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 β–O–4, β–5, and β–β 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 β–O–4 and β–β linkages over β–5 linkages, NHPI preferred β–O–4 linkages over β–5 and β–β linkages, and ABTS preferred β–O–4 and β–β linkages over β–5 linkages under the present conditions. NHPI and ABTS tended to mediate Cα–Cβ 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.