Dibenzodioxocins; a novel type of linkage in softwood lignins

Proc. Natl. Acad. Sci. U.S.A.

Hatfield

et al. 1999

162

Nuclear magnetic resonance (NMR) of isolated lignins from anArabidopsis mutant deficient in ferulate 5-hydroxylase (F5H) and transgenic plants derived from the mutant by overexpressing the F5H gene has provided detailed insight into the compositional and structural differences between these lignins. Wild-type Arabidopsis has a guaiacyl-rich, syringyl-guaiacyl lignin typical of other dicots, with prominent ␤-aryl ether (␤-O-4), phenylcoumaran (␤-5), resinol (␤-␤), biphenyl͞dibenzodioxocin (5-5), and cinnamyl alcohol end-group structures. The lignin isolated from the F5H-deficient fah1-2 mutant contained only traces of syringyl units and consequently enhanced phenylcoumaran and dibenzodioxocin levels. In fah1-2 transgenics in which the F5H gene was overexpressed under the control of the cauliflower mosaic virus 35S promoter, a guaiacyl-rich, syringyl͞guaiacyl lignin similar to the wild type was produced. In contrast, the isolated lignin from the fah1-2 transgenics in which F5H expression was driven by the cinnamate 4-hydroxylase promoter was almost entirely syringyl in nature. This simple lignin contained predominantly ␤-aryl ether units, mainly with erythro-stereochemistry, with some resinol structures. No phenylcoumaran or dibenzodioxocin structures (which require guaiacyl units) were detectable. The overexpression of syringyl units in this transgenic resulted in a lignin with a higher syringyl content than that in any other plant we have seen reported. transgenic plants ͉ mutant ͉ syringyl lignin ͉ heteronuclear single quantum coherence (HSQC) ͉ guaiacyl lignin T he biotechnological manipulation of lignin content and͞or structure in plants is seen as a route to improving the use of plant cell wall polysaccharides in various agricultural and industrial processes. The aims of these efforts range from enhancing cell wall digestibility in ruminants to reducing the energy demand and negative environmental impacts of chemical pulping and bleaching required in the papermaking process. To achieve these goals, many researchers (1-3) have down-regulated the expression of genes of the monolignol biosynthetic pathway in attempts to decrease lignin deposition. This approach has sometimes had negative side effects, including the collapse of tracheary elements under the tension generated by transpiration. One of the most desirable strategies for modifying lignin is to produce a less crosslinked, syringyl-rich lignin. It is hoped that this modified lignin would have improved industrial and͞or agricultural properties, yet remain fully functional in planta.Through the identification of the fah1-2 mutant, it has been shown that ferulate 5-hydroxylase (F5H) activity is required for syringyl lignin deposition in Arabidopsis (4). The Arabidopsis F5H gene was subsequently cloned by T-DNA tagging (5). F5H expression was shown to be rate limiting for syringyl lignin accumulation by overexpression of the gene in the fah1-2 mutant background (6). The lignin in transgenic plants, in which the F5H gene was expressed under the control of ca...

Section: Resultsmentioning

confidence: 62%

NMR characterization of lignins in Arabidopsis altered in the activity of ferulate 5-hydroxylase

Marita

Proc. Natl. Acad. Sci. U.S.A.

Hatfield

et al. 1999

162

“…Once thought to involve some 6-8% of native lignin units (29), noncyclic ␣-ethers E have recently been found at extremely low levels in isolated wood lignins (30). Recently discovered dibenzodioxocin structures D (31,32) probably contributed to erroneous estimates previously.…”

Section: Resultsmentioning

confidence: 99%

NMR characterization of altered lignins extracted from tobacco plants down-regulated for lignification enzymes cinnamylalcohol dehydrogenase and cinnamoyl-CoA reductase

Proc. Natl. Acad. Sci. U.S.A.

Hatfield²,

Piquemal³

et al. 1998

181

165

Homologous antisense constructs were used to down-regulate tobacco cinnamyl-alcohol dehydrogenase (CAD; EC 1.1. Lignins are phenolic polymers essential for mechanical support, defense, and water transport in vascular terrestrial plants (1-3), but they are a major obstacle to efficient utilization of plants for paper making or animal feed. A recent approach toward improved utilization has been the down-regulation of enzymes involved in the lignin monomer biosynthetic pathway (Fig.

“…Dimers composed of two G units (3,6,8) were the most abundant, with smaller amounts of HH (1) and HG (2, 4) 5−5′ dimeric structures. Dibenzodioxocins are considered to be the main biphenyl structures in lignin; 57 therefore, the 5−5′ thioacidolysis dimers can be considered mostly as being dibenzodioxocin degradation products, although simple biphenyl structures have also been reported in lignin; 27 how readily dibenzodioxocins release 5−5-dimers upon thioacidolysis has not yet been well documented.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Structural Characterization of Lignin Isolated from Coconut (Cocos nucifera) Coir Fibers

Rencoret

Marques

et al. 2013

J. Agric. Food Chem.

146

117

The structure of the isolated milled "wood" lignin from coconut coir has been characterized using different analytical methods, including Py-GC/MS, 2D NMR, DFRC, and thioacidolysis. The analyses demonstrated that it is a phydroxyphenyl-guaiacyl-syringyl (H-G-S) lignin, with a predominance of G units (S/G ratio 0.23) and considerable amounts of associated p-hydroxybenzoates. Two-dimensional NMR indicated that the main substructures present in this lignin include β− O−4′ alkyl aryl ethers followed by phenylcoumarans and resinols. Two-dimensional NMR spectra also indicated that coir lignin is partially acylated at the γ-carbon of the side chain with p-hydroxybenzoates and acetates. DFRC analysis showed that acetates preferentially acylate the γ−OH in S rather than in G units. Despite coir lignin's being highly enriched in G-units, thioacidolysis indicated that β−β′ resinol structures are mostly derived from sinapyl alcohol. Finally, we find evidence that the flavone tricin is incorporated into the coconut coir lignin, as has been recently noted for various grasses.