. The coupling of two free radicals generates an intermediate quinone methide. Nucleophilic attack followed by additional free radical condensations leads to incorporation of the nucleophile into the lignin macromolecule.
Figure 2. Two modes of hydroxycinnamic acid incorporation into the cell wall of forages. A, p-coumaric acid attached at the y-position of lignin, without crosscoupling. B , ferulic acid is capable of cross-linking between arabinoxylans and lignin. The asterisks indicate other potential sites of lignin attachment.
In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by UNIV OF ARIZONA on December 16, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch005
163 ester bond is located at a "non-quinone methide" lignin hydroxyl—a process that requires a discrete biological process. The coumarate moiety possesses a free phenolic hydroxyl, which does not participate in the lignification process. This is in contrast to the other predominant cinnamic acid present in forage cell walls, ferulic acid. Ferulates can undergo free radical condensation with lignin and be fully incorporated into the lignin structure (5). As it has been amply shown that ferulic acid is esterified to arabinoxylans in forages through the 5-position of a-Larabinofuranosyl moieties (6,7), herein lies an important mechanism for ligninhemicellulose covalent interactions in forages. Ferulic acid is esterified to arabinoxylans through the carboxyl group, and the free phenolic hydroxyl allows for free radical formation leading to incorporation into the lignin matrix (Figure 2B). How does this relate to woody cell wall structure? The studies with forages clearly indicate that biochemical processes are involved in the placement of pcoumaric acid on the lignin polymer, and the attachment of ferulic acid to arabinoxylans. Ferulate is subsequently incorporated into the lignin polymer whereas p-coumarate is not. It would seem logical to assume then that if forages undergo biochemically-controlled processes for the formation of lignin-polysaccharide interactions, a controlled process would also occur in wood. Therefore, the quinonemethide theory may not adequately describe L C formation in wood. The chemical reactivity of the lignin quinone methide has been the subject of several investigations (8-12). One of the overriding points of all of these studies is that the lignin quinone methide is somewhat unreactive towards nucleophiles such as carbohydrate hydroxyls. The reactivity of the quinone methide towards carboxylic acids (uronic acids) is much higher. This reactivity is in contrast to the reported types of major L C linkages in wood. In the case of softwood and hardwood glucuronoxylans, studies indicate that the predominant linkage is an alkali-stable ether linkage, not an ester linkage through the uronosyl moiety (2,13,14). Could a biological process be controlling this bond formation? N M R spectroscopy has been extremely useful in ascertaining the predominant linkages in lignin isolates. Brunow and and others (15,16) have listed the major and minor interunit linkages, and these are shown in Figure 3. As can be seen from a perusal of the structures, a-alkyl ethers are not shown—efforts to locate such structures have largely been unsuccessful. Indeed, of all the structures indicated the only one that could possibly involve a carbohydrate is Structure G, where an alkyl group is attached to the y-position of a P-O-4 unit. This concept will be revisited later in the section entitled "Points to Ponder." Do Different Wood Species and Wood Types Have Different Linkages? It is emphasized here that the choice of substrate can have profound influence on the presence and type of L C bonds found. Using temperate zone hardwoods as an example, their principal hemicellulose is an 0-acetyl-(4-0-methylglucurono)xylan, comprising 15-30% of the weight of these woods (17). The ( l - » 4 ) p-Dxylopyranosyl backbone carries occasional substitutions at the two position by 4-0methyl-a-D-glueopyranosiduronic acid, as well as randomly distributed acetyl groups. Approximately 7 acetate groups are present for every 10 xylose units, and there is approximately 1 uronosyl moiety for every 7 xylose units. It is interesting to note that this structure and composite makeup is quite conserved for all temperate zone hardwood species. This implies xylan plays an integral role in hardwood cell wall structure. Obst has investigated the L C bonds present in aspen; about 20% were labile to base, implying an ester linkage (14). This is in accordance with the observation that an almost quantitative yield of glucuronoxylan can be obtained from treating aspen wood meal with 24% K O H (18). The same base extraction can be done with paper birch (Betula papyrifera), providing a glucuronoxylan in 80% yield (17). These isolated materials have minimal lignin content (
