The effect of dissolved oxygen on kinetics during continuous biocatalytic oxidations

Conventional oxidation catalysts frequently exhibit limited selectivity, restricting their use in industrial syntheses. In particular, for the continuous synthesis of complex active pharmaceutical ingredients the high selectivity of biocatalytic oxidations is attractive. However, due to their dependence on gaseous molecular oxygen, which is poorly water soluble, such reactions are frequently limited by low dissolved oxygen concentrations. In order to better understand how oxygen-limitation influences the effectiveness with which an enzyme can be used, the continuous oxidation of glucose into gluconic acid, by glucose oxidase, was studied in a continuous stirred tank reactor. Results showed that a 3-fold increase in the oxygen content of the feed gas improved the reaction rate by twice as much as a 10-fold increase in enzyme concentration, confirming that the reaction is most sensitive to dissolved oxygen concentrations. Therefore, the enzyme could, on average, be used four times more effectively at an enzyme concentration of 0.1 g.L^-1 than at 1 g.L^-1, due to higher dissolved oxygen concentrations at steady state. However, at feed gas compositions greater than 60% oxygen, reaction rates began to drop due to enzyme deactivation. It was also found that the measurement of mass transfer coefficients is significantly affected by media composition, which may lead to inaccurate predictions of reaction rate.