No abstract

Feedback driven by enzyme catalyzed reactions occurs widely in biology and has been well characterized in single celled organisms such as yeast. There are still few examples of robust enzyme oscillators in vitro that might be used to study nonlinear dynamical behavior. One of the simplest is the urea-urease reaction that displays autocatalysis driven by the increase in pH accompanying the production of ammonia. A clock reaction was obtained from low to high pH in batch reactor and bistability and oscillations were reported in a continuous flow rector. However, the oscillations were found to be irreproducible and one contributing factor may be the lack of stability of the enzyme in solution at room temperature. Here, we investigated the effect of immobilizing urease in thiol-poly(ethylene glycol) acrylate (PEGDA) hydrogel beads, prepared using emulsion polymerization, on the ureaurease reaction. The resultant mm-sized beads were found to reproduce the pH clock and, under the conditions employed here, the stability of the enzyme was increased from hours to days.

No abstract

An intriguing step of the Bray−Liebhafsky oscillatory reaction, that is, iodine oxidation with hydrogen peroxide, was examined. This process is characterized by enormous stochasticity of reaction induction times in the presence of mild mixing. In the present investigations, it was found that even in the presence of mixing, stochasticity is seriously reduced with simple glass powder addition. Using transparent glass, standard deviations changed from 184.9 to 10.6 min for 19 °C, and from 128.2 to 1.9 min for 27 °C. Repeating experiments with amber glass further confirmed those results as standard deviations changed from the mentioned ones to 8.7 min for 19 °C and to 2.5 min for 27 °C. Inert glass particles enhanced heterogeneous nucleation of oxygen formed in chemical reactions. Together with the previous analysis of the involved kinetic barrier of the whole reaction, it is an additional strong evidence of a possible energetic coupling between nucleation of glass cavities and chemical reactions. Such processes are usually neglected and may considerably change the investigations of the reaction mechanisms in other complex systems. Reaction was followed by the potentiometric method at 19 and 27 °C. The adjusted stopped-flow titration method was used in order to confirm the high degree of iodine to iodate conversion in the presence of nucleation centers.

A physicochemical model of iodine oxidation with hydrogen peroxide is extended by heterogeneous processes, conceptually improving the understanding of the reaction mechanism.