PhD Thesis. Investigation of the oxidation of iron(iii) complexes of porphyrin derivatives by bromate. Dénesné Rácz Krisztina

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1 PhD Thesis Investigation of the oxidation of iron(iii) complexes of porphyrin derivatives by bromate Dénesné Rácz Krisztina University L. Eötvös, Institute of Chemistry 2008

2 I. Introduction Nonlinear phenomena such as temporal oscillations and spatial pattern formation have been described in a lot of biological and biochemical processes. Well-known examples are the oscillating glycolysis, the peroxidase-oxidase reaction and excitation waves in cardiac tissues or yeast extract. All of these phenomena are very complex which makes the fundamental understanding of their mechanism very difficult. On the contrary, simple oscillating chemical reactions like the Belousov-Zhabotinsky (BZ) reaction are easier to study in detail. Such simple chemical oscillations most often operate under unphysiological conditions, but their general properties are in many respects similar to the characteristics of complicated phenomena in living organism and can be used to extract information for bioprocesses, too. Therefore, there is a growing interest in systems with closer connections to bio-oscillations. One of the ways to create biomimetic model systems from simple chemical oscillators is to replace their components by compounds of biological importance. Such attempts are the modifications of the BZ reaction with malic and ascorbic acid as organic substrates or manganese as a catalyst, or the replacement of hexacyano ferrate ion by hemin in the H 2 O 2 SO Fe(CN) 6 system. Years ago, it was tried to replace ferroin, the most frequently used catalyst in the BZ reaction by porphyrin complexes of iron, for example hemin. However, several trials to apply porphyrine complexes or related compounds in the BZ reaction have been unsuccessful, mainly because of their poor solubility in polar solvents and their sensitivity against acidic media and generally to unphysiological conditions. Therefore, it is furtheron an open question whether it is possible to create nonlinear behaviour in systems containing compounds of such or similar chemical structure. In my PhD work, I studied the reaction of two porphyne derivatives, hemin and the iron complex of meso-tetra(4-sulfonatophenyl)porphine with bromate ions. Consequently, my work and the thesis consist of two main parts. The first part discusses the reaction between hemin and bromate. However, hemin is not stable in the acidic medium, necessary to the oxidation by bromate. This fact made the experiments difficult and enabled only the study of a narrow concentration region. To overcome this problem, we were surching for an iron complex of a similar porphyrin derivative which could react with bromate in a similar way but being acid-resistent. This compound is iron(iii) meso-tetra(4-sulfonatophenyl)porphine 2

3 chloride (Fe III TPPS). In both cases, the aim of my work was to study and clear up the kinetics and the mechanism of the reactions. II. Experimental TPPS and Fe III TPPS were prepared by the method of Gonsales and Srivastava, hypobromous acid was prepared as described by Noszticzius. Hemin was purchased from Fluka, all other reagents from Reanal and were of analytical reagent grade. Hemin was used in a stock solution prepared in 0.2 mol dm -3 NaOH. All experiments were carried out at 20.0 o C. The acidic medium of the reactions of hemin and TPPS was set by sulfuric acid. The slightly acidic ph of the reactions of Fe III TPPS was set using Britton-Robinson buffers. The constant ionic strength of the solutions in the reactions of Fe III TPPS was maintained by the buffer solutions. For the reactions of hemin, the large excess of sulfuric acid present in the solutions was enough to maintain the ionic strength at a constant value. In part of the mesurements, sodium sulfate was added to the solutions to maintain the sum of the concentration of H 2 SO 4 and Na 2 SO 4 at a constant value. Even in these cases, the variation of the ionic strength remained below 7%. The reactions were followed spectrophotometrically and potentiometrically. Spectrophotometric measurements: Spectrophotometric measurements were carried out in a Milton Roy Spectronic 3000 Array spectrometer. The solutions in its 1 cm cuvette were thermostatted and during kinetic measurements, stirred with magnetic stirrer. The absorption coefficient of hemin at 365 nm in the acidic ph range of the experiments was determined by extrapolating the absorption of the kinetic curves to t = 0 time when the acid was added to the alkaline solutions. The concentration of TPPS was followed at 414 and 435 nm, that of Fe III TPPS using the 394 nm peak. Potentiometric measurements: The ph of the solutions was measured using a Radelkis OP-808P combination glass electrode connected to a Radelkis OP-211/1 digital ph meter. For the determination of the bromide ion concentration, a Radelkis OP-Br-0711P Br - sensitive electrode and a Radelkis OP-0830P calomel reference electrode was used. The presence of bromate ions changed the value of the 3

4 measured potentials and therefore, bromide calibration curves were determined in the presence of identical amounts of bromate as in the kinetic experiments. Numerical simulations and parameter estimations were carried out on a PC using LSODE and CVODE integration packages for the hemin-bromate and the Fe III TPPS-bromate reaction, respectively. III. New scientific results and consequences 1. The kinetics of the reaction of hemin and acidic bromate was established. Concentration conditions were searched to find an optimum ph which was low enough for the oxidation of hemin by bromate but high enough to avoid the acidic decomposition of hemin. It was proven experimentally, that the oxidation is of autocatalytic nature. The concentration dependence of the maximum rate of the reaction (measured in the inflexion point) was determined by spectrophotometry. The reaction was found to be first order in hemin, bromate and hydrogen ion concentration. The rate constant was determined for two different ionic strengths. The length of the induction period is decreasing by increasing the concentration of bromate and hydrogen ions but is independent of the concentration of hemin. The effect of perturbation by bromide ions and elementary bromine was established: in low concentrations, they increase the length of the induction period however in high concentrations, they are decreasing it because of the increase in the reaction rate. 2. Based on the experimental results, the most possible mechanism of the hemin bromate reaction was established. For the reaction mechanism, following considerations were taken into account: the autocatalysis, the results of the kinetic and perturbation experiments and the reaction steps of the bromate reduction as known from the mechanism of the BZ reaction. Bromate undergoes the same changes in the reaction with hemin like in the BZ. In the reaction, BrO 2 radicals rather than elementary bromine play a crutial role and react with hemin. 3. The suggested mechanism was proven by model calculations. The results of computations account for the main experimental features of the reaction. The model 4

5 calculations allowed to give an estimation for the rate constant of the reaction between hemin and BrO 2 radicals. 4. The kinetics of the reaction of Fe III TPPS and bromate was established. Despite of the similar chemical structure of the two compounds, the kinetics of the reaction of hemin and Fe III TPPS with bromate are substantially different. It was proven by spectrophotometric measurements that in the reaction of Fe III TPPS with bromate, there is no autocatalysis. In neutral media, the first part of the reaction is separate from the following phases and therefore, its rate equation and rate constant were determined by measuring the initial rate of the reaction. The reaction is first order in Fe III TPPS and in bromate and independent of the hydrogen ion concentration. The rate constant was determined by two independent methods, both spectrophotometrically and potentiometrically and the two methods yielded the same results. By following the change of the ph in the reaction, it was established that the first part of the reaction consumes hydrogen ions while they are produced in the forthcoming phases. By perturbation experiments, it was stated that the role of bromide ions in the kinetics can be neglected (they are final products) however, elementary bromine and hypobromous acid play an essential role in the mechanism. By measuring the bromide ion concentration with potentiometric method it was stated that 1 mol bromide ion is produced during the conversion of 1 mol Fe III TPPS which means that oxidation of Fe III TPPS corresponds to an overall 6- electron redox process. 5. The possible mechanism of the reaction of Fe III TPPS and bromate was established. On the contrary to the reaction of hemin and bromate, Fe III TPPS is oxidized by bromate in three main consecutive reaction steps. Bromate is reduced in these three steps in three oxigen-transfer processes through the intermediates HBrO 2 and HOBr finally to bromide ions. By comparison with analogous reactions of Fe III TPPS with peroxides, Fe III TPPS is converted in this six-electron oxidation to a biliverdin-analogue iron(iii) complex. The possible structure of one of the two organic intermediates of the reaction was given, too. 6. Based on the suggested mechanism, model calculations were made which account for the main features of the reaction. The rate constant of the first step of the three consecutive reactions was determined experimentally, that of the second and third step were established by model calculations. Based on the suggested mechanism using the calculated rate constants and appropriate absorption coefficients, the model reproduced the 5

6 experimentally measured absorption time curves and their concentration dependence very well. List of publications: 1. Rácz, K., Burger, M., Nagy-Ungvárai, Zs.: Autocatalytic Oxidation of Hemin by Acidic Bromate. Int. J. Chem. Kinet., 2006, 38, Rácz, K., Burger, M., Lagzi, I., Ungvárai-Nagy, Zs.: Oxidation of a Water-Soluble Porphyrin Complex by Bromate, React. Kinet. Catal. Lett., (to be published) Poster: 1. Rácz, K., Burger, M., Nagy-Ungvárai, Zs.: Autocatalytic reaction between hemin and acidic bromate. Nonlinear Phenomena in Chemistry, ESF REACTOR Workshop, Budapest, Jan (poszter) 6