Reaction Selectivity in Electro-oxidation of Lignin Dimer Model Compounds and Synthetic Lignin with Different Mediators for the Laccase Mediator System (PZH, NHPI, ABTS)

Electro-oxidation of lignin dimer model compounds bearing beta-O-4, beta-5, and beta-beta linkages and synthetic lignin (guaiacyl-type dehydrogenation polymer [G-DHP]) in the presence of different mediators previously used in the laccase mediator system (LMS), that is, promazine hydrochloride (PZH), N-hydroxyphthalimide (NHPI), and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), was investigated. The characteristics of the electro-oxidations of the lignin dimer model compounds were reflected in the electro-oxidation of G-DHP. The three mediators effectively mediated the electro-oxidation of G-DHP, although they showed different reaction selectivities toward the three major lignin linkages of G-DHP. Specifically, PZH preferentially mediated the oxidation of beta-O-4 and beta-beta linkages over beta-5 linkages, NHPI preferred beta-O-4 linkages over beta-5 and beta-beta linkages, and ABTS preferred beta-O-4 and beta-beta linkages over beta-5 linkages under the present conditions. NHPI and ABTS tended to mediate C alpha-C beta cleavage in contrast to PZH, and the three mediators all mediated the conversion of coniferyl alcohol end-units. Consequently, PZH, NHPI, and ABTS were found to be promising mediators for the electrolytic mediator system of lignin.