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Cellobiose dehydrogenase (CDH) is an extracellular redox enzyme of ping-pong type, i.e. it has separate oxidative and reductive half reactions. Several wood degrading fungi produce CDH, but the biological function of the enzyme is not known with certainty. It can, however, indirectly generate hydroxyl radicals by reducing Fe(3+) to Fe(2+) and O2 to H2O2. Hydroxyl radicals are then generated by a Fenton type reaction and they can react with various wood compounds, including lignin. In this work we study the effect of CDH on a non-phenolic lignin model compound (3,4-dimethoxyphenyl glycol). The results indicate that CDH can affect lignins in three important ways. (1) It breaks beta-ethers; (2) it demethoxylates aromatic structures in lignins; (3) it introduces hydroxyl groups in non-phenolic lignins. The gamma-irradiated model compound gave a similar pattern of products as the CDH treated model compound, when the samples were analyzed by HPLC, suggesting that hydroxyl radicals are the active component of the CDH system.

The mechanism of oxidation of lignin by laccase and mediator has been investigated by cyclic voltammetry and bulk electrolysis. Electrochemical properties and reactivities of the two mediators 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HBT) were studied and their intermediates responsible for lignin oxidation were characterized. ABTS was oxidized first to the cation radical (ABTS+.) and then to the dication (ABTS2+). The two oxidized species are relatively stable and electrochemically reversible, with formal redox potentials (E0' vs Ag/AgCl) of 472 mV for ABTS/ABTS+. and 885 mV for ABTS+./ABTS2+. The dication was shown to be the intermediate responsible for the oxidation of the non-phenolic lignin model compound veratryl alcohol, whereas the cation radical reacted only with phenolic structures in lignin. Cyclic voltammetry of HBT shows only one oxidation peak at 878 mV, but unlike ABTS, the oxidized intermediate was not stable and decayed rapidly. The radical intermediate of HBT was shown to catalyze the oxidation of veratryl alcohol to veratraldehyde. The kinetics of homogeneous redox catalysis of mediators and veratryl alcohol were estimated. ABTS-mediated lignin oxidation at the redox potential of laccase (585 mV) was shown to be possible, but at a very slow rate, as previously reported for laccase and mediator.