Krisztina Kurin-Csörgei

Systematic design of chemical oscillators using complexation and precipitation equilibria.

Concentration oscillations are ubiquitous in living systems, where they involve a wide range of chemical species. In contrast, early in vitro chemical oscillators were all derived from two accidentally discovered reactions based on oxyhalogen chemistry. Over the past 25 years, the use of a systematic design algorithm, in which a slow feedback reaction periodically drives a bistable system in a flow reactor between its two steady states, has increased the list of oscillating chemical reactions to dozens of systems. But these oscillating reactions are still confined to a handful of elements that possess multiple stable oxidation states: halogens, sulphur and some transition metals. Here we show that linking a ‘core’ oscillator to a complexation or precipitation equilibrium can induce concentration oscillations in a species participating in the equilibrium. We use this method to design systems that produce periodic pulses of calcium, aluminium or fluoride ions. The ability to generate oscillations in elements possessing only a single stable oxidation state (for example, Na+, F-, Ca2+) may lead to reactions that are useful for coupling to or probing living systems, or that help us to understand new mechanisms by which periodic behaviour may arise.

Generation of pH-Oscillations in Closed Chemical Systems: Method and Applications

All pH-oscillators reported to date function only under open (flow reactor) conditions. We describe an approach to generating pH-oscillations in a closed system by starting from an open system pH-oscillator, finding semibatch conditions under which it oscillates with an inflow of a single reactant to an otherwise closed reactor containing the remaining components, and replacing this inflow with a layer of silica gel impregnated with the key reactant. We present data showing the successful application of this technique to the BrO(3)(-)-Mn(2+)-SO(3)(2-), IO(3)(-)-Fe(CN)(6)(4-)-SO(3)(2-), and BrO(3)(-)-Fe(CN)(6)(4-)-SO(3)(2-) systems. In all three cases, sulfite ion is the species that is replenished via dissolution from the gel layer.

pH-Regulated Chemical Oscillators

The hydrogen ion is arguably the most ubiquitous and important species in chemistry. It also plays a key role in nearly every biological process. In this Account, we discuss systems whose behavior is governed by oscillations in the concentration of hydrogen ion. The first chemical oscillators driven by changes in pH were developed a quarter century ago. Since then, about two dozen new pH oscillators, systems in which the periodic variation in pH is not just an indicator but an essential prerequisite of the oscillatory behavior, have been discovered. Mechanistic understanding of their behavior has grown, and new ideas for their practical application have been proposed and, in some cases, tested. Here we present a catalog of the known pH oscillators, divide them into mechanistically based categories based on whether they involve a single oxidant and reductant or an oxidant and a pair of reductants, and describe general mechanisms for these two major classes of systems. We also describe in detail the chemistry of one example from each class, hydrogen peroxide-sulfide and ferricyanide-iodate-sulfite. Finally, we consider actual and potential applications. These include using pH oscillators to induce oscillation in species that would otherwise be nonoscillatory, creating novel spatial patterns, generating periodic transitions between vesicle and micelle states, stimulating switching between folded and random coil states of DNA, building molecular motors, and designing pulsating drug delivery systems. We point out the importance for future applications of finding a batch pH oscillator, one that oscillates in a closed system for an extended period of time, and comment on the progress that has been made toward that goal.

The 1,4-cyclohexanedione-bromate-acid oscillatory system II. Chemical waves

In the 1,4-cyclohexanedione (CHD)-bromate-sulfuric acid system when spread into a thin layer and ferroin is added, after an induction period, trigger waves start to propagate with a velocity of about a few mm/min. Since gaseous compounds do not form in the reaction of CHD with bromate, bubbles do not appear in the reaction zone and thus very sharp leading edges of the oxidizing wave fronts are observable.

Mechanistic studies of oscillatory copper(II) catalyzed oxidation reactions of sulfur compounds

Abstract Trace amounts of copper ion catalyst induce exotic phenomena in the oxidation of several inorganic sulfur compounds by peroxides in aqueous solution. Simple and complex oscillations and several kinds of bistability are observed when the copper(II)-catalyzed oxidation of S 2 O 3 2− by either H 2 O 2 or S 2 O 8 2− is carried out in a CSTR and when SCN − ions are oxidized with H 2 O 2 in the presence of copper ions under either batch or flow conditions. For the S 2 O 3 2− –H 2 O 2 –Cu(II) reaction, a four-step model is proposed, in which formation of the intermediate HOS 2 O 3 − and attack on that species by S 2 O 3 2− and H 2 O 2 play key roles. When this core of reactions is extended with additional steps, computer simulations yield good agreement between the experimentally observed and calculated pH oscillations, bistability and batch behavior. In the oscillatory S 2 O 3 2− –S 2 O 8 2− –Cu(II) flow reaction, Cu(I), Cu(II) and Cu(III) species as well as SO ⋅ − 4 and S 2 O ⋅ − 3 are postulated to participate in a free radical mechanism, which successfully simulates the oscillations. To model the experimentally observed oscillations and bistability in the H 2 O 2 –SCN − –Cu(II) system, we have proposed a complex mechanism involving 30 reactions and 26 independent species.

Model for the oscillatory reaction between hydrogen peroxide and thiosulfate catalysed by copper(II) ions

A mechanistic model is presented for the CuII-catalysed oxidation of thiosulfate by hydrogen peroxide, which gives oscillatory behaviour in a flow reactor. A simple four-step model, in which formation of the intermediate HOS2O3– and attack on that species by H2O2 and S2O32– play key roles, suffices to give oscillatory behaviour. By extending this core set of reactions with additional steps that describe acid–base equilibria and reactions between H2O2 and sulfur species, we obtain better agreement with the observed oscillatory behaviour as well as the ability to simulate the observed batch behaviour and the bistability and complex multi-peaked oscillations found in flow systems. Our results suggest that mechanistic descriptions of sulfur-containing hydrogen peroxide oscillators should emphasize the capability of the sulfur substrate to be partially oxidized to a relatively stable intermediate prior to its total oxidation to sulfate.

The 1,4-cyclohexanedione-bromate-acid oscillatory system I. Its organic chemistry

The organic chemistry of the 1,4-cyclohexanedione (CHD)-bromate-sulfuric acid oscillatory system has been revealed by following the reaction of 1,4-CHD with bromate using a GC/MS technique. We could identify 1,4-dihydroxybenzene as an intermediate, 1,4-benzoquinone as the main oxidation, and mono- and dibromocyclohexanedione as the main bromination products. Acid bromate does not cleave the alicyclic ring.

Oscillations in the Concentration of Fluoride Ions Induced by a pH Oscillator

Sustained oscillations in the concentration of free fluoride ions can be generated when the BrO3--SO32--Mn(II) oscillator is coupled either to Al3+-F- complex formation or to the Ca2+-F- precipitation process in a flow reactor. By careful analysis of the relevant equilibria and optimization of the reactant concentrations, one can obtain [F-] oscillations of several orders of magnitude as measured with an ion-selective electrode. The BrO3--SO32--Mn(II)-Al(NO3)3-NaF system also exhibits bistability, that is, simultaneously stable steady states of high and low [F-].

A new chemical system for studying pattern formation: Bromate–hypophosphite–acetone–dual catalyst

A modified version of the short-lived BrO3(-)-H2PO2(-)-Mn(II)-N2 oscillator, the BrO3(-)-H2PO2(-)-acetone-dual catalyst system, where the catalyst pair can be Mn(II)-Ru(bpy)3SO4, or Mn(II)-ferroin, or Mn(II)-diphenylamine, shows long-lasting batch oscillations in the potential of a Pt electrode and in colour, accompanying periodic transitions between the oxidised and reduced forms of the catalysts. Experimental conditions for the oscillations are established. The origin of the batch oscillations and the role of the catalyst pair in the oscillatory behaviour are discussed. The new system is ideally suited to the study of waves and patterns in reaction-diffusion systems, since in addition to the longevity of its spatial behaviour in batch, it produces no gaseous or solid products and exhibits significant photosensitivity.

Temperature-triggered chemical oscillators. A peculiar temperature effect in perturbed uncatalyzed bromate-driven reactions

Some uncatalyzed bromate oscillators with p-substituted phenol derivatives as the organic substrates when perturbed with chloride ions show a remarkable behavior, namely, a sudden transition, at a well-defined threshold temperature, from the non-oscillatory to the oscillatory state, and back. The high sensitivity of the reacting systems towards chloride ion perturbation and the conditions for the occurrence of the phenomenon are described.