Oxidation of l-Leucine by Alkaline Diperiodatoargentate (III

P.G. Department of Studies in Chemistry, Karnatak UniVersity, Dharwad-580 003, India. The oxidation of L-Leucine by alkaline diperiodatoargentate(III)...
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Ind. Eng. Chem. Res. 2006, 45, 8029-8035

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Oxidation of L-Leucine by Alkaline Diperiodatoargentate(III) Deamination and Decarboxylation: A Kinetic and Mechanistic Study Chanabasayya V. Hiremath, Suresh D. Kulkarni, and Sharanappa T. Nandibewoor* P.G. Department of Studies in Chemistry, Karnatak UniVersity, Dharwad-580 003, India

The oxidation of L-Leucine by alkaline diperiodatoargentate(III) (DPA) at 298 K and a constant ionic strength of 1.0 mol dm-3 was studied spectrophotometrically. The oxidation products are pentanoic acid and Ag (I). The stoichiometry is [L-Leucine]:[DPA] ) 1:2. The reaction is of first order in [DPA] and has less than unit order in both [L-Leucine] and [alkali] and a retarding effect in [IO4-]. The oxidation reaction in alkaline medium has been shown to proceed via a L-Leucine-DPA complex, which further reacts with one molecule of DPA in a rate-determining step followed by other fast steps to give the products. The main products were identified by spot test and IR. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The probable active species of oxidation have been identified. 1. Introduction

2. Experimental Section

L-leucine

is an essential amino acid. It forms active sites of enzymes and helps in maintaining their proper conformation by keeping them in proper ionic states. So, oxidation of L-leucine may help in understanding some aspects of enzyme kinetics. Amino acids not only act as the building blocks in protein synthesis, but they also play a significant role in metabolism. Amino acids can undergo many types of reaction depending on whether a particular amino acid contains nonpolar groups or polar substituents. The oxidation of amino acids is of interest as the oxidation products differ for different oxidants.1,2 Thus, the study of amino acids becomes important because of their biological significance and selectivity toward the oxidant. Diperiodatoargentate(III) (DPA) is a powerful oxidizing agent in alkaline medium with the reduction potential3 1.74 V. It is widely used as a volumetric reagent for the determination of various organic and inorganic species.4 Jayaprakash Rao5,6 et al. have used DPA as an oxidizing agent for the kinetics of oxidation of various organic substrates. They normally found that the order with respect to both the oxidant and substrate was unity, and [OH-] was found to enhance the rate of reaction. It was also observed that they did not arrive at the possible active species of DPA in alkali, and on the other hand, they proposed mechanisms by generalizing the DPA as [Ag(HL)L](x+1)-. However, Kumar7-9 et al. put forth an effort to give evidence for the reactive form of DPA in a large scale of alkaline pH values. In the present investigation, we have obtained the evidence for the reactive species for DPA in alkaline medium. The literature survey reveals that there are no reports on mechanistic studies of L-leucine oxidation by DPA. Thus, L-leucine has been selected as a substrate in order to explore the mechanism of oxidation by DPA in alkaline medium and to check the reactivity of amino acids toward DPA. The title reaction is studied to investigate the redox chemistry of the Ag(III) in such media and to arrive at a plausible mechanism. * To whom correspondence should be addressed. Tel.: +91-8362770524. Fax: +91-836-2747884. E-mail: [email protected].

2.1. Materials and Reagents. All chemicals used were of reagent grade, and double distilled water was used throughout the work. A solution of L-leucine (Sisco-Chem Ltd.) was prepared by dissolving an appropriate amount of recrystallized sample in double distilled water. The required concentration of L-leucine was used from its aqueous stock solution. A stock solution of IO4- was prepared by dissolving a known weight of KIO4 (Riedel-de Haen) in hot water; the stock solution was used after keeping for 24 h. Its concentration was ascertained iodometrically10 at neutral pH, maintained using a phosphate buffer. KNO3 and KOH were used to maintain the ionic strength and alkalinity of the reaction, respectively. An aqueous solution of AgNO3 was used to study the product effect, Ag(I). 2.1.1. Preparation of DPA. DPA was prepared by oxidizing Ag(I) in the presence of KIO4, as described elsewhere:11 the mixture of 28 g of KOH and 23 g of KIO3 in 100 cm3 of water along with 8.5 g AgNO3 was heated just to boiling, and 20 g of K2S2O8 was added in several lots with stirring and then allowed to cool. It was filtrated through a medium porosity fritted glass filter, and 40 g of NaOH was added slowly to the filtrate, whereupon a voluminous orange precipitate agglomerated. The precipitate was filtered as above and washed three to four times with cold water. The pure crystals were dissolved in 50 cm3 of water and warmed to 80 °C with constant stirring, and thereby, some solid was dissolved to give a red solution. The resulting solution was filtered when it was hot, and on cooling at room temperature, the orange crystals separated out and were recrystallized from water. The complex was characterized from its UV spectrum, exhibited three peaks at 216, 255, and 362 nm. These spectral features were identical to those reported earlier for DPA.11 The magnetic moment study revealed that the complex is diamagnetic. The compound prepared was analyzed12 for silver and periodate by acidifying a solution of the material with HCl, recovering and weighing the AgCl for Ag, and titrating the iodine liberated when an excess of KI was added to the filtrate for IO4-. The aqueous solution of DPA was used for the required [DPA] in the reaction mixture. During the kinetics, a constant concentration, viz., 1 × 10-5 mol dm-3, of KIO4 was used throughout the study unless otherwise stated. Thus, the pos-

10.1021/ie060612d CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006

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sibility of oxidation of L-leucine by periodate was tested, and it was found that there was no significant interference due to KIO4 under experimental conditions. Precaution was also taken to avoid the dissolution of O2 and CO2 in the solution by maintaining an inert atmosphere with N2 throughout the study. 2.2. Kinetic Measurements. Kinetic measurements were performed on a Varian CARY 50 Bio UV-vis spectrophotometer. The kinetics was followed under pseudo-first-order conditions where [L-leucine] > [DPA] at 25 ( 0.1 °C, unless specified. The reaction was initiated by mixing the DPA into the L-leucine solution which also contained required concentrations of KNO3, KOH, and KIO4. The progress of reaction was followed spectrophotometrically at 360 nm by monitoring the decrease in the absorbance due to DPA (molar absorbency index, “” to be 13 900 ( 100 dm3 mol-1 cm-1). It was verified that there is a negligible interference from other species present in the reaction mixture at this wavelength. In view of the modest concentration of alkali used in the reaction medium, attention was also directed to the effect of the reaction vessel surface on the kinetics. Use of polythene/ acrylic wares and quartz or polyacrylate cells gave the same results, indicating that the surface does not have any significant effect on reaction rates. In view of the ubiquitous concentration of carbonate in the basic medium, the effect of carbonate was also studied. Added carbonate had no effect on the reaction rates. However, fresh solutions were nevertheless used for carrying out each kinetic study. Regression analysis of experimental data to obtain regression coefficient “r” and the standard deviation “s”, of points from the regression line, was performed with the Microsoft Excel 2003 program.

Figure 1. First-order plots of oxidation of L-leucine by DPA in aqueous alkaline medium at 298 K: [DPA] (mol dm-3) (1) 1.0 × 10-5, (2) 3.0 × 10-5, (3) 5.0 × 10-5, (4) 8.0 × 10-5, (5) 10 × 10-5; [L-leucine] ) 8.0 × 10-3; [OH-] ) 0.50; I ) 1.0; [IO4-] ) 1.0 × 10-5 mol dm-3). Table 1. Effect of [DPA], [L-Leucine], [IO4-], and [OH-] on Diperiodatoargentae(III) Oxidation of L-Leucine in Alkaline Medium at 298 Ka [DPA] × 105 [L-leu] × 103 [IO4-] × 105 [OH-] (mol dm-3) (mol dm-3) (mol dm-3) (mol dm-3) 1.0 3.0 5.0 8.0 10 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

3. Results 3.1. Stoichiometry and Product Analysis. Different sets of reaction mixtures containing varying ratios of DPA to L-leucine in the presence of a constant amount of OH- and KNO3 were kept for 3 h at 298 K in a nitrogen atmosphere in a closed vessel. The remaining concentration of DPA was estimated spectrophotometrically at 360 nm. The results indicate that 2 mol of DPA consumed 1 mol of L-leucine (2:1) as shown in eq 1.

The main oxidation products were identified as pentanoic acid by spot test13 and ammonia by Nessler’s reagent, and the product CO2 was qualitatively detected by bubbling N2 gas through the acidified reaction mixture and passing the liberated gas through tube containing limewater. The nature of the carboxylic acid was confirmed by the IR spectrum, which showed a carbonyl (>CdO) stretch at 1708 cm-1 and OsH stretching of the acid at 3042 cm-1. The formation of free Ag+ in solution was detected by adding KCl solution to the reaction mixture, which produced white turbidity due to the formation of AgCl. It was observed that pentanoic acid does not undergo further oxidation under the present kinetic conditions. 3.2. Reaction Orders. The reaction orders were determined from the slopes of log kobs vs log(concentration) plots by varying

a

8.0 8.0 8.0 8.0 8.0 3.0 5.0 8.0 10 30 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 3.0 5.0 8.0 10 1.0 1.0 1.0 1.0 1.0

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 0.1 0.3 0.5 0.8 1.0

kobs × 103 (s-1) found

calcd

2.46 2.43 2.44 2.46 2.45 1.52 1.94 2.44 2.81 3.78 2.44 2.09 1.70 1.39 1.20 1.70 2.16 2.44 2.64 2.78

2.48 2.48 2.48 2.48 2.48 1.45 1.98 2.48 2.71 3.59 2.48 1.98 1.64 1.31 1.16 1.68 2.30 2.48 2.60 2.64

I ) 1.0 mol dm-3.

the concentration of the reductant and alkali in turn while keeping other conditions constant. The DPA concentration was varied in the range of 1.0 × 10-5 to 1.0 × 10-4 mol dm-3, and the linearity of the plots of log absorbance vs time up to 80% completion of the reaction as in Figure 1 (r g 0.9984, s e 0.000 81) indicates a reaction order of unity in [DPA]. This is also confirmed by varying [DPA], which did not result in any change in the pseudo-first-order rate constants, kobs as in Table 1. The L-leucine concentration was varied in the range 3.0 × 10-3 to 3.0 × 10-2 mol dm-3 at 298 K while keeping other reactant concentrations and conditions constant. The kobs values increased with the increase in concentration of L-leucine, indicating an apparent less than unit order dependence on [L-leucine] (r g 0.9997, s e 0.000 78) as in Table 1. The effect of alkali on the reaction has been studied in the range of 0.101.0 mol dm-3 at constant concentrations of L-leucine and DPA and a constant ionic strength of 1.0 mol dm-3. The rate constants increased with increasing [alkali], and the order was found to be less than unity (r g 0.9990, s e 0.000 71) as in Table 1. 3.2. Effect of [Periodate]. Periodate concentration was varied from 1.0 × 10-5 to 1.0 × 10-4 at constant [DPA], [L-leucine], and ionic strength. It was observed that the rate constants decreased by increasing [IO4-]; the values are reported in Table 1.

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scavenger had been added initially, was kept in an inert atmosphere for 1 h. Upon diluting the reaction mixture with methanol, a precipitate resulted, suggesting that there is participation of free radicals in the reaction. The blank experiment of either DPA or L-leucine alone with acrylonitrile did not involve polymerization under the same condition as those induced with the reaction mixture. Initially added acrylonitrile decreases the rate, also indicating the free radical intervention, which is also the case in earlier work.14 4. Discussion

Figure 2. Effect of dielectric constant of the medium on oxidation of L-leucine by diperiodatoargentate(III) at 298 K.

3.3. Effect of Products. Initially added products, such as Ag(I) and pentanoic acid, do not affect the rate of reaction. 3.4. Effect of Ionic Strength (I) and Dielectric Constant of the Medium (D). The addition of KNO3 was used to increase the ionic strength of the reaction medium. There is a negligible effect on the rate of reaction at constant [DPA], [L-leucine], [OH-], and [IO4-]. The dielectric constant of the medium, “D”, was varied by varying the tert-butyl alcohol and water percentage. The decrease in dielectric constant of the reaction medium increased the rate, and the plot of log kobs vs 1/D was linear with a positive slope as given in Figure 2 (r g 0.9980, s e 0.000 61). 3.5. Effect of Temperature (T). The influence of temperature on the kobs values was studied at 25, 30, 35, and 40 °C. The rate constants, k, of the slow step of Scheme 1 were obtained from the intercepts of 1/kobs vs 1/[L-leucine] plots at four different temperatures. The values are given in Table 2. The activation parameters for the rate-determining step were obtained by the least-squares method of a plot of log k vs 1/T and are presented in Table 2. 3.6. Test for Free Radicals (Polymerization Study). The intervention of free radicals was examined as follows: The reaction mixture, to which a known quantity of acrylonitrile

In the later period of the 20th century, the kinetics of oxidation of various organic and inorganic substrates by Ag(III) species, which may be due to the strong versatile nature of a two-electron oxidant, was studied. Among the various species of Ag(III), Ag(OH)4-, diperiodatoargentate(III), and ethylenebis (biguanide) (EBS) silver(III) were of maximum attention to the researchers due to their relative stability.15 The stability of Ag(OH)4- is very sensitive toward traces of dissolved oxygen and other impurities in the reaction medium, and it had not drawn much attention. However, the other two forms of Ag(III)4-9,16 are considerably stable; the DPA is used in highly alkaline medium, and EBS is used in highly acidic medium. It is known that L-leucine exists in the zwitterion17 form in aqueous medium. In highly acidic medium, it exists in the protonated form, whereas in highly basic medium is exists as the anionic form according to the following equilibria.

( )

+ R-CH(NH2)COOH h R-CH NH3 COOzwitterion R-CH(NH2)COOH + OH- h R-CH(NH2)COO- + H2O A literature survey11 reveals that the water soluble diperiodatoargentate(III) (DPA) has a formula [Ag(IO6)2]7- with dsp2 configuration of square planar structure, similar to a diperiodatocopper(III) complex with two bidentate ligands, a periodate to form a planar molecule. When the same molecule is used in alkaline medium, it is unlikely to exist as [Ag(IO6)2]7- as a periodate is known to be in various protonated forms18 depending on the pH of the solution as given in following multiple

Table 2. Thermodynamic Activation Parameters for the Oxidation of L-Leucine by DPA in Aqueous Alkaline Medium with Respect to the Slow Step of Scheme 1 (A) Effect of Temperature T (K) 298 303 308 313

(B) Activation Parameters (Scheme 1)

103 k

(s-1)

4.29 4.98 5.67 6.58

parameters

values

∆Hq (kJ mol-1) ∆Sq (J K-1 mol-1) ∆Gq (kJ mol-1) log A

19.4 ( 0.5 -225 ( 25 86.4 ( 4.0 1.46 ( 0.15

(C) Effect of Temperature To Calculate K1, K2, and K3 for the Oxidation of L-Leucine by Diperiodatoargentate(III) in Alkaline Medium T (K)

K1 (dm3 mol-1)

K2 × 104 (mol dm-3)

K3 × 10-2 (dm3 mol-1)

298 303 308 313

0.120 ( 0.006 0.31 ( 0.01 0.55 ( 0.02 0.89 ( 0.05

3.81 ( 0.15 3.05 ( 0.12 2.28 ( 0.10 1.65 ( 0.08

2.44 ( 0.10 3.42 ( 0.11 5.12 ( 0.24 8.20 ( 0.32

(D) Thermodynamic Quantities Using K1, K2, and K3 thermodynamic quantities

values from K1

values from K2

values from K3

∆H (kJ mol-1) ∆S (J K-1 mol-1) ∆G298 (kJ mol-1)

99 ( 4 316 ( 15 5.1 ( 0.2

-43.7 ( 0.8 -212 ( 10 19.5 ( 0.8

62.5 ( 3.1 255 ( 15 -13.7 ( 0.5

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of L-leucine combines with Ag(II) species in a fast step to give an aldehyde, which reacts with another molecule of MPA in a fast step to give the products. Thus, all these results indicate a mechanism of the type in Scheme 1. Since Scheme 1 is in accordance with the generally wellaccepted principle of noncomplementary oxidations taking place in a 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 as given in the Experimental Section. This type of radical intermediate has been observed in earlier work.21 However, the Michaelis-Menten plot proved the complex formation between oxidant and reductant, which explains the less than unit order in [L-leucine]. On the basis of the square planar structure of DPA, the structures of MPA and the complex may be proposed as below.

Scheme 1. Detailed Mechanism for the Oxidation of L-Leucine by Alkaline DPA

equilibria (2)-(4).

H5IO6 h H4IO6- + H+

(2)

H4IO6- h H3IO62- + H+

(3)

H3IO62- h H2IO63- + H+

(4)

Periodic acid (H5IO6) exists in acid medium and also as H4IO6- at pH 7. Thus, under the present alkaline conditions, the main species are expected to be H3IO62-and H2IO63-. At higher concentrations, periodate also tends to dimerize.19 On the contrary, the authors5,6 in their recent past studies have proposed the DPA as [Ag(HL)2]x- in which “L” is a periodate with an uncertain number of protons and “HL” is a protonated periodate of an uncertain number of protons. This can be ruled out by considering the alternative form18 of IO4- at pH > 7 which is in the form H3IO62- or H2IO63-. Hence, DPA could be [Ag(H3IO6)2]- or [Ag(H2IO6)2]3- in alkaline medium. Therefore, under the present conditions, diperiodatoargentate(III) may be depicted as [Ag(H3IO6)2]-. The similar speciation of periodate in alkali was proposed20 for diperiodatonickelate(IV). Since the rate of reaction was enhanced by [OH-], the added periodate retarded the rate, and the reaction had first order dependency in [DPA] and fractional order dependency in [L-leucine], the following mechanism (see Scheme 1) has been proposed by considering [L-leucine] to be in the anionic form which explains all other experimental observations. In the prior equilibrium step 1, the [OH-] deprotonates the DPA to give a deprotonated diperiodatoargentate(III); in the second step, displacement of a ligand, periodate, takes place to give free periodate which is evidenced by decrease in the rate with increase in [periodate] (Table 1). It may be expected that a lower Ag(III) periodate species such as monoperiodatoargentate(III) (MPA) is more important in the reaction than the DPA. The inverse fractional order in [H3IO6]2- might also be due to this. In the pre-rate-determining stage, this MPA combines with a molecule of an anionic species of L-leucine to give a complex, which decomposes in a slow step to give the free radical derived from L-leucine and Ag(II) species. The formation of intermediate Ag(II) species is evidenced by earlier work.7-9 This free radical

Spectroscopic evidence for the complex formation between oxidant and substrate was obtained from UV-vis spectra of L-leucine (8.0 × 10-3), DPA (5.0 × 10-5), [OH-] ) 0.50 mol dm-3, and a mixture of both. A bathochromic shift of about 12 nm from 347 nm to 359 nm in the spectra of DPA was observed. The rate law for Scheme 1 could be derived as rate ) -

d[DPA] ) dt kK1K2K3[DPA][L-leu][OH-]

[H3IO6 ] + K1[OH ][H3IO62-] + K1K2[OH-] + K1K2K3[OH-][L-leu] 2-

-

kobs ) kK1K2K3[L-leu][OH-] [H3IO6 ] + K1[OH ][H3IO62-] + K1K2[OH-] + K1K2K3[OH-][L-leu] 2-

-

(5) The rate law (5) can be rearranged into the following form, which is suitable for verification

[H3IO62-] [H3IO62-] 1 + ) + kobs kK K K [OH-][L-leu] kK2K3[L-leu] 1 2 3 1 1 + (6) kK3[L-leu] k According to eq 6, other conditions being constant, the plots of 1/kobs vs [H3IO6]2-, 1/[OH-], and 1/[L-leucine] were linear as shown in Figure 3. From the intercepts and slopes of such plots, the reaction constants K1, K2, K3, and k were calculated as (0.12 ( 0.05) dm3 mol-1, (3.81 ( 0.12) × 10-4 mol dm-3, (2.44 ( 0.10) × 102 dm3 mol-1, and (4.29 ( 0.12) × 10-3 s-1, respectively. The values of K1 and K2 obtained are also in

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Figure 3. Verification of rate law (5) on the oxidation of L-leucine by diperiodatoargentate(III) at 298 K: 1/kobs vs 1/[L-leucine] (-[-); 1/kobs vs [H3IO6]2(-9-); 1/kobs vs 1/[OH-] (-2-). Table 3. Activation Parameters for Some Amino Acids (for isokinetic temperature) amino acids L-proline L-alanine L-isoleucine L-leucine

k1 × 103 (s-1) at 298 K

k2 × 103 (s-1) at 303 K

∆Sq (J K-1 mol-1)

∆Hq (kJ mol-1)

∆Gq (kJ mol-1)

ref

3.32 3.04 0.011 4.29

3.84 3.66 0.012 4.98

-211 -212 -231 -225

25.0 24.0 14.9 19.4

88.0 87.2 83.9 86.4

31 32 33 present work

agreement with earlier literature.22 These constants were used to calculate the rate constants and compared with the experimental kobs values and found to be in reasonable agreement with each other, which fortifies Scheme 1. The equilibrium constant K1 is far greater than K2. This may be attributed to the greater tendency of DPA to undergo deprotonation compared to the formation of hydrolyzed species in alkaline medium. The negligible effect of ionic strength on the rate explains qualitatively the reaction between one negatively charged ion and a neutral molecule, as seen in Scheme 1. The effect of the solvent on the reaction rate has been described in detail in the literature.23 Increasing the content of tert-butyl alcohol in the reaction medium leads to an increased effect on the rate of reaction, which seems to be contrary to the expected reaction between neutral and anionic species in media of lower relative permittivity. However, an increase in the rate of the reaction with decreasing relative permittivity may be due to the stabilization of the complex (C) at low relative permittivity, which is less solvated than DPA at higher relative permittivity because of its larger size. The thermodynamic quantities for the different equilibrium steps, in Scheme 1 can be evaluated as follows. The L-leucine and hydroxide ion concentrations given in Table 1 were varied at different temperatures. The plots of 1/kobs vs 1/[L-leucine] (r g 0.9990, s e 0.001 22), 1/kobs vs [H3IO6]2- (r g 0.9928, s e 0.005 71), and 1/kobs vs 1/[OH-] (r g 0.9994, s e 0.000 81) should be linear as shown in Figure 3. From the slopes and intercepts, the values of K1 are calculated at different temperatures. A van’t Hoff’s plot was made for the variation of K1

with temperature [i.e., log K1 vs 1/T (r g 0.9994, s e 0.1101)], and the values of the enthalpy of reaction ∆H, entropy of reaction ∆S, and free energy of reaction ∆G were calculated. These values are also given in Table 2. A comparison of the latter values 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 the rate-determining step is fairly slow and involves high activation energy.24 In the same manner, K2 and K3 values were calculated at different temperatures and the corresponding values of thermodynamic quantities are given in Table 2. The values of ∆Sqand ∆Hq were both favorable for electron transfer processes. The favorable enthalpy was due to release of energy on solution changes in the transition state. The low value of the enthalpy of activation obtained might be due to the involvement of prior equilibrium steps,25a as given in Scheme 1. The high negative value of ∆Sq (-225 J K-1 mol-1) suggests that the intermediate complex is more ordered than the reactants.25b The observed modest enthalpy of activation and a higher rate constant for the slow step indicates that the oxidation presumably occurs via an inner-sphere mechanism. This conclusion is supported by earlier observations.26,27 The activation parameters for the oxidation of some amino acids by DPA are summarized in Table 3. The entropy of the activation for the title reaction falls within the observed range. Variation in the rate within a reaction series may be caused by change in the enthalpy or entropy of activation. Changes in the rate are caused by changes in both ∆Hq and ∆Sq, but these quantities vary extensively in a parallel fashion. A plot of ∆Hq

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[DPA]T ) [DPA]f + [Ag(H3IO6)(H2IO6)]2- + [Ag(H3IO6)(H2O)2] + [C]

{

) [DPA]f 1 + K1[OH-] +

K1K2[OH-] [H3IO62-]

+

}

K1K2K3[OH-][L-leu] [H3IO62-]

[DPA]f ) [DPA]T[H3IO62-] [H3IO62-] + K1[OH-][H3IO62-] + K1K2K3[OH-][L-leu] (II) [OH]T ) [OH]f + [Ag(H3IO6)(H2IO6)]2- + [Ag(H3IO6)(H2O)2] Figure 4. Plot of log k2 at 303 K versus log k1 at 298 K (conditions as given in Table 3): (1) L-alanine; (2) L-proline; (3) L-leucine; (4) L-isoleucine.

versus

∆Sq

-

) [OH]f + K1[DPA][OH ] +

is linear according to the following equation.

∆Hq ) β∆Sq + constant β is called the isokinetic temperature; it has been asserted that apparently linear correlations of ∆Hq with ∆Sq are sometimes misleading and the evaluation of β by means of the above equation lacks statistical validity.28 Exner29 advocates an alternative method for the treatment of experimental data. 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 240.3 K by plotting log k1 at 298 K versus log k2 at 303 K (r g 0.9997, s e 0.006) in Figure 4. The value of β (240.3 K) is lower than experimental temperature (298 K). This indicates that the rate is governed by the entropy of activation.30 The linearity and the slope of the plot obtained may confirm that the kinetics of these reactions follows a similar mechanism, as previously suggested. 5. Conclusions Among the various species of Ag(III) in alkaline medium, monoperiodatoargentate(III) is considered to be the active species of Ag(III). The pH of the medium is crucial. The activation parameters with respect to the slow step of the reaction and thermodynamic parameters were computed. The overall mechanistic sequence described is consistent with product studies and kinetic studies. Appendix According to Scheme 1

kK1K2K3[DPA][OH-][L-leu] -d[DPA] ) k[C] ) dt [H3IO6]2(I)

[H3IO62-]

In view of the low concentration of DPA and H3IO62- used,

[OH]T ) [OH]f

(III)

[L-leu]T ) [L-leu]f + [C] ) [L-leu]f + K1K2K3[DPA][OH-][L-leu] [H3IO62-] In view of the low concentration of DPA, OH-, and H3IO62used,

[L-leu]T ) [L-leu]f

β ) T1T2(b - 1)/T2b - T1

Rate )

K1K2[DPA][OH-]

(IV)

Substituting (II), (III), and (IV) in (I) and omitting the subscripts T and f, we get Rate ) kK1K2K3[DPA][L-leu][OH-] [H3IO6 ] + K1[OH ][H3IO62-] + K1K2[OH-] + K1K2K3[OH-][L-leu] 2-

-

Nomenclature (DPA) ) diperiodatoargentate(III) (MPA) ) monoperiodatoargentate(III) [L-leu] ) L-leucine “” ) molar absorption coefficient kobs ) observed rate constant r ) regression coefficient s ) standard deviation k ) rate constant w.r.t slow step of the mechanism K1, K2, and K3 ) equilibrium constants ∆H ) change in enthalpy of reaction ∆S ) change in entropy of reaction ∆G ) change in free energy of reaction ∆Sq ) entropy of activation ∆Hq ) enthalpy of activation ∆Gq ) free energy of activation D ) dielectric constant of the medium I ) ionic strength of the medium T (K) ) absolute temperature in kelvin

Ind. Eng. Chem. Res., Vol. 45, No. 24, 2006 8035

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ReceiVed for reView May 18, 2006 ReVised manuscript receiVed August 31, 2006 Accepted September 16, 2006 IE060612D