Lignin polymer, structure

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). 

Figure 16. Structure of lignin polymer formed in anaerobic solvent.

Attachment of Hydroxycinnamic Acids to Structural Cell Wall Polymers. Peroxidase mediation may also result in binding the hydroxycinnamic acids to the plant cell wall polymers (66,67). For example, it was reported that peroxidases isolated from the cell walls of Pinus elliottii catalyze the formation of alkali-stable linkages between [2-14C] ferulic acid 1 and pine cell walls (66). Presumably this is a consequence of free-radical coupling of the phenoxy radical species (from ferulic acid 1) with other free-radical moieties on the lignin polymer. There is some additional indirect support for this hypothesis, since we have established that E-ferulic acid 1 is a good substrate for horseradish peroxidase with an apparent Km (77 /tM), which is approximately one fifth of that for E-coniferyl alcohol (400 /iM) (unpublished data). 

In conjunction with a study on the reactivity of dimeric quinone me-thides, Elder et al. (56) examined the physical and electronic structure of guaiacylglycerol-/ -coniferyl ether, which is substituted in a manner representative of the lignin polymer. Calculations were performed using AMBER (Assisted Model Building with Energy Refinement) (24), which is a force-field method, and the energetic minimum was determined to be a folded structure similar to that reported by Gravitis and Erins (55). 

Although most previous research on the use of lignin in structural polymers has dealt with its contribution to polyphenolics and polyurethanes (8), alternatives for crosslinking it with other polymer systems exist as well. These have been summarized recently (9). Among these alternatives are lignin derivatives with acrylate functionality. 

Phenylalanine, tyrosine, and tryptophan are converted to a variety of important compounds in plants. The rigid polymer lignin, derived from phenylalanine and tyrosine, is second only to cellulose in abundance in plant tissues. The structure of the lignin polymer is complex and not well understood. Tryptophan is also the precursor of the plant growth hormone indole-3-acetate, or auxin (Fig. 22-28a), which has been implicated in the regulation of a wide range of biological processes in plants. 

Terashima, N., Atalla, R. H., Ralph, S. A., Landucci, L. L., Lapierre, C., and Monties, B., 1996, New preparations of lignin polymer models under conditions that approximate cell well lignification I. Synthesis of novel lignin polymer models and their structural characterization by 13C NMR., Holzforsch. 49 521-527. 

Lignin polymers are often found in most plant structures in association with cellulose. The structure of lignin is not well defined, but lignin appears to be made up of polymers of propylbenzene with hydroxy and methoxy groups attached. Lignin is primarily hydrocarbon in nature and makes up a major portion of insoluble dietary fiber. 

Banoub J.H., Benjelloun-Mlayah B., Ziarelli F., Joly N., Delmas M. Elucidation of the complex molecular structure of wheat straw lignin polymer by atmospheric pressure photoionization quadrupole time-of-flight tandem mass spectrometry. Rapid Communications in Mass Spectrometry 21 2867-2888 (2007). 

This study was a preliminary effort to detect and characterize structures in the lignin polymer that result from condensation reactions during alkaline treatment. The only method capable of observing this complex polymer with the necessary detail on an atomic scale is 13C-NMR spectroscopy. Enormous progress has been made in the capabilities of NMR especially in the last few years, and very few of the modern techniques have yet been applied to lignin. 

Oxidation with alkaline CuO gave large amounts of meta-hydroxy derivatives of benzoic acid and benzene dicarboxylic acids (Hayatsu et al., 1980a). This suggests the presence of phenol ethers in the polymer structure. Interestingly, terrestrial polymers such as lignin, humic acid, and coal yield mainly para-rather than meta-hydroxy derivatives by this method. 

The alcohols formed from some cinnamic acid derivatives, namely /7-coumaryl alcohol, coniferyl alcohol (LI), and sinapyl alcohol (L2), commonly known as monolignols, undergo dimerization reactions that yield lignans such as (-l-)-pinoresinol (L3), (-l-)-sesamin (L4), (-)-matairesinol (L5), and podophyllotoxin (L6) (Fig. 13). Several thousand lignans are found to occur in nature. Lignins, the structural components of plant cell walls, are polymers of monolignols and/or lignans. 

Another case in point is the solvent extraction of lignins (Nimz, 1966). Are the propylbenzene oligomers found in the extract degradation products of the original lignin polymer, or are they precursors co-occurring with the polymer In any case, their identities have helped establish the structure of native lignin. 

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