Solvation model for the oxidation of methionine by imidazolium fluorochromate in aqueous acetic acid medium

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1 J. Serb. Chem. Soc. 71 (1) (2006) UDC JSCS 3395 Original scientific paper Solvation model for the oxidation of methionine by imidazolium fluorochromate in aqueous acetic acid medium BINCY JOHN, M. PANDEESWARAN, D. S. BHUVANESHWARI and K. P. ELANGO * Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram , India ( drkpelango@rediffmail.com) (Received 29 September 2004, revised 13 April 2005) Abstract: The oxidation of methionine by imidazolium fluorochromate (IFC) were studied, in the presence of chloroacetic acid, in water acetic acid mixtures of varying molar compositions. The reaction is first order with respect to methionine, IFC and acid. The reaction rates were determined at different temperatures and the activation parameters were computed. The reaction rate increases with increasing mole fraction of acetic acid in the mixture and specific solvent solvent solute interactions were found to predominate (86 %). A solvation model and a probable mechanism for the reaction are postulated. Keywords: kinetics, oxidation, mechanism, methionine. INTRODUCTION Extensive studies on the mechanism of the oxidation of methionine (Met) by several oxidants have been reported. This sulphur-containing, essential amino acid is reported to behave differently, in comparison to other amino acids, towards many oxidants. 1 3 This may well be due to the presence of an electron-rich sulphur centre, which is easily oxidisable. Imidazolium fluorochromate (IFC) is a mild, stable and selective oxidant and has been reported only recently. 4 A literature search showed that there seems to be very few reports of the use of IFC. In this article, the kinetics and mechanism of the oxidation of methionine by IFC in aqueous acetic acid mixtures of varying molar compositions are reported. The aim was to utilise solvent variation studies to understand the mechanism of the oxidation of this biologically important amino acid because it may reveal the mechanism of its metabolism. EXPERIMENTAL Methionine (Merck) was used as supplied. IFC was prepared by the reported method 4 and its purity was checked by iodometric assay. Monochloroacetic acid was used as a source of hydrogen ions. Acetic acid was purified by the standard method. * Corresponding author doi: /JSC J 19

2 20 JOHN et al. Kinetic measurements The reaction was studied under pseudo-first-order conditions by keeping an excess of the substrate over the oxidant. The solvent was pure water unless otherwise stated. The reactions were studied at constant temperature ( 0.1 ºC) in the presence of chloroacetic acid (0.3 M) and were followed up to 80 % completion by monitoring iodometrically the decrease in the IFC concentration. Pseudo-first-order rate constancts, k obs, were evaluated from the linear plots of log IFC against time (r 2 > 0.98). Duplicate kinetic runs showed that the rate constants are reproducible to within 3 %.Thespecificrateconstant,k 2, were computed from the relation: k 2 = k obs / Met. Stoichiometry and product analysis The stoichiometry of the reaction was determined by performing the experiment under the conditions of IFC > Met. The disappearance of IFC was monitored until constant titre values were obtained. Me S R + O2CrFOImH Me S(O) R + OCrFOImH Product analysis was carried out under kinetic conditions. The oxidation of Met resulted in the formation of the corresponding sulphoxide. The sulphoxide was determined by a reported method. 5 The yield of the sulphoxide was 85 5%. RESULTS AND DISCUSSION The reactions were first order with respect to IFC. Further, the values of k obs were independent of the initial concentration of IFC (Table I). The reaction rate increased linearly with increasing concentration of Met. The order of the reaction with respect to Met was also one. The reaction is catalysed by hydrogen ions and the order with respect to H + was one. Therefore, the rate law can be represented as: d IFC /dt = k Met IFC H + The oxidation of Met under a nitrogen atmosphere failed to induce the polymerization of acrylonitrile. Further, the rate of oxidation decreased with the addition of Mn(II). Therefore, a one-electron oxidation giving rise to free radicals is unlikely. The rate constants for the oxidation of Met at 25, 35 and 45 ºC (at Met = M, IFC = Mand H + = M) are , and s 1, respectively. The activation parameters were calculated as: H # = kj mol 1, S # = 166 2Jmol 1 K 1 and G # = kj mol 1. These activation parameters are comparable with those for the oxidation of Met by other halochromates. 1,3 The influence of the solvent on the rate of oxidation of methionine by IFC was studied in water organic solvent mixtures with different mole fractions of the organic co-solvent, viz. acetic acid. The results in Table II indicate that the rate constant (k 2 ) is remarkably sensitive to the composition of the mixed solvent. The rate constant increases with increasing mole fraction of acetic acid in the mixture. The influence of the relative permittivity, r, of a solvent on the rate can be described by the equation of Laidler and Eyring. 6 Aplotoflogk 2 versus 1/ r is linear (r = 0.979, sd = 0.07, = 0.24) with a positive slope. The solvent effect was

3 SOLVATION MODEL FOR OXIDATION OF METHIONINE 21 also analysed using the Grunwald Winstein equation. 7 Aplotoflogk 2 versus the solvent ionizing power, Y, in water acetic acid mixtures is linear (r = 0.929, sd = 0.12, = 0.43) with a negative slope, which indicates an S N 2 type transition state. Such a transition state is stabilized with increasing content of organic solvent in the medium and, hence, the rate of the oxidation increases. The statistical results indicate that the correlations between log k 2 and the macroscopic solvent parameters, such as relative permittivity and ionizing power, are poor. Thus, no single solvent parameter can completely explain the solvent effect on the reactivity. TABLE I. Rate constants for the oxidation of methionine by IFC at ºC 10 2 Met /M 10 3 IFC /M 10 Acid /M 10 4 k obs /s a a contained M Mn(II) Idealized theories often predict that the solvent relative permittivity serves as a quantitative measure of the solvent polarity. However, this approach is often inadequate since these theories regard a solvent as a non-structured continuum, not composed of individual solvent molecules with their own solvent solvent interactions and they do not take into account specific solute solvent interactions, such as hydrogen bonding and electron pair donor electron pair acceptor interactions, which often play a dominating role in solute solvent interactions. No single macroscopic physical parameter could possibly account for the multitude of solute solvent interactions on the molecular microscopic level. Thus, bulk solvent properties, such as the relative permittivity and ionizing power, poorly describe the microenvironment around the reacting species, which governs the stability of the transition state and hence the rate of the reaction. Hence, there have been a variety of attempts to quantify different

4 22 JOHN et al. aspects of solvent polarity and then use the resultant parameters to interpret solvent effects on reactivity through multiple regression. Various treatments for the above mentioned solvent solvent solute interactions based on Linear Solvation Energy Relationships (LSER) have been developed. 8 TABLE II. Effect of added acetic acid on the oxidation of methionine by IFC at ºC. [Met] = M, IFC =1-3 Mand H + = M mole fraction of CH 3 COOH k 2 /dm 3 mol -1 s The effect of solvent on reactivity may be understood from the stand point of specific and non-specific solvation effects. This kind of dual dependency of reactivity on the solvent composition is illustrated by the Kamlet Taft solvatochromic comparison method. 9 This method may be used to unravel, quantify, correlate and rationalize multiple interacting solvent effects on reactivity. Thus the rate data were correlated with the solvatochromic parameters in the form of the following LSER. log k = A 0 + s * +a + b where * in an index of the solvent dipolarity/polarizability, which measures the ability of the solvent to stabilize a charge or a dipole by virtue of its dielectric effect, is the solvent hydrogen bond donor acidity (HBD), is the solvent hydrogen bond acceptor basicity (HBA) in a solute to solvent hydrogen bond and A 0 is the regression value of the solute property in the reference solvent cyclohexane. The regression coefficients s, a and b measure the relative susceptibilities of the solvent dependent solute property, log k 2, to the respective solvent parameter. These solvatochromic parameters for the solvent mixtures used in the present study were taken from the literature. 10 The rate of oxidation in the solvent mixture studied show excellent correlations with solvent via the above LSER. The correlation results obtained are given below. log k 2 = ( 1.75) 2.22( 0.09) * ( 0.92) +8.81( 1.35) (R 2 = 0.996, sd = 0.02, = 0.08, P =31%,P =55%,P *=14%) Such an excellent correlation, with a variance of over 99 % for the investigated aqueous organic solvent mixture, indicates the existence of non-specific and specific solvent solute interactions. From the values of the regression coefficients, the contribution of each parameter (P x ), on a percentage basis, to the reactivity were calculated. 11 The observation of this systematic multiple regression analysis leads to the following preliminary conclusions: i) In water acetic acid mixtures, the rate of the reaction is strongly influenced by specific solute solvent interactions, as indicated by the percentage contributions of the and parameters.

5 SOLVATION MODEL FOR OXIDATION OF METHIONINE 23 ii) The positive sign of the coefficients of the and terms suggests that the specific interactions between the transition state and the solvent, through HBD and HBA properties, are larger than those between the reactants and the solvent. iii) The contribution of the solvent HBA basicity,, to the total solvent effect is predominant. The positive sign of the coefficient of this term suggests that the solvent mixture solvates the transition state more than the reactants. Since acetic acid is an amphiprotic solvent, increasing its mole fraction in the mixture may stabilize the transition state through better solvation and consequently increase the rate of the oxidation. iv) The solvent dipolarity/polarizability, *, plays a very minor role. The negative sign of the coefficient of this term suggests that the reactants are more extensively solvated by the solvent mixture than the transition state. 12 Considering these points, the solvation model for the oxidation of methionine by IFC in water acetic acid mixtures can be represented as: dipolarity/ HBD (31 %) and polarizability HBA (55 %) interactions (14 %) interactions Reactants Transition state Products Furthermore, a dynamic exchange of solvent molecules between the solvation shell of the transition state and the bulk exists. 13 As the concentration of the organic component in the solvent mixture increases, the greater is the number of organic solvent molecules introduced into the solvation shell. As the mole fraction of the organic component in the solvent mixture increases, the greater is the stabilization of the transition state through HBD/HBAsolute solvent interactions and, consequently, the rate of the reaction increases. Mechanism Under the experimental conditions employed in the present study, methionine is oxidized to the corresponding sulphoxide stage only. Based on the above kinetic observations, i.e., first order dependence on Met, IFC and Acid, the following mechanism is proposed for the reaction. The linear increase in the rate with acidity suggests the involvement of protonated IFC in the rate-determining step. In the first step, IFC is protonated. The protonated IFC attacks the substrate, in a slow step, to form a complex, which subsequently decomposes to give the products in a fast step. The proposed scheme envisages an oxygen atom transfer from the oxidant, which is in agreement with earlier observations. The electrophilic attack on the sulphide sulphur can be viewed as an S N 2 reaction. An S N 2 like transition state is supported by the observed solvent effect.

6 24 JOHN et al. K IFC + H + IFCH + k Met + IFCH + [Complex] slow fast [Complex] Products The above mechanism leads to the following rate law: Rate = k Met IFCH + (or) Rate =Kk Met IFC H +. The rate law in its final form accounts for the observed kinetics. CONCLUSION The oxidation of methionine by IFC is first order with respect to Met, IFC and Acid. Under the employed experimental conditions, methionine is oxidized to the corresponding sulphoxide stage only. The entropy of activation is negative, suggesting the formation of a complex in the slow step. The reaction is highly influenced by the amount and nature of the organic co-solvent added to the mixture. Linear and multiple regression analysis may be used to separate and quantify the specific and non-specific solvational effects on the oxidation of methionine by IFC. Using the above method, the significance of these solvent solvent solute interactions on the reactivity has been established. From the regression coefficients, information on the solvent reactant and the solvent transition state interactions is obtained and a solvation model is proposed. IZVOD SOLVATACIONI MODEL OKSIDACIJE METIONINA IMIDAZOLIJUM-FLUOROHROMATOM U VODENIM RASTVORIMA SIR]ETNE KISELINE BINCY JOHN, M. PANDEESWARAN, D. S. BHUVANESHWARI i K. P. ELANGO Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram , India Prou~avana je oksidacija metionina imidazolijum-fluorohromatom (IFC) u prisustvu hlor-sir}etne kiseline, a u sme{ama vode i sir}etne kiseline razli~itih molskih udela. Reakcija je prvog reda po metioninu, IFC i kiselini. Na razli~itim temperaturama odre ivana je konstanta brzine reakcije i izra~unati su parametri aktivacije. Brzina reakcije raste sa pove}awem molskog udela sir}etne kiseline u sme{i i utvr eno je da dominiraju (86 %) specifi~ne interakcije rastvara~ rastvara~ rastvorena supstanca. Predlo`en je model rastvarawa i verovatan mehanizam reakcije. (Primqeno 29. septembra 2004, revidirano 13. aprila 2005)

7 SOLVATION MODEL FOR OXIDATION OF METHIONINE 25 REFERENCES 1. A. Goyal, S. Kothari, Indian J. Chem. 42B (2003) V.Sharma,P.K.Sharma,K.K.Banerji,Indian J. Chem. 36A (1997) V.Sharma,P.K.Sharma,K.K.Banerji,J. Indian Chem. Soc. 74 (1997) A. Pandurangan, G. A. Rajkumar, B. Arabindoo, V. Murugesan, Indian J. Chem. 38B (1999) 99 5.S.Goel,S.Kothari,K.K.Banerji,J. Chem. Res. (M) (1996) E.S.Amis,J.F.Hinton,Solvent Effect of Chemical Phenomena, Vol. 1, Academic Press, New York, A. H. Fainberg, S. Winstein, J. Am. Chem. Soc. 78 (1956) C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, M. J. Kamlet, J. M. Abboud, M. H. Abraham, R. W. Taft, J. Org. Chem. 48 (1983) Y. Marcus, J. Chem. Soc. Perkin Trans. 2 (1994) G. Fatima Jeyanthi, K. P. Elango, Int. J. Chem. Kinet. 35 (2003) S. Saravanakanna, K. P. Elango, Int. J. Chem. Kinet. 34 (2002) Y. Marcus, Ion Solvation, Wiley, New York, 1985.

Kinetics and mechanism of the oxidation of methionine by quinolinium chlorochromate

J. Serb. Chem. Soc. 70 (2) 145 151 (2005) UDC 577.112.386:531.3:66.094.3 JSCS 3257 Original scientific paper Kinetics and mechanism of the oxidation of methionine by quinolinium chlorochromate M. PANDEESWARAN,