A kinetic and mechanistic study on the oxidation of l-methionine and N-acetyl l-methionine by cerium(IV) in sulfuric acid medium

2.1 Materials

Double distilled water was used for preparing the solutions. l-methionine and N-acetyl l-methionine (SRL) were used as such. Stock aqueous solutions of l-methionine and N-acetyl l-methionine were prepared by dissolving it in water. The Ce(IV) stock solution was obtained by dissolving cerium(IV) ammonium sulfate (E. Merck) in 0.98 mol dm⁻³ sulfuric acid and standardized with iron(II) ammonium sulfate solution (Jeffery et al., 1996). Other chemicals and reagents, such as sodium sulfate, sulfuric acid, acetonitrile, acetone, hydrated copper sulfate and aluminum sulfate used were of analytical grade with 99.9% purity.

2.2 Kinetic measurements

Kinetic studies were carried out in sulfuric acid medium in the temperature range (288.1–298.1 K) under pseudo first order conditions with a large excess of l-methionine and N-acetyl l-methionine over Ce(IV). The reaction was followed by estimating the unreacted Ce(IV) as a function of time by titrating against ferrous ammonium sulfate solution employing ferroin as an indicator (Walden et al., 1933).

No precautions were taken to exclude the diffused light entering into the reaction mixture (Krishna and Sinha, 1959). The Ce(IV) solution was thermally quite stable (Grant, 1964) in the visible region and undergoes photochemical decomposition (Heidt and Smith, 1948) only in the UV region. Since, the oxidation of Kolp and Thomas (1949) water even at 333 K by Ce(IV) was immeasurably slow and insignificant, no further precautions were taken to account for this.

From the titration values, plots of log [Ce(IV)] vs. time were made and from the slope of such plots, the pseudo first order rate constants k¹ (s⁻¹) were obtained. All the first order plots were linear, with a correlation coefficient of 0.996–0.999. The results were reproducible within an accuracy of ±5%

3.1 Effect of [l-methionine] and N-acetyl l-methionine

At a constant [Ce(IV)] (8 × 10⁻³ mol dm⁻³), [H⁺] (5 × 10⁻² mol dm⁻³) and [Na₂SO₄] (1 × 10⁻¹ mol dm⁻³) the kinetic runs were carried out with various (1–10 × 10⁻² mol dm⁻³) concentrations of l-methionine and N-acetyl l-methionine which yielded rate constants whose values depended on [l-methionine] and [N-acetyl l-methionine]. The pseudo first order rate constants k¹ (s⁻¹) thus obtained were found to increase with [l-methionine] and [N-acetyl l-methionine] (Table 1). This shows that the reaction obeys first order with respect to [l-methionine] and [N-acetyl l-methionine]. This was confirmed by the linear plots of k¹ (s⁻¹) vs. [l-methionine] Fig. 1a and [N-acetyl l-methionine] Fig. 1b.

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Figure 1a

Shows the linear plot of k′ (s⁻¹) vs. [l-methionine] confirms the first order reaction with respect to [l-methionine].

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Figure 1b

Shows the linear plot of k′ (s⁻¹) vs. [N-acetyl l-methionine] confirms the first order reaction with respect to [N-acetyl l-methionine].

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The values of k₂ (mol dm⁻³ s⁻¹) was evaluated from the slope of k¹ (s⁻¹) vs. [l-methionine] (Fig. 1a) and [N-acetyl l-methionine] plots (Fig. 1b) The k₂ (mol dm⁻³ s⁻¹) values thus obtained from such plots were in agreement with the corresponding values calculated from the factor k¹ (s⁻¹)/[l-methionine] and [N-acetyl l-methionine] (Table 2).

3.2 Effect of [Ce(IV)]

The reaction rate was measured at constant [l-methionine] and [N-acetyl l-methionine] (Table 3). 2 × 10⁻² mol dm⁻³, [H⁺] 5 × 10⁻² mol dm⁻³ and Na₂SO₄ 1 × 10⁻¹ mol dm⁻³ but with various [Ce(IV)] (5–14×10⁻³ mol dm⁻³). The pseudo first order rate constants k¹ were found to be independent of [Ce(IV)], confirmed the first order dependence of rate on [Ce(IV)]. The log[Ce(IV)]_T was plotted against ‘t’, very good straight line plots were obtained indicating that the reaction was first order with respect to [Ce(IV)].

3.3 Effect of [H⁺]

The reaction rates were measured at constant [l-methionine] and [N-acetyl l-methionine] (2.0 × 10⁻² mol dm⁻³), [Ce(IV)] (8.0 × 10⁻³ mol dm⁻³) and [Na₂SO₄] (1 × 10⁻¹ mol dm⁻³) but with various [H⁺] (4–11.0 × 10⁻² mol dm⁻³) (Table 4). The insignificant rate variations with [H⁺] shows the absence of $\text{Ce}{({\text{SO}}_4)}_2{\text{HSO}}_4^{-}$ and ${\text{H}}_3\text{Ce}{({\text{SO}}_4)}_4^{-}$ .

3.4 Effect of [ionic strength]

The influence of ionic strength on the reaction rate was maintained by the addition of sodium sulfate on the reaction rate. It was observed that the rate was not influenced by the ionic strength (Table 5).

3.5 Effect of [ ${\text{HSO}}_4^{-}$ and ${\text{SO}}_4^{-2}$ ]

Concentration of [ ${\text{HSO}}_4^{-}$ ] (KHSO₄) and ${\text{SO}}_4^{-2}$ (K₂SO₄) were varied in the range of (1–8 × 10⁻¹ 0.8 mol dm⁻³) at fixed [substrate] (2.0 × 10⁻² mol dm⁻³), [Ce(IV)] (8.0 × 10⁻³ mol dm⁻³) and [H⁺] (5 × 10⁻² 0.05 mol dm⁻³). It was observed that the rate was not influenced by the increasing concentration of ${\text{SO}}_4^{-2}$ and ${\text{HSO}}_4^{-}$ (Table 5) confirming that the reaction occurs between an ion Ce⁴⁺ and a neutral molecule (substrate) (Laidler, 1965).

3.6 Effect of dielectric constant

In order to determine the effect of dielectric constant (polarity) of the medium on rate, the oxidation of substrates by Ce(IV) was studied in acetonitrile as well as acetone mixtures of various compositions (Table 6). The data clearly reveals that the rate decreased with an increase in acetonitrile and acetone contents of solvent i.e., with decrease in dielectric constant of the solvent mixture. This indicates that there is a charge development in the transition state involving a more polar activated complex than the reactants (Sharma et al., 1995). The reaction is between a neutral molecule (substrate) and an ion (Ce⁴⁺) and the absence of ion–ion or dipole–dipole type mechanism which is also supported by the negative ΔS^# values obtained in this work.

3.7 Test for free radical intermediates

The intervention of free radicals was examined as follows. The reaction mixture, to which a known quantity of acrylonitrile scavenger had been added initially, was kept in an inert atmosphere for 2 h. Upon diluting the reaction mixture with methanol, white precipitate has been formed, indicating the presence of free radical intervention in the reaction.

3.8 Rate and activation parameters

The effect of temperature on k¹ (s⁻¹) was studied in the range (288.1–298.1 K) for l-methionine and (277.7–287.7 K) for N-acetyl l-methionine, (Table 1) and the results were shown in Table 7. From Arrhenius plot, for l-methionine (Fig. 2(a)) and N-acetyl l-methionine, (Fig. 2(b)) the value of energy of activation (Ea) was calculated. Hence, the values of ΔS^#, ΔH^#, ΔG^# (Table 7) were computed from the Eyring’s plot for l-methionine (Fig. 3(a)) and N-acetyl l-methionine, (Fig. 3(b)). The large negative value of entropy of activation (ΔS^#) obtained is attributed to the severe restriction of solvent molecules (electrostriction) around the transition state (Anis, 1992) and it indicates that the complex is more ordered than the reactants (Hiremath et al., 2007).

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Figure 2

(a) Arrhenius plot for l-methionine). (b) Arrhenius plot for N-Acetyl l-methionine shows the linear plot of log k₂ vs. 1/T. From Arrhenius plot, energy of activation (Ea) is calculated.

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Figure 3

(a) Eyring’s plot for l-methionine) and (b) Eyring’s plot for N-acetyl l-methionine shows the linear plot of 5 + log k₂/T vs. 1/T × 10⁻³. From Eyring’s plot the value of ΔS#, ΔH#, ΔG# is computed.

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3.9 Stoichiometry and product analysis

Different reaction mixtures with different sets of concentration of reactants, where [Ce(IV)] was in excess over [substrate] at constant ionic strength and acidity were kept for 24 h at 293 K in an inert atmosphere. The unreacted Ce(IV) was titrated against standard ferrous ammonium sulfate by using ferroin as an indicator. The results indicated that 2 mol of Ce(IV) was consumed by 1 mol of substrate, (i.e. 2:1) according to Eq. (1).

The main reaction products are Ce(III) and methionine sulfoxide, N-acetyl methionine sulfoxide. Sulfoxides of the substrates were confirmed by the following reactions. The reaction mixtures were allowed to stand for a few hours. Then, sodium bicarbonate was added and stirred vigorously, followed by a dropwise addition of benzoyl chloride solution. The precipitate N-benzoyl methionine sulfoxide was confirmed by its m.p. 183 °C (Goswami et al., 1981; Meenakshisundaram and Vinothini, 2003). The procedure is similar to the one employed in the oxidation of l-methionine by aqueous Cr(VI) (Olatunji and Ayoko, 1988). Further this has been confirmed by the acetone–ethanol mixture resulted in the precipitate of methionine sulfoxide, which was identified by its m.p. 238 °C.

Further work on other related amino acids with Ce(IV) is in progress.