Optimizing the chloroperoxidase–glucose oxidase system: The effect of glucose oxidase on activity and enantioselectivity

• The chloroperoxidase–glucose oxidase system was studied in the oxidation of thioanisole.
• The ratio of both enzymes influences product formation rate and enantioselectivity.
• An unexpected non-selective oxidation was found which was related to glucose oxidase.
• The non-selective oxidation depended on the concentration of co-solvent.
• The system affords the desired product with full conversion and high enantioselectivities.

The optimum application of chloroperoxidase from Caldariomyces fumago in oxidations with hydrogen peroxide depends on the mode of addition of the oxidant. The use of the previously reported combination of chloroperoxidase and glucose oxidase, for in situ generation of hydrogen peroxide, was studied in more detail using thioanisole as a model substrate. Maximum yields and enantiopurities were observed at high chloroperoxidase reaction rates and not at low hydrogen peroxide formation rates, as would be expected considering the instability of CPO at high hydrogen peroxide concentrations. Glucose oxidase catalyzed aerobic sulfoxidation, affording racemic sulfoxide, was observed as an unexpected and novel side-reaction. It was attributed to oxidation by a flavin hydroperoxide formed by reaction of the free flavin cofactor associated with glucose oxidase with dioxygen. The rate of this side-reaction depended on the amount of co-solvent in the system and the enantiopurity of the oxidation product could thus be improved by lowering the co-solvent concentration.

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