Reappraising a Controversy: Formation and Role of the Azodication (ABTS2+) in the Laccase-ABTS Catalyzed Breakdown of Lignin

In fermentations of lignocelluloses, redox potentials (If not indicated otherwise, redox potentials in Volt are taken versus Normal Hydrogen Reference Electrodes (NHE).) E0 of laccases/plant peroxidases by 0.79/0.95 V enable oxidations of phenolic substrates and transformations of synthetic and substrate-derived compounds to radicals that mediate attacks on non-phenolic lignin (models) by 1.5 V. In consecutive one-electron abstractions, the redox mediator 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) is oxidized by electroor wet-chemistry to the green cation radical (ABTS•+, 0.68 V) and the red dication (ABTS2+, 1.09 V). The enzyme/ABTS couple generates the stable ABTS•+ whose low E0 cannot explain the couple’s contemporary attack on non-phenolic lignins. This paradoxon indicates the non-confirmed production of the ligninolytic ABTS2+ by the enzymes. During incubations of live sapwood chips in ABTS/H2O2 to prove their constitutive peroxidase, the enzyme catalyzed the formation of the expected green-colored ABTS•+ solution that gradually turned red. Its spectrophotometric absorbance peaks at λ = 515–573 nm resembled those of ABTS2+ at 518–520 nm. It is shown that portions of an enzyme-free ABTS•+ preparation with inactivated enzyme are reduced to ABTS during their abiotic oxidation of low-MW extractives from lignocelluloses to redox mediating radicals. The radicals, in turn, apparently transform the remaining ABTS•+ to red derivatives in the absence of functional oxidoreductases. Ultrafiltration and Liquid-Chromatography suggest the presence of a stable ABTS2+ compound absorbing at 515 nm, red protein/ABTS adducts, and further ABTS moieties. Therefore, ABTS mediated lignin degradations could result from chain reactions of ABTS•+-activated lignocellulose extractives and fissured rather than complete ABTS2+ molecules.