Reactivities of various mediators and laccases with kraft pulp and lignin model compounds

Abstract

Laccase-catalyzed oxygen delignification of kraft pulp offers some potential as a replacement for conventional chemical bleaching and has the advantage of requiring much lower pressure and temperature. However, chemical mediators are required for effective delignification by laccase, and their price is currently too high at the dosages required. To date, most studies have employed laccase from Trametes versicolor. We have found significant differences in reactivity between laccases from different fungi when they are tested for pulp delignification in the presence of the mediators 2,2-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HBT). A more detailed study of T. versicolor laccase with ABTS and HBT showed that HBT gave the most extensive delignification over 2 h but deactivated the enzyme, and therefore a higher enzyme dosage was required. Other mediators, including 1-nitroso-2-naphthol-3,6-disulfonic acid, 4-hydroxy-3-nitroso-1-naphthalenesulfonic acid, promazine, chlorpromazine, and Remazol brilliant blue, were also tested for their ability to delignify kraft pulp. Studies with dimeric model compounds indicated that the mechanisms of oxidation by ABTS and HBT are different. In addition, oxygen uptake by laccase is much slower with HBT than with ABTS. It is proposed that the dication of ABTS and the 1-oxide radical of HBT, with redox potentials in the 0.8-to 0.9-V range, are required for pulp delignification.

Degradation of a non-phenolic β-O-4 substructure and of polymeric lignin model compounds by laccase of Coriolus versicolor in the presence of 1-hydroxybenzotriazole

“…The benzylic positions of IIÎ V were highly oxidized, but a CK-carbonyl analog of I could not be detected in the reaction mixture. A con£icting result was also reported for the oxidation of a L-O-4 dilignol by the laccase^ABTS couple [11,22,23]. We conclude that the electron densities on the aromatic rings due to the electron donation of alkoxyl groups cause unexpected products.…”

“…The molecular weight distributions of soluble IR Clabeled lignins treated with laccase with or without 1-HBT were determined by scintillation counting of 1-ml GPC fractions that were collected. The fractions were assayed for IR C in a scintillation £uid [22] in a Packard Tri-Carb liquid scintillation analyzer Model 2500 TR.…”

Laccase-Mediated Oxidative Degradation of the Herbicide Dymron

“…As expected, the oxidation rate increased with the NNDS concentration ( Figure 9). NNDS can be easily oxidized by laccase, the oxidized NNDS does not exist stably in an aqueous solution, and the once oxidized NNDS probably is not recycled in the laccase/mediator system (19). This explains why excess amounts of the mediator were required for effective oxidation of dymron in the present study.…”

“…Up to now, many synthetic mediators were reported to facilitate laccase/mediator systems and selection of a mediator greatly affects the oxidation of nonphenolic compounds. We investigated four different mediators, ABTS, HBT (1-hydroxybenzotriazole), NNDS (1-nitroso-2-naphthol-3,6-disulfonic acid) (19), and violuric acid, for the degradation of dymron ( Figure 4). In all cases, dymron was successfully degraded by the laccase/mediator system.…”

Selective targeting of a laccase from Stachybotrys chartarum covalently linked to a carotenoid‐binding peptide

“…Laccases (EC 1. 10.3.2) are multi-copper phenol oxidases (7-9) capable of oxidizing a variety of aromatic compounds using redox-active small molecule 'mediators' (10) that shuttle electrons between the enzyme (acceptor) and target molecule (donor), with the concomitant reduction of O 2 as shown in eqn 1-3:…”