ELECTROCHEMICAL ANALYSIS OF THE INTERACTIONS OF LACCASE MEDIATORS WITH LIGNIN MODEL COMPOUNDS

The mechanism of oxidation of lignin by laccase and mediator has been investigated by cyclic voltammetry and bulk electrolysis. Electrochemi cal properties and reactivities of the two mediators 2,2'-azinobis-(3- ethylbenzthiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HB T) were studied and their intermediates responsible for lignin oxidati on were characterized, ABTS was oxidized first to the cation radical ( ABTS(+.)) and then to the dication (ABTS(2+)). The two oxidized specie s are relatively stable and electrochemically reversible, with formal redox potentials (E-0' vs Ag/AgCl) of 472mV for ABTS/ABTS(+.) and 885m V for ABTS(+.)/ABTS(2+). The dication was shown to be the intermediate responsible for the oxidation of the non-phenolic lignin model compou nd veratryl alcohol, whereas the cation radical reacted only with phen olic structures in lignin. Cyclic voltammetry of HBT shows only one ox idation peak at 878 mV, but unlike ABTS, the oxidized intermediate was not stable and decayed rapidly. The radical intermediate of HBT was s hown 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 po tential of laccase (585 mV) was shown to be possible, but at a very sl ow rate, as previously reported for laccase and mediator. (C) 1998 Els evier Science B.V.