Article pubs.acs.org/JPCA
Radicals in the Bray−Liebhafsky Oscillatory Reaction Dragomir R. Stanisavljev,* Maja C. Milenković, Ana D. Popović-Bijelić, and Miloš D. Mojović Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, P.O. Box 47, 11158 Belgrade, Serbia ABSTRACT: This study investigates the formation of free radicals in the Bray−Liebhafsky (BL) oscillatory reaction. The results indicate that radicals are produced during both monotonous and oscillatory dynamics observed as the change of the electron paramagnetic signal (EPR) of the spin-probe TEMPONE. EPR spin-trapping with DEPMPO suggested that the most abundant radical produced in the BL reaction is an iodine-centered radical. The EPR spectrum of the DEPMPO/ iodine-centered radical adducts has not been previously reported. This study may aid in establishing a more realistic reaction mechanism of the BL reaction and related chemical oscillators.
1. INTRODUCTION Despite the fact that the Bray−Liebhafsky (BL) oscillatory reaction has been known for almost a hundred years,1,2 its exact mechanism is still raveled. BL is one of the simplest chemical oscillators consisting initially of potassium iodate and hydrogen peroxide in acidic water solution. Because of its simple initial composition, it makes a good model system for understanding more complex biochemical oscillators, which may be involved in regulation of a number of important processes in living cells.3 As originally proposed,1,2,4,5 the dynamic of the whole reaction may be represented by the periodic domination of two complex processes: 2IO3− + 2H+ + 5H 2O2 = I 2 + 5O2 + 6H 2O
(1)
I 2 + 5H 2O2 = 2IO3− + H+ + 4H 2O
(2)
radicals may be obtained with specially designed chemicals able to make a characteristic interaction with unpaired electrons. In this article, we investigated the existence of radical intermediates in the BL reaction using electron paramagnetic resonance (EPR) spectroscopy. The first set of experiments involved the use of a well-known stable nitroxyl radical compound, the spin-label 4-oxo-2,2,6,6-tetramethylpiperidineN-oxyl (TEMPONE), and the second set included the use of the spin trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline Noxide (DEPMPO).17
2. EXPERIMENTAL SECTION The BL reaction was carried out in a 5.2 mL volume at 62 °C. The reaction mixture contained initially 7.20 × 10−2 M KIO3, 4.80 × 10−2 M H2SO4, and 4.07 × 10−1 M H2O2. Before and during the oscillatory evolution, the aliquots (60 μL) of the reaction mixture was extracted and immediately mixed with 6 μL of 1 mM TEMPONE. For the EPR spin trapping measurements, the spin-trap DEPMPO was used. The BL reaction was carried out in a small volume (204 μL) at 62 °C and initially contained 7.16 × 10−2 M KIO3, 4.68 × 10−2 M H2SO4, 9.65 × 10−2 M H2O2, and 50 mM DEPMPO. Twenty-seven minutes after the addition of DEPMPO, EPR spectra of a 60 μL aliquot was recorded. All chemicals were obtained from Merck, except DEPMPO from Enzo Life Sciences. The EPR spectra were recorded at room temperature (22 °C) using a Varian E104-A X-band ESR spectrometer (field center 3410 G, scan range 200 G, microwave frequency 100 kHz, modulation amplitude 2 G, microwave power 10 mW, and time constant 0.032 s). The spectra were processed by EW software (Scientific Software), and simulations were performed using Bruker WinEPR SimFonia software.
Process 1 represents reduction of iodate to iodine. After a certain period of time, the accumulated iodine is predominantly oxidized back to iodate during process 2. Periodic domination of the second process gives the oscillatory character to the whole BL reaction. Establishing the exact BL reaction mechanism has been shown to be a difficult task since the identification of all reaction intermediates closely relies on fast, sensitive, and selective experimental techniques. Consequently, the formation of free radicals in this peculiar reaction is still debated. Namely, there are opposing data that suggest that BL reaction could be modeled with6−10 or without11−15 steps involving free radicals. The redox chemistry of iodine and peroxide suggests that various radical intermediates, such as IO2•, IO•, I•, HO•, and HOO•, may be formed in the BL reaction. The existence of radicals in the BL system is only indirectly anticipated from the influence of assumed radical scavengers on the reaction dynamics.7,16 However, the possible reactions of the scavenger with the BL components was not considered and the changes in reaction dynamics may not be conclusively attributed to removing radicals from the system. The more direct evidence of © XXXX American Chemical Society
Received: March 8, 2013 Revised: April 1, 2013
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system, strong oxidizing free radicals with high standard reduction potentials21,22 (E0r(HO•/H2O) = 2.538 V, E0r(IO•/ IOH) = 2.3 V, E0r (I•/I−) = 1.33 V, and E0r (HOO•/H2O2) = 1.436 V) may easily oxidize TEMPONE. Although the one electron oxidation of TEMPONE with HIO is thermodynamically possible (due to the high value of the standard reduction potential E0r (HIO/(1/2I2)) = 1.35 V),22 it will probably proceed with the initial formation of iodine radical E0r (HIO/ I•) = 0.641 V,22 which is a thermodynamically unfavorable process for TEMPONE oxidation. Standard reduction potential of the couple E0r(IO2•/IO2H) may be only estimated to be 10 min) after which a detectable amount of EPR-active DEPMPO adduct is formed. The formed peroxy radicals in eq 5 are also good reducing agents (E0r(O2/HOO•) = −0.186 V),22 which can effectively reduce iodate, forming oxygen and additional IO2•:29
Figure 3. (a) Experimental and (b) simulated EPR spectrum of the DEPMPO/radical adduct formed in the BL reaction (later denoted as DEPMPO/BL adduct). Simulation was obtained using following hyperfine splitting constants: a(P) = 32.5 G, a(N) = 9.8 G, and a(H) = 12.5 G.
Since radicals may originate from all initially present components of the BL reaction (H2O2, H2SO4, and KIO3), further studies, related to detecting the main source of trapped radical, were performed. Spectral simulation confirmed that the DEPMPO/BL adduct obtained in the BL reaction does not arise from any oxygen centered radicals that may be formed from H2O2 since the DEMPO/BL adduct splitting constants, a(P) = 32.5 G, a(N) = 9.8 G, and a(H) = 12.5 G (Figure 3b), are markedly different from those reported in the literature for oxygen-centered radicals.26 To test if the source of radicals may be H2SO4 (since DEPMPO is able to form adducts with SO3−• anion radical),25 it was substituted in the BL reaction mixture with HClO4. The obtained EPR spectrum showed to be the same as the one in Figure 3a, indicating that the detected radical is not formed from H2SO4. However, the substitution of KIO3 with KClO3 or KBrO3 did not give rise to the characteristic DEPMPO/BL spectrum. These results indicate that the trapped BL radical originates from KIO3 and most probably is one of the iodine-centered radicals. It should also be pointed out here that the same EPR signal appeared during predominantly monotonous evolution (without oscillations) at lower initial peroxide concentration and temperature.
HOO• + IO3− + H+ → IO2• + O2 + H 2O
(6)
•
This reaction, together with the HOO recombination, could also explain formation of oxygen during reduction stages of the reaction and efficient removing of HOO•, which was not observed during spin-trapping. Although only one radical was trapped with DEPMPO, the production of other radicals, as a result of fast chain reactions, is possible as indicated in experiments with TEMPONE. Even though they are present in small amounts, because of their extreme reactivity, radicals may couple various processes and thus form important reaction intermediates.9
4. CONCLUSIONS The presented results strongly suggest the existence of free radicals in the mechanism of the BL reaction. The radicals are formed during the monotonous and the oscillatory dynamics C
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(17) Frejaville, C.; Karoui, H.; Tuccio, B.; Moigne, F.; Culcasi, M.; Pietri, S.; Lauricella, R.; Tordo, P. 5-Diethoxyphosphoryl-5-methyl-1pyrroline N-Oxide (DEPMPO): a New Phosphorylated Nitrone for the Efficient in Vitro and in Vivo Spin Trapping of Oxygen-Centred Radicals. J. Chem. Soc. Chem. Commun. 1994, 1793−1794. (18) Krishna, M. C.; Grahame, D. A.; Samuni, A.; Mitchell, J. B.; Russo, A. Oxoammonium Cation Intermediate in the NitroxideCatalyzed Dismutation of Superoxide. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 5537−5541. (19) Goldstein, S.; Mernyi, G.; Russo, A.; Samuni, A. The rol Eof Oxoammonium Cation in the SOD-Mimic Activity of Cyclic Nitroxides. J. Am. Chem. Soc. 2003, 125, 789−795. (20) Chapman, A.; Killian, G.; Gelerinter, E.; Jarrett, M. Alternations of Oxygen Uptake and the Redox State of Ubiquinone in Rabbit Sperm Exposed to a Variety of Physiologic Treatments. Biol. Reprod. 1985, 33, 859−869. (21) Stanisavljev, D. Consideration of the Thermodynamic Stability of Iodine Species in the Bray−Liebhafsky Reaction. Ber. Bunsenges. Phys. Chem. 1997, 101, 1036−1039. (22) Stanisavljev, D.; Milenković, M.; Mojović, M.; Popović-Bijelić, A. Oxygen Centered Radicals in Iodine Chemical Oscillators. J. Phys. Chem. 2011, 115, 7955−7958. (23) Schmitz, G. Inorganic Reactions of Iodine(III) in Acidic Solutions and Free Energy of Iodous Acid Formation. Int. J. Chem. Kinet. 2008, 40, 647−652. (24) Stanisavljev, R. D. Energy Dynamics in the Bray−Liebhafsky Oscillatory Reaction. J. Phys. Chem. A 2010, 114, 725−729. (25) Minisci, F., Ed. Free Radicals in Biology and Environment, Proceedings of the NATO Advanced Research Workshop, Bardolino, Italy, 21−26 June 1996; Kluwer Academic Publishers: The Netherlands, 1996. (26) Karoui, H.; Chalier, F.; Finet, J. P.; Tordo, P. DEPMPO: an Efficient Tool for the Coupled ESR-Spin Trapping of Alkylperoxyl Radicals in Water. Org. Biomol. Chem. 2011, 9, 2473−2480. (27) Morton, J. B.; Preston, K. F. Magnetic Properties of Free Radicals, Part a: Atom, Inorganic Radicals and Radicals in Metal Complexes. Landolt−Bornstein 1977, 9, 249−250. (28) Miyajima, N.; Akatsu, T.; Ikoma, T.; Ito, O.; Rand, B.; Tanabe, Y.; Yasuda, E. A Role of Charge Transfer Complex with Iodine in the Modification of Coal Tar Pitch. Carbon 2000, 38, 1831−1838. (29) Noyes, R. M.; Furrow, S. D. The Oscillatory Briggs−Rauscher Reaction. A Skeleton Mechanism for Oscillations. J. Am. Chem. Soc. 1982, 104, 45−48.
regimes indicated by the spin probe TEMPONE. On the basis of kinetic considerations, we assumed that the EPR spintrapping experiments by DEPMPO indicate the creation of IO2• radical whose spin-adduct has not been reported. This study of the Bray−Liebhafsky oscillator may contribute to a better understanding of its mechanism, and it also supports the free radical approach in describing detailed chemistry of other systems with similar dynamics.
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AUTHOR INFORMATION
Corresponding Author
*(D.R.S.) E-mail: dragisa@ffh.bg.ac.rs. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Ministry of Science of the Republic of Serbia under the project no. 172015 and III41005.
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REFERENCES
(1) Bray, W. C. A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis. J. Am. Chem. Soc. 1921, 43, 1262−1267. (2) Bray, W. C.; Liebhafsky, H. A. Reactions Involving Hydrogen Peroxide, Iodine and Iodate Ion. J. Am. Chem. Soc. 1931, 53, 38−44. (3) Goldbeter, A. Biochemical Oscillations and Celular Rhythms; Cambridge University Press: Cambridge, U.K., 2002. (4) Liebhafsky, H. A. Reactons Involving Hydrogen Peroxide, Iodine and Iodate Ion. III. The Reduction of Iodate Ion by Hydrogen Peroxide. J. Am. Chem. Soc. 1931, 53, 896−911. (5) Liebhafsky, H. A. Reactions Involving Hydrogen Peroxide, Iodine and Iodate Ion. IV. The Oxidation of Iodine to Iodate Ion by Hydrogen Peroxide. J. Am. Chem. Soc. 1931, 53, 2074−2090. (6) Degn, H. Evidence of a Branched Chain Reaction in the Oscillating Reaction of Hydrogen Peroxide, Iodine, and Iodate. Acta Chem. Scand. 1967, 21, 1057−1066. (7) Treindl, L.; Noyes, R. M. A New Explanation of the Oscillations in the Bray−Liebhafsky Reaction. J. Phys. Chem. 1993, 97, 11354− 11362. (8) Noyes, R. M. Some Models of Chemical Osillators. J. Chem. Educ. 1989, 66, 190−191. (9) Epstein, I. R.; Pojman, J. A. Waves and Patterns. In Introduction to Nonlinear Chemical Dynamics; Oxford University Press: New York, 1998. (10) Sharma, R.; Noyes, R. M. Oscillations in Chemical Systems. A Detailed Molecular Mechanism for the Bray−Liebhafsky Reaction of Iodate and Hydrogen Peroxide. J. Am. Chem. Soc. 1976, 98, 4345− 4361. (11) Schmitz, G. Cinetique de la Reaction de Bray. J. Chim. Phys. 1987, 84, 957−966. (12) Kolar-Anić, Lj.; Schmitz, G. Mechanism of the Bray-Liebhafsky reaction: Effect of the oxidation of iodous acid by hydrogen peroxide. J. Chem. Soc. Faraday Trans. 1992, 88, 2343−2349. (13) Schmitz, G. Kinetics and Mechanism of the Iodate Reaction and Other Related Reactions. Phys. Chem. Chem. Phys. 1999, 1, 1909− 1914. (14) Schmitz, G. Iodine Oxidation by Hydrogen Peroxide in Acidic Solutions, Bray−Liebhafsky Reaction and Other Related Reactions. Phys. Chem. Chem. Phys. 2010, 12, 6605−6615. (15) Schmitz, G.; Furrow, S. Kinetics of the Iodate Reduction by Hydrogen Peroxide and Relation with Briggs−Rauscher and Bray− Liebhafsky Oscillating Reactions. Phys. Chem. Chem. Phys. 2012, 14, 5711−5717. (16) Olexova, A.; Mrakavova, M.; Melichercik, M.; Treindl, L. The Autocatalytic Oxidation of Iodine with Hydrogen Peroxide in Relation to the Bray−Liebhafsky Oscillatory Reaction. Collect. Czech. Chem. Commun. 2006, 71, 91−106. D
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