Lignin monomers

Quinone methides play an important role in lignification. They are produced directly, as intermediates, when lignin monomers, be they hydroxycinnamyl alcohols, hydroxy-cinnamaldehydes, or hydroxycinnamates, couple or cross-couple at their 8- positions. A variety of postcoupling quinone methide rearomatization reactions leads to an array of structures in the complex lignin polymer (Fig. 12.2). 

Lu, F. Ralph, J. Preliminary evidence for sinapyl acetate as a lignin monomer in kenaf. J. Chem. Soc., Chem. Commun. 2002, 90-91. 

Gupta JK, Sharma P, Kern HW et al (1990) Degradation of synthetic lignins and some lignin monomers by the yeast Rhodotorula glutinis. World J Microb Biotechnol 6 53-58... 

Fig. 2.2 A. Chemical structure of the three types of lignin monomer units H, p-hydroxyphenyl- G, guaiacyl and S, syringyl. Note the methyl groups (boxed) in the methoxy moieties of the G and S monomers. B. Pie chart showing the proportions of pine xylem ESTs that putatively encode enzymes of Ci metabolism, glycolysis, and the TCA cycle.

The products of low temperature pyrolysis are char and low molecular components, see pathway 1 in Figure 53. At moderate temperature levels, the formation of a variety of lignin monomers (see Figure 49) occurs via pathway 2. And at high temperatures (> 500°C), fragmentation reactions take place, forming CO, H2, and reactive vapours. [67]... 

Lignans are phenylpropanoid dimers in which the monomers are linked by the central carbon (C8) atoms of the propyl side chains (Fig. 12.1) [10]. Many lignans are formed from coniferyl alcohol, a typical lignin monomer, and the coupling of two coniferyl alcohol radicals proceeds under the control of a unique asymmetric... 

Table II. Lignin Monomer Composition, Obtained by Nitrobenzene Oxidation of Lignin from Normal and Brittle Fescue Grown at Lusignan (Same as in Table I)...

The only biological function which has been repeatedly confirmed is the role of peroxidases in lignin monomer polymerization (1). But even in this case, the role of the various isozymes is not yet clear, although anionic, cell wall bound peroxidases generally seem to be involved in lignification... 

In plant cell walls, lignin monomers seem to be present in vivo in the form of cinnamyl alcohols. In vitro, their acid precursors can also be oxidized by peroxidases (3). In order to gain further insight into the possible... 

Lignin. A polymer found in wood (25—30%). The structure of the lignin monomer is still not completely known. Its similarity to coniferyl alcohol, noted more than 75 years ago (Ref 1), is confirmed by the fact that it can be oxidized to vanillin and hydrogenated to compds of the cyclohexylpropyl type. 

Meyer, K., Shirley, A. M., Cusumano, J. C., Bell-Lelong, D. A., and Chappie, C., 1998, Lignin monomer composition is determined by the expression of a cytochrome P450-dependent monooxygenase in Arabidopsis, Proc. Natl. Acad. Sci. USA 95 6619-6623. 

Comprehensive information on the network of pathways responsible for the synthesis of numerous secondary metabolites can be found in Chapter 21. In addition, information on this aspect is also available in articles by Shimada et al. [2003], Toshiaki [2003], Tanner et al. [2003], Boatright et al. [2004], Hoffmann et al. [2004], Dixon et al. [2005], Niemetz and Gross [2005], Xie and Dixon [2005], and Ferrer et al. [2008], Nonetheless, the complete dissection of phenolic metabolic pathway is far from being complete. For example, recent reports underline that important questions still remain to be answered in the field of protoanthocyanidins and tannins [Xie and Dixon, 2005], and that the exact nature of the biosynthetic pathway(s) leading to lignin monomers has not been fully elucidated [Boudet, 2007]. 

Phenolic compounds and flavonoids are a unique category of plant phytochemicals especially in terms of their vast po ential health-benefiting properties. They represent the most abundant and the most widely represented class of plant natural products. A substantial amount of research has been carried out over the past two decades yet large information gaps still exist. For example, the inventory of these compounds is still incomplete, although there is continuous effort to provide new structures. In addition the dissection of the metabolic pathways for certain phenolic compounds remains to be resolved. Recent reports underline that important questions that still need to be answered in the field of proanthocyanidin and tannin biosynthesis [Xie and Dixon, 2005], and even the exact nature of the biosynthetic pathway(s) leading to lignin monomers is not fully elucidated. 

VM, percentage matter volatilized in pyrolysis CHYDR, carbohydrates with pentose and hexose subunits PHLM, phenols and lignin monomers LDIM, lignin dimers LIPID, lipids, alkanes, alkenes, bound fatty acids, and alkylmonoesters ALKY, alkylaromatics NCOMP, mainly heterocyclic N-containing compounds STEROL, sterols PEPTI, peptides SUBER, suberin FATTY, free fatty acids in % of total ion intensity. 

Microbial transformations of DOM were also studied by incubation experiments. The DOM extracted from maize straw and forest floors was incubated for 90 days and samples taken before and after incubation were analyzed by Py-FIMS and complementary UV absorbance, fluorescence emission spectroscopy, FTIR-spectroscopy, 1H NMR spectroscopy, and 13C natural abundance (Kalbitz et al., 2003). The Py-FI mass spectra showed increases in the proportions of phenols and lignin monomers at the expense of lignin dimers and alkylaromatics during... 

Correspondingly, the thermograms in Figure 14.11a show that the curve for the summed ion intensities of carbohydrates, phenols, lignin monomers, and peptides from the arable Ap horizon (AR-Ap) peak at relatively low temperature and exceed the corresponding curves from the other horizons. The abundance of these compounds was greatly reduced in the groundwater of this field. In conserved extensive... 

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