Periodic and Bursting pH Oscillations in an Enzyme Model Reaction

pH-feedback systems to program autonomous self-assembly and material lifecycles

pH-responsive systems have gained importance for the development of smart materials and for biomedical applications because they can switch between different states by simple acid/base triggers. However, such equilibrium systems lack the autonomous behaviour that is so ubiquitous in living systems that self-regulate out of equilibrium. As a contribution to the emerging field of autonomous chemical systems, we have developed pH-feedback systems (pH-FS) based on the coupling of acid- and base-producing steps in chemical reaction networks. The resulting autonomous nonlinear pH curves can be coupled with a variety of pH-sensitive building blocks to program the lifecycles of the associated transient state at the level of self-assemblies and material systems. In this article, we discuss the different generations of such pH-feedback systems, the principles of their coupling to self-assemblies with lifecycles and highlight emerging concepts for the design of autonomous functional materials. The specificity, robustness, and flexible operation of such pH-FS can also be used to realize chemo-structural and chemo-mechanical feedbacks that extend the behaviour of such materials systems toward complex and functional life-like systems.

Bursting Oscillations in the Revised Mechanism of the Hemin – Hydrogen Peroxide – Sulfite Oscillator

The reaction mechanism for the hemin-mediated oxidation of sulfite by hydrogen peroxide was reinvestigated. This has become necessary due to an erroneous stoichiometric factor used during a previous numerical analysis. A simple correction of the previous model failed to reproduce the experimentally observed bursting oscillations of the pH value. Therefore, we propose an extended mechanism which includes not only the autocatalytic production of H+ and pH-dependent equilibria of hemin species, but also the decomposition of hemin in media of high H2O2 concentration. The dynamic properties of this 7-variable reaction mechanism are in good agreement with previous experimental and numerical observations. A closer analysis reveals that the pH-dependent dimerisation equilibrium of hemin provides only a minor contribution to the onset of oscillations. Therefore, this equilibrium can be neglected yielding a 6-variable version of the revised 7-variable model, while maintaining the local bifurcation structure. The revised mechanisms are considered realistic models that describe the observed dynamical features of the hemin-containing pH oscillator.

Designing an enzymatic oscillator: Bistability and feedback controlled oscillations with glucose oxidase in a continuous flow stirred tank reactor

The reaction of glucose with ferricyanide catalyzed by glucose oxidase from Aspergillus niger gives rise to a wide range of bistability as the flow rate is varied in a continuous flow stirred tank reactor. Oscillations in pH can be obtained by introducing a negative feedback on the autocatalytic production of H+ that drives the bistability. In our experiments, this feedback consists of an inflow of hydroxide ion at a rate that depends on [H+] in the reactor as k0[OH-]0[H+]/(K+[H+]). pH oscillations are found over a broad range of enzyme and ferricyanide concentrations, residence times (k0 (-1)), and feedback parameters. A simple mathematical model quantitatively accounts for the experimentally found oscillations.