Lignin free radical content

The free radical content in kraft lignin has been determined by Steelink (39) as about 0.0001 per OCH3. Even with a model e value of 200,000 as obtained for a tetra-/ T/-butylquinonemethide radical (P), only about 7% of the absorption of kraft lignin at 420 m/z would be explained. 

Ranby, B., Krinstad, K.P., Cowling, E.B., Lin, S. The free radical content in wood and lignins. Acta Chem. Scand. 23, 3257-3275 (1969)... 

These data indicate that mechanical, chemical, and biological attack of lignin create radical centers. We turned our attention next to an EPR analysis of chemically modified lignins. Reducing a number of samples (23) with NaBH4 seemed to have very little effect on the spin content. However, when samples were converted to their metal salts, a marked increase in free radical centers was observed. Acidification of the salts returned the spin content to its original level. 

Thus, a small concentration of ortho- or parabenzoquinone species in an environment of phenolic functions could explain the radical enhancement upon basification. The residual spin content of the neutral or acid form of lignin is almost nil in whole wood, very small in native lignins, but significant in kraft and other chemically modified lignins. Such a stable free radical could be attributed to (a) the small equilibrium concentration of I in Equation 1, (b) a semiquinone polymer patterned after synthetic models (4y 25) containing donor and acceptor groups, or (c) radicals entrapped and stabilized in a polymeric matrix (5,15). 

Free Radical Characteristics and Reactions in Weathered Wood. Wood, wood fiber components, and isolated lignin contain certain amounts of free radicals that are detectable by ESR spectroscopy (SS, S9). Unexposed green wood with 69% moisture content (in dark and in vacuo) was found (77a) to contain no free radicals. A trace amount of free radicals may be produced in the presence of oxygen, and most of these free radicals are generated in wood during mechanical preparation (90) as well as in wood exposed to electromagnetic irradiation. ESR studies revealed that wood interacts readily... 

To investigate the importance, not only of laccase mediators, but also of lacca-ses per se, several laccases were studied for the oxidation of the nonphenolic lignin dimer I. In the presence of the redox mediators 1-HBT or violuric acid, it was found that the oxidation rates of dimer I by the laccases differed considerably. In oxidation of dimer I, both 1-HBT and violuric acid were to some extent, consumed. The consumption rate followed the same order of laccases as the oxidation rates of dimer I. The oxidation rate of dimer I was found to be dependent on both k, jt and the stability of the laccase in question. Both 1-HBT and violuric acid inactivated the laccases— violuric acid to a greater extent then 1-HBT. The presence of dimer I in the reaction mixture slowed down this inactivation. Inactivation seems to be mainly due to the reaction of the redox mediator free-radical with the laccases. No relationship between the carbohydrate content of the laccases and their inactivation was found. When the redox potential of the laccases is in the range of 750-800 mV, i.e., about that of the redox mediator, a further increase in redox potential does not affect k(,jt and the oxidation rate of dimer I [147]. 

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