КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 5, с. 560-567
УДК 541.128:546.712:547.455.643
INTERACTION OF SOME ANTIOXIDANTS WITH BELOUSOV-ZHABOTINSKY REACTION BASED ON CATECHOL-BrO--Mn2+-H2SO4 SYSTEM © 2013 I. A. Shah, G. M. Peerzada*, N. Bashir
Department of Chemistry, University of Kashmir Srinagar, India *E-mail: peerzada_gmp@yahoo.co.in Received 17.02.2012
Temporal evolution of a new Mn(II) catalyzed Belousov—Zhabotinsky (BZ) chemical oscillator with catechol (1,2-dihydroxybenzene) as organic substrate is reported within narrow range of concentrations of initial reagents at 30°C. After optimizing the oscillation parameters the system was perturbed with the antioxidants like ascorbic acid and inosine. It is found that ascorbic acid acts as co-substrate within certain concentration limit, whereas inosine acts as a quencher of oscillations. Addition of ascorbic acid to the BZ system decreases induction time thus acting synergistically to help the reaction to enter quickly into the oscillatory regime. A good linear dependence of induction time on the concentration of ascorbic acid (R2 = 0.9948) and inosine (R = 0.955) is reported. Inosine has been found to increase the induction time and quench the oscillations. It is mentioned that the magnitude of induction time decreases to a greater extent with ascorbic acid as compared to the magnitude of its increase with the same concentration of inosine. This is pointing to the fact that ascorbic acid is stronger antioxidant than inosine as depicted by their interaction with catechol-based BZ chemical oscillator. Temporal evolution of the BZ reaction with the injection of antioxidants at different stages of reaction is also reported.
DOI: 10.7868/S0453881113050158
Oscillations and dynamic phenomena are a rule rather than exception in biosystems. These are also familiar in mechanical systems and electrical circuits. Direction of motion of an object or an electric current may repeatedly reverse itself with or without damping amplitude of oscillations and repetitive standing or travelling waves may be generated in a continuous medium. Chemical systems are less prone to oscillations, and their evolution usually leads to monotonic change in chemical parameters [1]. However, it is well established that oscillations do occur in some chemical systems. These oscillatory chemical systems belong to the category having nonlinear dynamics. The similarity between life processes that exhibit oscillatory behavior and the oscillating chemical systems suggest that the biological and non-biological systems conform to the same law. The chemical oscillatory systems represent the scaled-down version of their biological analogues.
Basically, an oscillating chemical reaction is the one where some chemical species such as reaction intermediates exhibit fluctuations, periodic or aperiodic depending upon the reaction conditions [2]. For a chemical reaction to be the source of an auto-oscillatory system, at least the following requirements must be met [3]: 1) the system should be far from thermodynamic equilibrium, 2) there should be at least one au-
tocatalytic step, 3) the system should possess at least two steady states under initial conditions. The chemical oscillatory reactions are complex that have been primarily examined in physio-chemical terms with a view to elucidate the intricate underlying mechanisms rather than from the application point of view. Many authors have presented different models to explain the mechanistic details of these complex phenomena. Development of a reasonable and well supported mechanism for the Belousov—Zhabotinsky (BZ) reaction [4—14] provided an intellectual frame work for the experimental and theoretical investigation of bromate driven chemical oscillators. The dynamics of these reactions was studied in closed systems and chemical oscillations in these systems can be observed for a limited time, until thermodynamic equilibrium is achieved. However, with the introduction of continuous-flow, stirred tank reactors (CSTR) and the pulse perturbation technique [15], many fundamental principles of chemical oscillations came to surface with better mechanistic understanding of the chemical oscillatory reactions. It also helped to design and monitor new chemical oscillators and opened new avenues for oscillatory reactions in chemical analysis. As a result, there has been a gradual shift from theoretical to practical approaches. The first paper that considered the
use of chemical oscillations for analytical potential was published by Tikhonova et al. in 1978 [16]. Since then some efforts have been made to probe the analytical potential of oscillatory chemical reactions [17, 18]. Analyte pulse perturbation technique (APP) is employed to determine the trace amounts of analyte by perturbing the oscillation pulse. The determination of the analyte depends upon its interaction with the oscillatory reaction and the change in some oscillation parameter is correlated to the concentration of the analyte. Many substances have been determined in real samples [15, 17, 18].
In modeling reaction mechanism of oscillatory chemical reaction, two common approaches are available. These include radical based and non-radical based [8, 19—24] pathways. The existence of these transient species is anticipated directly by some electro-analytical techniques or else by the use of pulse perturbants. Antioxidants can act as perturbants of an oscillation pulse and can interact with both radical as well as non radical intermediate species. Antioxidant compounds in food play an important role as a health-protecting factor. Scientific evidences suggest that antioxidants reduce the risk for chronic diseases including cancer and heart disease. The various antioxidants like vitamin C, vitamin E, carotenes, phenolic acids, phytate and phytoestrogens obtained from plants have been recognized as having the potential to reduce disease risk. Some compounds such as gallates, have strong antioxidant activity, while others such as monophenols are weak antioxidants. Use of antioxi-dants as pulse perturbants has many fold applications. These can be helpful in establishing indirectly the mechanism of the interacting oscillatory chemical reaction, hence approving or disapproving the presence of a particular reaction intermediate. Using structurally related antioxidants on the same oscillatory chemical reaction will be helpful in evaluation of their antioxidant activity [25, 26] under the given reaction conditions. This method of the evaluating antioxidant activity would be easier, sensitive, economic and highly precise as it involves much lower detection limits. The interaction of an antioxidant with a particular oscillatory chemical reaction will help in understanding its chemistry and hence its role in biological systems. This would also be beneficial in understanding the mechanism of some diseases in our body.
In the present investigation the optimal conditions of catechol-based BZ chemical oscillator have been worked out. Catechol has sufficient solubility in aqueous acids and exhibits nonlinear behavior as a substrate in BZ reaction over a narrow range of its concentration. The compound is present in the living systems as a part or an important constituent in the biomolecules such as antioxidants (flavanoides), hormones (dopamine, epinephrine, norepinephrine) and in many stimulant drugs such as catecholamines. It is
also a potential toxic substance and poisons biomole-cules like urease. This catechol-based BZ system was then perturbed with antioxidants such as ascorbic acid and inosine and its temporal evolution was studied po-tentiometrically. The interaction of antioxidants with BZ systems based on substrates or the derivatives found in living systems would help one to understand the mechanism of their interaction, one may arrive at certain conclusions regarding the role of such interactions in living systems. This investigation has a two pronged approache: (i) to establish the order of reactivity of the aforesaid antioxidants, (ii) to visualize the effect of antioxidants on the oscillatory behavior of BZ reaction.
EXPERIMENTAL
All chemicals used were of analytical grade with high degree of purity and were used without further purification. The reagents used were catechol (99%, "Qualigens"), potassium bromate (99.6%, "Merck"), manganese(II) sulphate monohydrate (98%, "BDH"), sulfuric acid (98%, "Merck"), ascorbic acid (99.0%, "Sigma-Aldrich"), and inosine (99.0%, "Sigma-Ald-rich"). The voltage from the BZ oscillator was recorded by the Dual channel/pH/ion conductivity meter PC 5500 ("Eutech Instrument"). The dynamics of the reaction was monitored with a shiny platinum electrode and a saturated calomel electrode (SCE) as indicator and reference electrode respectively. One half-cell contained the reaction mixture under investigation with platinum electrode dipped into it and another half-cell contained potassium chloride solution with SCE dipped into it. The two half-cells were connected through a salt bridge containing saturated solution of potassium nitrate. The temperature of the reaction mixture was maintained by a high precision Serological Water Bath maintained at 30 ± 0.1°C. All the solutions used were first kept under thermostatic conditions for 20 min to acquire the desired temperature. The 2 mL each of Mn2+ ion solution and catechol were mixed in one reaction cell and the reaction started by injecting 2 mL of potassium bromate into the reaction mixture. No stirring was done to the reaction mixture. For each trial the electrodes and the salt bridge were washed thoroughly with double distilled water. The redox potential was recorded from the time BrO- ion solution has been added to reaction mixture. Ascorbic acid and inosine were used as perturbants and their small volumes (0.2 mL) of different concentrations were injected at different stages of the reaction.
RESULTS AND DISCUSSION
BZ reaction ha
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