Demethylating, lignin

Fig. 10-5. Scheme for production of methyl mercaptan (MM), dimethyl sulfide (DMS), and demethylated lignin. 

Aryl methyl ethers. A great deal of attention has been directed to the demethylation of aryl methyl ethers on account of interest in the degradation of lignin and related compounds by both aerobic and anaerobic organisms. 

LiP catalyzes the oxidation of a low-molecular-weight redox mediator, veratryl alcohol, which in mrn mediates one-electron oxidation of lignin to generate aryl cation radicals [100]. The radicals facilitate a wide variety of reactions such as carbon-carbon cleavage, hydroxylation, demethylation, and so on. Dezotti et al. [101] reported enzymatic removal of color from extraction stage effluents using lignin and horseradish peroxidases immobilized on an activated silica gel support. 

MnP catalyzes hydrogen-peroxide-dependent oxidation of Mn to Mn, which in turn oxidizes phenolic components of lignin [102]. Oxidative demethylation, dechlorination, and decolorization of bleach plant effluent by the MnP of Phanerochaete chrysosporium has been demonstrated [103,104]. 

Tablf I. Peroxidase and Oxidase Production by Selected Wood-Inhabiting Fungi, and their Capacity to Demethylate and Solubilize Lignin... 

Fungus Units x 1000/1 mg Mycelium Lignin- Poly-Blue peroxidase1 Oxidase Laccase3 Demethyl- ation4 Solubi- lization5... 

Nitrogen values more than triple on adding ammonium nitrate. The mor the straw is rotted, the less nitrogen can be hydrolyzed with hydrochloric acid. A part of the nitrogen in the lignin fractions is fixed as a-amino nitrogen. The quantity decreases from 60% of total nitrogen content in fresh straw to about 20% in rotted straw. A remarkable decrease could be observed for the methoxyl content. This fact can be explained by an enzymatic demethylation process. 

The decomposition of lignin is caused by oxidation and demethylation furthermore, fixation of nitrogen occurs. Nitrogen can be derived from peptides, amino acids, or ammonia. AD these compounds can be formed by the decomposition of plant proteins or during the autolysis of the microorganisms. 

Other experiments show that phenolcarboxylic acids labelled in the carboxyl group form radioactive polymers by simultaneously forming Cl4(>2. This means that not all C1402 is split off, and the polymerization does not occur only through quinones as intermediate steps. Demethylation of the lignin decomposition products is a further important reaction because o-benzoquinones can be formed under certain conditions. 

We studied the demethylation of lignin decomposition products labelled in the C atom of the methoxyl group. After demethylation the resulting 1,2-diphenols form p- or o-benzoquinones by further oxidation under mild conditions depending upon the substitution of the side chain, as shown in Figure 5. In the case of demethylation of partially methylated 1,2,3-triphenols, formation... 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Other preliminary experiments on alkali lignin included oxidations by barium peroxide and alkali (5, 6), alkali fusion, and alkali fusions in the presence of calcium peroxide, sodium borate perhydrate, and monopersulfate compound. Ether extractives and water extractives were examined, but in all cases too many of the oxidation products obtained were new and unidentifiable, and it was impossible to evaluate the experiments adequately with the available techniques. Vanillic acid appeared to be the chief oxidation product under conditions which did not demethylate further or destroy the aromatic nature of the oxidation products. Some oxidation conditions yielded p-hydroxybenzyl moieties as products, and some gave no trace of these products whatever. More detailed studies of the ether-insoluble, water-soluble components of the several oxidation mixtures were postponed until adequate procedures were developed for analytical isolation and identification. 

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