A. Potthast, T. Rosenau, K. Fischer
Oxidation of Benzyl Alcohols by the Laccase-Mediator System (LMS) a Comprehensive Kinetic Description
Summary
Investigations into the reaction kinetics of the laccase-mediator system (LMS) have been carried out.
Two widely used mediators, 2,2?-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS, 3) and 1-hydroxybenzotriazole (HOBT, 4), were compared by means of a model reaction, the oxidation of 2,4-dimethoxybenzyl alcohol (DMBA, 1) to 2,4-dimethoxybenzaldehyde (DMA, 2). The consumption of
dioxygen was recorded electrochemically, substrate consumption and product formation were monitored
by GLC.
With ABTS as the mediator, the LMS reaction proceeded in two clearly distinguishable stages. The
first phase is characterized by a fast decrease in oxygen with zero-order kinetics and no detectable formation
of 2,4-dimethoxybenzaldehyde (2). ABTS is converted into oxidized species, the cation radical
6 and the dication 7, respectively. In the second phase, oxygen consumption was considerably slower
and followed a second-order kinetics, while the benzaldehyde was produced according to a zero-order
rate law. According to the kinetic studies, the ABTS dication, but not the enzyme itself, is acting as the
actual oxidant. The rate of oxidation product formation increased with increasing mediator / benzyl alcohol
ratio. Less oxygen than the equivalent amount was consumed in the second reaction stage indicating
that the oxidized ABTS formed in the first stage acts as an oxidant reservoir, being reduced to ABTS in
turn.
The LMS reaction with HOBT (4) as the mediator did not exhibit distinguishable phases, and was
characterized by a comparatively slow oxygen uptake with zero-order kinetics throughout. Enzymatic
oxidation of HOBT to the HOBT radical (5), which acts as the actual oxidant towards the benzyl alcohol,
was the rate-determining step. The production of 2,4-dimethoxybenzaldehyde thus followed a zeroorder
rate law as well. The reaction rate increased with increasing HOBT / benzyl alcohol ratios. Increasing
concentrations of 4 caused less oxygen to be consumed per equivalent of benzaldehyde formed, indicating
the occurrence of another reaction pathway at high mediator charges. At low HOBT / benzyl alcohol
ratios the HOBT radical (5) acts as one-electron oxidant and is reduced to HOBT in a reversible
process. In contrast, at higher HOBT / benzyl alcohol ratios 5 acts as a three-electron oxidant, being irreversibly
reduced to benzotriazole. At commonly employed mediator concentrations, a superposition of
both mechanisms results. The pure borderline cases can only be observed at HOBT / benzyl alcohol
ratios below 1 and above 6, respectively.
Holzforschung, Walter de Gruyter
Print ISSN: 0018-3830
Volume: 55, 12/2001
Pages: 47 - 56
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