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Z. Phys. Chem. 223 (2009) 299–317 . DOI 10.1524.zpch.2009.5432 © by Oldenbourg Wissenschaftsverlag, München

Kinetic and Mechanistic Investigations on Oxidation of L-tryptophan by Diperiodatocuprate(III) in Aqueous Alkaline Medium By Nagaraj P. Shetti and Sharanappa T. Nandibewoor* P.G. Department of Studies in Chemistry, Karnatak University, Dharwad, Karnataka-580 003, India. (Received July 14, 2008; accepted December 8, 2008)

Kinetics . Mechanism . L-tryptophan . Oxidation . Diperiodatocuprate(III) The oxidation of L-tryptophan (L-TRP) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.20 mol dm-3 was studied spectrophotometrically at 298 K. The reaction between DPC and L-tryptophan in alkaline medium exhibits 1:4 stoichiometry (L-tryptophan: DPC). The reaction is of first order in [DPC] and has less than unit order in [L-TRP] and negative fractional order in [periodate] and [alkali]. Intervention of free radicals was observed in the reaction. The oxidation reaction in alkaline medium has been shown to proceed via a DPC- L-tryptophan complex, which decomposes slowly in a rate determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, 1H NMR, 13CNMR and LC-MS spectral studies. The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to slow step of the mechanism were computed and discussed and thermodynamic quantities were also determined.

1. Introduction Amino acids act not only as the building block in protein synthesis but they also play a significant role in metabolism and have been oxidized by a variety of oxidizing agents [1]. The study of the oxidation of amino acids is of interest because of their biological significance and selectivity towards the oxidant to yield the different products [2–3]. L-tryptophan [L-TRP] is an essential amino acid and is needed to maintain optimum health. It is particularly plentiful in bananas, dried dates, milk, cottage

* Corresponding author. E-mail: [email protected]

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cheese, meat, fish, turkey, and peanuts. This amino acid is required for the production of niacin (vitamin B3) a precursor of serotonin, a neurotransmitter, that is important for normal nerve and brain function. Serotonin is one of the useful chemicals in stabilizing emotional moods, pain control, inflammation, intestinal peristalsis, etc. It is also important in controlling hyperactivity in children, assists in alleviating stress, helps with weight loss and reducing appetite. A shortage of L-tryptophan may be a contributing factor to heart artery spasms. In recent years, the study of highest oxidation state of transition metals has intrigued many researchers. Transition metals in a higher oxidation state can be stabilized by chelation with suitable polydentate ligands. Metal chelates such as diperiodatocuprate(III) [3], diperiodatoargentate(III) [4] and diperiodatonickelate(IV) [5] are good oxidants in a medium with an appropriate pH value. Periodate and tellurate complexes of copper in its trivalent state have been extensively used in the analysis of several organic compounds [6]. The kinetics of selfdecomposition of these complexes was studied in some detail [7]. Copper(III) is shown to be an intermediate in the copper(II) catalyzed oxidation of amino acids by peroxydisulphate [8]. The oxidation reaction usually involves the copper(II)copper(I) couple and such aspects are detailed in different reviews [9]. The use of diperiodatocuprate(III) (DPC) as an oxidant in alkaline medium is new and restricted to a few cases due to the fact of its limited solubility and stability in aqueous medium. DPC is a versatile one-electron oxidant for various organic compounds in alkaline medium and its use as an analytical reagent is now well recognized [10]. Copper complexes have occupied a major place in oxidation chemistry due to their abundance and relevance in biological chemistry [11]. Copper(III) is involved in many biological electron transfer reactions [12]. They have also been used [13] in the differential titration of organic mixtures, in the estimation of chromium, calcium and magnesium from their ores, antimony, arsenic and tin from their alloys. Since multiple equilibria between different copper(III) species are involved, it would be interesting to know which of the species is the active oxidant. In earlier reports [3,14] on DPC oxidation, periodate had a retarding effect and the order in [OH-] was found to be less than unity in most of the reactions. However in the present study we have observed entirely different kinetic observations. Literature survey reveals that there are no reports on the oxidation of Ltryptophan (L-TRP) by diperiodatocuprate(III). The present study deals with the title reaction to investigate the redox chemistry of DPC in alkaline media, to compute the thermodynamic quantities of various steps of Scheme 1 and to arrive at a suitable mechanism on the basis of kinetic and spectral results.

2. Experimental 2.1 Materials and reagents All chemicals used were of reagent grade and double distilled water was used throughout the work. The copper(III) periodate complex was prepared [15] and

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standardized by a standard procedure [16]. The UV-vis spectrum with maximum absorption at 415nm verified existence of copper(III) complex. The solution of L-tryptophan (S.D. Fine Chem.) and copper sulphate (BDH) were prepared by dissolving known amounts of the samples in distilled water. Periodate solution was prepared and standardized iodometrically [17]. Required alkalinity and ionic strength were maintained by KOH (BDH) and KNO3 (Analar) respectively in reaction solutions.

2.2 Kinetic measurements Since, the initial rate of reaction is fast, the kinetic measurements were performed on a Varian CARY 50 Bio UV-Visible Spectrophotometer attached to a rapid kinetic accessory (HI-TECH SFA-12). The kinetics was followed under pseudo-first order condition where [TRP] > [DPC] at 298±0.1 K, unless specified. The reaction was initiated by mixing the DPC to L-trp solution which also contained required concentration of KNO3, KOH and KIO4 The progress of reaction was followed spectrophotometrically at 415 nm by monitoring the decrease in absorbance due to DPC with the molar absorbancy index, 'ε' to be 6235±100 dm3 mol-1 cm-1 (Literature ε = 6230 [18]). It was verified that there is a negligible interference from other species present in the reaction mixture at this wavelength. The pseudo-first order rate constants, 'kobs' were determined from the log(absorbance) versus time plots. The plots were linear up to 80% completion of reaction under the range of [OH-] used. The orders for various species were determined from the slopes of plots of log kobs versus respective concentration of species except for [DPC] in which non variation of 'kobs' was observed as expected to the reaction condition. During the kinetics a constant concentration viz. 5.0!10-4 mol dm-3 of KIO4 was used throughout the study unless otherwise stated. Since periodate is present in the excess in DPC, the possibility of oxidation of L-tryptophan by periodate in alkaline medium at 298 K was tested. The progress of the reaction was followed iodometrically. However, it was found that there was no significant reaction under the experimental conditions employed compared to the DPC oxidation of L-tryptophan. The total concentration of periodate and OH- was calculated by considering the amount present in the DPC solution and that additionally added. Kinetics runs were also carried out in N2 atmosphere in order to understand the effect of dissolved oxygen on the rate of reaction. No significant difference in the results was obtained under a N2 atmosphere and in the presence of air. In view of the ubiquitous contamination of carbonate in the basic medium, the effect of carbonate was also studied. Added carbonate had no effect on the reaction rates. LC-ESI-MS analysis was carried out using reverse phase high performance chromatography (HPLC) system with a phenomena C-18 column, UV-visible detector and series mass analyzer. 5 μL of acidified product and methanol solution was injected. The mobile phase consisted of ammonium acetate (eluent A) and acetonitrile at a flow rate of 1 ml.min . Gradient elution was run to separate

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Scheme 1. Stoichiometry of the reaction.

substrate and product. Gradient 0min.50% A-20min.50%A-20min.75%B20min.25%B. UV detection at 254 nm.

2.3 Stoichiometry and product analysis Different sets of reaction mixtures containing excess of DPC to L-tryptophan in presence constant amounts of OH- and KNO3 were kept for 6 hrs in closed vessel under inert atmosphere. The remaining DPC concentration was estimated by spectrophotometrically at 415 nm. The results, 1:4 stoichiometry as given in Scheme 1. The main product, indole-3-acetic acid was separated [19] by TLC, using the mixtures of methyl acetate, isopropanol and 25% ammonium hydroxide in the ratio of 45:35:20(v.v). It is characterized by its melting point (164 °C) and IR and NMR spectra respectively. The IR spectrum of the product showed in Fig. 1, a sharp band at 3389 cm-1, assigned to indole -NH and a series of bands in the region of 2730 to 3127 cm-1 were due to the hydrogen bonded -OH stretching frequencies. An intense sharp band at 1701 cm-1 was due to C = O stretching frequencies of carboxylic group. The absorbance of broad bands in the region of 3086 to 3310 cm-1 and presence of only an intense sharp band at 3389 cm-1 confirms the absence of free NH2 and presence of indole-NH group. The product indole-3-acetic acid was further confirmed by its 1HNMR and 13CNMR in Fig. 2 respectively. 1HNMR (300 MHz, DMSO d6) spectra showed peaks in ppm (1H, s, δ-10.88) carboxylic OH (D2O exchanged); (2Ar H, d, δ-7.49 and 7.47); (2Ar H, t, δ-7.36 and 7.33); (1Ar H, s, 6.98); (1H, s, δ-4.48) NH D2O exchanged; and (1H, s, δ-2.5). Where as 13CNMR (300 MHz, DMSO d6) peaks at δ: 174.29, 136.88, 127.94, 124.75, 121.97, 119.39,119.33, 112.25, 108.46 and 31.82 ppm are assigned to carbons designated with numbers 1,2, 3, 4, 5, 6, 7, 8, 9 and 10 respectively. LC-ESI-MS analysis of isolated acidified product indicated the presence of Indole-3-acetic acid as molecular ion peak of m.z 176 (m+1) (Fig. 3). The byproducts were identified as ammonia by Nessler's reagent [20] and the CO2 was qualitatively detected by bubbling nitrogen gas through the acidified reaction mixture and passing the liberated gas through tube containing limewater. The presence of Cu(OH)2 was also tested. Regression analysis of experimental data to obtain the regression coefficient r and standard deviation S of point from the regression line was performed using Microsoft Excel-2003 programme.

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Fig. 1. IR spectra of indole-3-acetic acid in KBr. Indole NH-band at 3389 cm-1, hydrogen bonded -OH stretching band between (2730 to 3127) cm-1 and carboxylic group (C = O) stretching at 1701 cm-1.

3. Results 3.1 Reaction orders The reaction orders were determined from the slope of log rate constants versus concentration plots by varying the concentrations of L-tryptophan, alkali and periodate in turn while keeping all other concentrations and conditions constant.

3.2 Effect of [Diperiodatocuprate (III)] The oxidant DPC concentration was varied in the range of 1.0!10-5 to 1.0! 10-4 mol dm-3 and the fairly constant kobs values indicate that order with respect to [DPC] was unity (Table 1). This was also confirmed by linearity of the plots of log [absorbance] versus time (r ≥ 0.986, S ≤ 0.014) up to 80% completion of the reaction as shown in Fig. 4.

3.3 Effect of [L-tryptophan] The effect of L-tryptophan on the rate of reaction was studied at constant concentrations of alkali, DPC and periodate at a constant ionic strength of 0.20 mol dm-3.

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Fig. 2. 13CNMR spectra of indole-3-acetic acid in DMSO d6. (1) = 174.29, (2) = 136.88, (3) = 127.94, (4) = 124.75, (5) = 121.97, (6) = 119.39, (7) = 119.33, (8) = 112.25, (9) = 108.46, (10) = 31.82.

The substrate, L-TRP was varied in the range of 1.0!10-4 to 1.0!10-3 mol dm-3. The kobs values increased with increase in concentration of L-tryptophan (Table 1). The order with respect to [L-TRP] was found to be less than unity (r ≥ 0.995, S ≤ 0.009). This is also confirmed in the plots of kobs versus [L-TRP]0.82 which is linear rather than the direct plot of kobs versus [L-TRP] (Fig. 5).

3.4 Effect of [Alkali] The effect of increase in concentration of alkali on the reaction was studied at constant concentrations of L-tryptophan, DPC and periodate at a constant ionic strength of 0.20 mol dm-3 at 25 °C. The rate constants decreased with increase in alkali concentration (Table 1), indicating negative fractional order dependence of rate on alkali concentration (r ≥ 0.988, S ≤ 0.007).

3.5 Effect of [Periodate] The effect of increasing concentration of periodate was studied by varying the periodate concentration from 1.0!10-4 to 1.0!10-3 mol dm-3 keeping all other

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Fig. 3. LC-ESI-MS analysis of isolated product indicated the presence of Indole-3-acetic acid as molecular ion peak of m.z 176 (m+1).

reactant concentrations constant. It was found that the added periodate had a retarding effect on the rate of reaction. The order with respect to periodate concentration was negative less than unity (r ≥ 0.0984, S ≤ 0.004) (Table 1).

3.6 Effect of Ionic Strength (I) and Dielectric Constant on the Medium (D) The addition of KNO3 at constant [DPC], [L-TRP], [OH-] and [IO4-], was found that increasing ionic strength of the reaction medium increases the rate of the reaction. Dielectric constants of the medium, D, was varied by varying t-butyl alcohol and water percentage. The D values were calculated from the equation D = DWVW + DBVB, where DW and DB are dielectric constants of pure water and t-butyl alcohol, respectively, and VW and VB are the volume fractions of components water and t-butyl alcohol, respectively, in the total volume of the mixture.

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Fig. 4. First order plots on the oxidation of L-tryptophan by DPC in aqueous alkaline medium at 298 K. (105 DPC (mol dm-3): (1) 1.0; (2) 3.0; (3) 5 .0; (4) 7.0; (5) 10.0.

Table 1. Effect of [DPC], [L-TRP], [IO4-] and [OH-] on diperiodatocuprate(III) oxidation of Ltryptophan in alkaline medium at T = 298 K, I = 0.2 mol dm-3. [DPC]!105 (mol dm-3) 1.0 3.0 5.0 7.0 10.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

[L-TRP]!104 (mol dm-3) 5.0 5.0 5.0 5.0 5.0 1.0 3.0 5.0 7.0 10.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

[IO4-]!104 (mol dm-3) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 1.0 3.0 5.0 7.0 10.0 5.0 5.0 5.0 5.0 5.0

[OH-] (mol dm-3) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.02 0.06 0.1 0.15 0.2

kobs!103 (s-1) kcal×103 (s-1) 5.84 5.81 5.79 5.75 5.91 1.33 3.80 5.79 6.82 8.81 11.4 8.06 5.79 4.83 3.71 8.23 6.58 5.79 5.19 4.49

5.92 5.92 5.92 5.92 5.92 1.43 3.89 5.92 7.61 9.6 11.4 7.81 5.92 4.76 3.68 7.94 6.78 5.92 5.11 4.49

The decrease in dielectric constant of the reaction medium increased the rate of the reaction.

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Fig. 5. Plot of kobs versus [L-TRP]0.82 and kobs versus [L-TRP].

3.7 Effect of Initially Added Products The externally added products, Indole 3-acetic acid and copper(II) (CuSO4) in the concentration range of 1.0!10-5 to 1.0!10-4, did not have any significant effect on the rate of the reaction.

3.8 Polymerization Study The intervention of free radicals in the reaction was examined as follows. The reaction mixture, to which a known quantity of acrylonitrile monomer was initially added, was kept for 2 hour in an inert atmosphere. On diluting the reaction with methanol, a white precipitate was formed, indicating the intervention of free radicals in the reaction [18]. The blank experiments of either DPC or Ltryptophan alone with acrylonitrile did not induce any polymerization under the same conditions as those induced for reaction mixture. Initially, added acrylonitrile decreased the rate of reaction indicating free radical intervention, which is the case in earlier work [21].

3.9 Effect of Temperature (T) The kinetics was studied at four different temperatures (288, 293, 298 and 303) K under varying concentrations of L-tryptophan, alkali and periodate, keeping other conditions constant. The rate constants were found to increase with increase in temperature. The rate constants (k) of the slow step of Scheme 1 were obtained from the slopes and intercepts of 1.kobs versus 1.[L-TRP], [H3IO62-] and [OH-] plots at four different temperatures and were used to calculate the activation parameters. The energy of activation corresponding to these constants was evaluated from the Arrhenius plot of log k versus 1.T (r ≥ 0.976, S ≤ 0.009) and other activation parameters obtained are tabulated in Table 2–5.

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Table 2. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in aqueous alkaline medium with respect to the slow step of Scheme 2: Effect of temperature. k!102 (s-1) 1.14 1.98 2.68 3.24

Temperature (K) 288 293 298 303

Table 3. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in aqueous alkaline medium with respect to the slow step of Scheme 2: Activation Parameters. Parameters Ea (kJ mol-1) ΔH# (kJ mol-1) ΔS# (JK-1 mol-1) ΔG# (kJ mol-1) log A

Values 50.7±1.0 48±2 -113±10 82±4 7.3±0.2

Table 4. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in aqueous alkaline medium with respect to the slow step of Scheme 2: Effect of temperature to calculate K4, K5 and K6 for the oxidation of L-tryptophan by diperiodatocuprate(III) in alkaline medium. Temperature (K) 288 293 298 303

K4!102 (mol dm-3) 6.5±0.3 7.2±0.3 9.1±0.4 11.6±0.5

K5!104 (mol dm-3) 6.8±0.3 3.7±0.2 3.0±0.1 2.5±0.1

K6!10-2 (dm3 mol-1) 17.9±0.3 21.4±0.5 25.5±0.6 27.9±0.7

Table 5. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in aqueous alkaline medium with respect to the slow step of Scheme 2: Thermodynamic quantities using K4, K5 and K6. Thermodynamic quantities ΔH (kJ mol-1) ΔS (JK-1 mol-1) ΔG298 (kJ mol-1)

Values from K4 29±2 76±4 6±0.2

Values from K5 -47±3 -224±10 20±1

Values from K6 22±1 139±6 -19±1

4. Discussion The water-soluble copper(III) periodate complex is reported [22] to be [Cu(HIO6)2(OH)2]7-. However, in an aqueous alkaline medium and at a high pH range employed in the study, periodate is unlikely to exist as HIO64- (as present in the complex) as is evident from its involvement in the multiple equilibria [23] (1)–(3), depending on the pH of the solution.

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(1) (2) (3) -

Periodic acid exists as H5IO6 in acid medium and as H4IO 6 near pH 7. Hence, under alkaline conditions as employed in this study, the main species are expected to be H3IO62-and H2IO63-. Thus, at the pH employed in this study, the soluble copper(III) periodate complex might be [Cu(OH)2(H3IO6)2]3-, a conclusion also supported by earlier work [3,14]. The reaction between the diperiodatocuprate(III) complex and L-tryptophan in alkaline medium has the stoichiometry 1:4 (L-TRP: DPC) with a first order dependence on [DPC] and an apparent order of less than unit order in [substrate], a negative fractional order dependence both on the [periodate] and [alkali]. No effect of added products was observed. Based on the experimental results, a mechanism is proposed for which all the observed orders in each constituent such as [oxidant], [reductant], [OH-] and [IO4-] may be well accommodated. It is known that L-tryptophan exists in the form of Zwitterion [24] in aqueous medium. In highly acidic medium, it exists in the protonated form, where as in highly basic medium it is in the fully deprotonated form [24]. In most of the reports [3] on DPC oxidation, periodate had a retarding effect and OH- had an increasing effect on the rate of the reaction. However in the present kinetic study, different kinetic results have been obtained. In this study both OH- and periodate retarded the rate of the reaction. The result of decrease in rate of reaction with increase in alkalinity (Table 1) can be explained in terms of prevailing equilibrium of formation of [Cu(OH)2(H3IO6)2]3- from [Cu(OH)2(H3IO6)(H2IO6)]4- hydrolysis as given in the following Eq. (4). (4) 2-

Also decrease in rate with increase in [H3IO6 ] (Table 1) suggests that equilibrium of copper(III) periodate complex to form monoperiodatocuptrate(III) (MPC) species as given in Eq. (5) is established. (5) Such type of equilibria (4) and (5) have been well noticed in literature [3,14]. It may be expected that a lower periodate complex such as monoperiodatocuptrate(III) (MPC) is more important in the reaction than the DPC. The inverse fractional order in [H3IO62-] might also be due to this reason. Therefore, MPC might be the main reactive form of the oxidant. The less than unit order in [L-TRP] presumably results from formation of a complex (C) between the MPC species and L-tryptophan prior to the formation of the products. This complex (C) decomposes in a slow step to form a free radical derived from L-tryptophan. This free radical species further reacts with

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Scheme 2. Detailed Scheme for the oxidation of L-tryptophan by alkaline diperiodatocuprate (III).

another molecule of MPC in a fast step to form indole-3-acetal intermediate. The indole-3-acetal then reacts with two more molecules of MPC in a further fast step to form the products such as indole-3-acetic acid, Cu(II) and periodate as given Scheme 2. Since Scheme 2 is in accordance with the generally well-accepted principle of non-complementary oxidations taking place in sequence of one-electron steps, the reaction between the substrate and oxidant would afford a radical intermediate. A free radical scavenging experiment revealed such a possibility (see infra). This type of radical intermediate has also been observed in earlier work [14]. The direct plot of kobs versus [L-TRP] was drawn to know the parallel reaction if any along with interaction of oxidant and reductant. However the plot of kobs versus [L-TRP] was not linear. Thus, in Scheme 2, the parallel reaction and

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Scheme 3. The probable structure of the complex [C].

Fig. 6. Spectroscopic evidence for the complex formation between DPC and L-Tryptophan. (1) UV-vis spectra of DPC complex (262 nm); (2) UV-vis spectra of mixture of DPC and Ltryptophan (269 nm).

involvement of two molecules of L-TRP in the complex are excluded. The probable structure of the complex is given in Scheme 3. Spectroscopic evidence for the complex formation between oxidant and substrate was obtained from UV-Vis spectra of L-tryptophan (5.0!10-4), DPC (5.0!10-5), [OH-] = 0.10 mol dm-3) and mixture of both. A bathochromic shift of about 7 nm from 259 to 266 nm in the spectra of DPC was observed in Fig. 6. The Michaelis-Menten plot also proved the complex formation between DPC and L-tryptophan, which explains the less than unit order dependence on [LTRP]. Such a complex between an oxidant has been observed in other studies [25]. Scheme 2 leads to the rate law (6).

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(6)

(7) This explains all the observed kinetic orders of different species. The rate law (7) can be rearranged in to the following form, which is suitable for verification: (8) According to Eq. (8), other conditions being constant, plots of 1.kobs versus [OH-] (r ≥ 0.995, S ≤ 0.014), 1.kobs versus 1.[L-TRP] (r ≥ 0.999, S ≤ 0.012) and 1.kobs versus [H3IO62-] (r ≥ 0.999, S ≤ 0.011) should be linear and are found to be so (Fig. 7). The slopes and intercepts of such plots lead to the values of K4, K5, K6 and k as (9.1±0.4)!10-2 mol dm-3, (3.0±0.1)!10-4 mol dm-3, (25.50±0.6)!102 dm3 mol-1 and (2.68±0.01)!10-2 s-1 respectively. The value of K4 is in good agreement with earlier literature [18]. These constants were used to calculate the rate constants and compared with the experimental values and found to be in reasonable agreement with each other (Table 1), which fortifies the Scheme 2. The equilibrium constant K4 is far greater than K5. This may be attributed to the greater tendency of DPC to undergo hydrolysis compared to the dissociation of hydrolyzed species in alkaline medium. The effect of ionic strength and dielectric constant of the medium on the rate explains qualitatively the reaction between two negatively charged ions, as seen in Scheme 2. The thermodynamic quantities for the first, second and third equilibrium steps of Scheme 2 can be evaluated as follows. The [H3IO6 2-], [L-TRP] and [OH-] (as in Table 1) were varied at four different temperatures. The plots of 1. kobs versus [OH-], 1.kobs versus 1.[L-TRP] and 1.kobs versus [H3IO62-] should be linear (Fig. 7). From the slopes and intercepts, the values of K4, K5 and K6 were calculated at different temperatures and these values are given in the Table 4. The vant Hoff's plots were made for variation of K4, K5 and K6 with temperature (log K4 versus 1.T (r ≥ 0.985, S ≤ 0.004), log K5 versus 1.T (r ≥ 0.973, S ≤ 0.007) and (log K6 versus 1.T (r ≥ 0.992, S ≤ 0.008) and the values of enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of reaction ΔG, were calculated for the first, second and third equilibrium steps. These values are given in Table 5. A comparison of the thermodynamic quantities of first step of Scheme 2 with those obtained for the slow step of the reaction shows that these values mainly refer to the rate limiting step, supporting the fact that the reaction before rate determining step is fairly fast and involves low activation energy [26].

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Fig. 7. Verification of rate law (9) for the of oxidation of L-tryptophan by DPC at T = 298 K.

The moderate values of ΔH# and ΔS# were both favorable for electron transfer processes. The negative value of ΔS# suggests that the intermediate complex is more ordered than the reactants [27] The observed modest enthalpy of activation and as well as a higher rate constant of the slow step indicate that the oxidation presumably occurs via inner-sphere mechanism. This conclusion is supported by earlier observation [13,28]. The activation parameters for the oxidation of some acids by DPC are summarized in Table 6. According to Exner [29], if the rates of several reactions in a series have been measured at two temperatures and log k2 (at T2) is linearly related to log k1 (at T1), i.e., log k2 = a + b log k1, he proposes that β can be evaluated from the equation. We have calculated the isokinetic temperature to be 311 K by plotting log k2 at 298 K versus log k1 at 303 K (r ≥ 0.997, S ≤ 0.006) as in Fig. 8. The value of β (311 K) is higher than experimental temperature (298 K). This indicates

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Table 6. Activation parameters for oxidation of some amino acids (for isokinetic temperature). Amino acids

L-Aspartic acid Vanillin L-Lysine L-Proline L-Tryptophan

k1!102 k2!102 (dm3mol-1s-1) (dm3mol-1s-1) at T = 298 K at T = 303 K 1.3 1.6 3.7 4.2 2.1 4.1 2.7 3.16 1.1 1.98

ΔH# (J.K.mol)

ΔS# (kJ.mol)

Reference

41.5 13.1 72.0 29.0 48.0

-142 -227 -32.0 -181 -113

31 32 33 34 Present work

Fig. 8. Plot of log k2 at 303 K versus log k1 at 298 K for isokinetic temperature (Table 6). (1) L-Tryptophan ; (2) L-Aspartic acid ; (3) L-Lysine ; (4) L-Proline ; (5) Vanillin.

that the rate is governed by the enthalpy of activation. [30] The linearity and the slope of the plot obtained may confirm that the kinetics these reactions follows a similar mechanism, as previously suggested.

5. Conclusion Among various species of DPC in alkaline medium, monoperiodatocuptrate(III) (MPC) [Cu(OH)2(H3IO6)]- is considered as active species for the title reaction. The results indicate that, the role of pH in the reaction medium is crucial. Rate constant of slow step and other equilibrium constants involved in the mechanism are evaluated and activation parameters with respect to slow step of reaction were computed. The overall mechanistic sequence described here is consistent with product studies, mechanistic and kinetic studies.

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Appendix According to Scheme 2, (9) (10) where T and f refer to total and free concentrations.

(11)

(12) Similarly, (13)

(14)

In view of low concentrations of DPC used, the second, third and fourth terms in the above equation are neglected. Therefore, (15) Similarly, (16) Substituting Eqs. (12), (15) and (16) in (9) and omitting the subscripts T and f we get, (17)

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