Lignins phenoxyl radicals

We found that when phenols (or lignins) are oxidized in alkaline media the initial step is a one-electron abstraction from the phenolic hydroxyl group forming a phenoxyl radical. We discovered an electrochemical method which allows us to determine roughly the free enthalpy of this particular reaction (phenol —> phenoxyl radical). This method has been... 

Such processes are always accompanied by a DP loss, either by electrophilic attack of ozone, by an ozone-catalyzed cleavage of the glycosidic bond or by attack of secondary radical species [15]. Residual lignin also plays a crucial role in ozone bleaching. Model studies showed that lignin with free phenolic hydroxyl groups accelerated carbohydrate oxidation, probably by activation of oxygen via phenoxyl radicals, whereas etherified phenolic model compounds had a protective effect [16,17]. 

Figure 2.10. The structure of lignin (Adler, 1977). (a) Building units of lignin, (b) Enzymatic dehydrogenation of coniferyl alcohol results in a number of resonance-stabilized phenoxyl radicals. Subsequent polymerization creates the variety of linkages between phenylpropane units that is characteristic of lignin, (c) Common substmctures and their proportions, as found in Picea abies and Betula verrucosa milled wood lignins. The proportion of individual bond types is noted beside each substructure for softwoods (S/w) and hardwoods (H/w).

Some potential substrates for CDH such as quinones and phenoxyl radicals are oxidation products of peroxidases and laccases formed during lignin degradation. This suggests the possibility of futile redox cycling where products of oxidation are sinq>ly reduced by CDH to produce the original substrate. It has... 

Unlike the peroxidation of the hydrocarbon polymers, the oxidation of lignin occurs by a stoichiometric process and not a chain reaction. Because phenols are antioxidants, the phenoxyl radicals formed are too stable to participate in a peroxidation chain reaction and the aromatic system is converted to quinoid compounds and ultimately humus. Both abiotic transition metal ion catalysed peroxidation and biological oxidation ire involved in the conversion of lignin to hiunus. 

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