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Periodic and Bursting pH Oscillations in an Enzyme Model Reaction

Oscillatory behaviour in the pH value was observed during the oxidation of sulfite by hydrogen peroxide mediated by hemin in a continuous-flow stirred tank reactor. The dynamics of this reaction was studied for a variety of flow rates of the reactants. As the flow rates increase, the oscillations evolve from relaxation oscillations to more complex shapes, displaying, among others, bursting behaviour. A reaction mechanism is proposed that involves the autocatalytic oxidation of HSO3- by H2O2, while equilibria between pH-dependent forms of hemin account for the consumption of protons. These equilibria provide the feedback steps which give rise to oscillatory dynamics. The contributions of the individual H+-consuming reactions to the dynamic behaviour are evaluated. Numerical simulations show that two pH-dependent reactions suffice to obtain bursting oscillations which resemble those observed in the experiments.