Aromatic units in lignin

FD Chan, KL Nguyen, AFA Wallis. Estimation of the aromatic units in lignin by nucleus exchange A reassessment of the method. J Wood Chem Technol 15 473 91, 1995. 

FIGURE 11.3 General reaction scheme for the oxidation of an aromatic unit in lignin and its alkaline hydrolysis. 

FIGURE 11.8 Initial reactions between an aromatic unit in lignin and ozone. In addition to oxidative cleavage of the aromatic ring (ozonolysis), a direct formation of radical intermediates takes place. 

Reactivity of Aromatic Sites in Lignin. It is of considerable interest to estimate the significance of the protodedeuteration data in predicting the reactivities of the phenylpropane units in lignin. 

Obviously then, the relative reactivities of the aromatic positions in lignin units do not fit a uniform pattern but depend largely on the specific nature of the electrophile. Consequently, the information obtainable from protodedeuteration data is somewhat limited at the moment. Since the protodedeuteration rate constants can be determined conveniently and precivSely, they can probably be used better in the future to predict reactivities in other electrophilic displacements as the interrelations between these reactions become more thoroughly understood. 

Shikimic acid pathway chemical pathway common in plants, bacteria, and fungi, where aromatic amino acids (e.g., tryptophan, phenylalanine, tyrosine) are synthesized, thereby providing the parent compounds for the synthesis of the phenylpropanoid units in lignins. 

An example of the types of difficulties encountered in quantitative lignin analysis can be found in the calculation of the abundance of condensed units in lignins based on H NMR spectroscopy [25,26]. The calculation is based on an integration of the signal due to aromatic protons (corrected for the contribution of certain vinyl protons), together with a determination of the number of phenylpropane units... 

Phenylpropane units in lignin are linked by aliphatic and aromatic carbon bonds and ether bonds. Wood lignin has a definite structure that cannot be represented by a single formula. A plausible schematic structure for lignin is shown in Figure 10... 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

The initially formed phenoxy radicals randomly combine to form a variety of bonds. Scheme 8.20 shows major linkages between units in softwood lignin. Hardwood lignins are similar, but contain varying quantities of the 3,5-dimethoxylated aromatic rings. 

Figure 9. Molar yields, obtained after 20 minutes at pH 3.0 with 0.13 units mL lignin peroxidase in the presence of H2O2 and veratryl alcohol, of C -Cg, B-O-4 and aromatic ring cleavage products from 4.ethoxy-3-methoxyphenylgfycerol-B-syringyl ether moieties in dehydrogenative copolymer of conifery] alcohol and 4.ethoxy 3-methoryphet lgfycerol-B-syringaresinol (54).

Lignin is a noncellulosic resinous component of wood. It is the second most abundant renewable natural resource. It has alcohol and ether units with many aromatic units. Much of lignin is sheetlike in structure. 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

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