Lignin decomposition products

Figure 1. Structure of identified lignin decomposition products...

Guaiacylglycerol-/3-coniferyl ether is a dimeric decomposition product of coniferous lignin dehydrodivanillin may be also formed by dimerization of vanillin after formation from lignin. Among the lignin decomposition products are mono-, di- and triphenol derivatives which are derived from the different types of lignin of needle trees, deciduous trees, or graminees. 

In Figure 2 the further reactions of the lignin decomposition products, which involve biological oxidation in the soil or in cultures or microorganisms, are summarized. All of the secondary reaction products could be identified. 

In order to eluddate the mechanism of the further transformations of the primary lignin decomposition products including the cleavage reactions, we synthesized some of the important primary products labelled with Cu and introduced these into the cultures of fungi or enzymes. Thus, it could be shown, for example, that the breakdown of the side chain of ferulic acid occurs at the double bond vanillic acid is found. During polymerization in the presence of phenoloxidases, in the case of carboxyl-labelled ferulic acid, about 60% of the activity is split off as Cli02. The polymers labelled in the 2 and 3 position in the side chain or in the methoxyl group contain the whole applied activity. 

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... 

Dr. Flaig. We could show that, by oxidative polymerization of lignin decomposition products with side chains containing one or more carbon atoms, polymer-like humic substances are formed. The polymerization can be carried out with mild oxidizing agents or in the presence of phenoloxydases. 

Dr. Flaig. The mechanism of the reaction between amino acids and oxidized lignin decomposition products can be described by the scheme of Trautner, which involves oxidative deamination. 

Even-Haim (1966) states that plants are capable of absorbing different organic molecules via roots, and make use of growth factors contained in humified organic molecules, but direct absorption by the plant of humus constituents or lignin decomposition products has never been demonstrated with absolute certainty. Using labelled vanillic acid he demonstrated that under sterile conditions wheat roots absorbed very small amounts of this substance but it was not found in the leaves. Likewise, Fiihr and Sauerbeck (1966) found that sunflower roots took up only traces of C-labelled humic acids and related compounds. 

The essential reactions (5) during the further transformations of the decomposition products of lignin are ... 

Dr. Flaig. The polymeric products of lignin decomposition formed in the presence of phenol oxidases are not water soluble. 

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