Effects of Microheterogeneous Environments of SDS, TX-100, and

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Effects of Microheterogeneous Environments of SDS, TX-100, and Tween 20 on the Electron Transfer Reaction between L‑Leucine and AuCl4−/AuCl3(OH)− Pratik K. Sen,*,† Nasimul Gani,† and Biswajit Pal‡ †

Department of Chemistry, Jadavpur University, Kolkata 700032, India Department of Chemistry, St. Paul’s C. M. College, 33/1 Raja Rammohan Roy Sarani, Kolkata 700009, India

‡

S Supporting Information *

ABSTRACT: The effect of an ionic (SDS) and two nonionic (TX-100 and Tween 20) surfactants on the electron transfer reaction between L-leucine and gold(III) complexes has been investigated in acetate buffer. H+, Cl−, and decreasing dielectric constant of the medium have inhibiting effects on the reaction rate. The reaction product has been identified as isovaleraldehyde by 1H NMR. Two different Au(III) species and the zwitterion form of the amino acid react where Au(III) undergoes a one step two-electron transfer process. In the presence of surfactants, the postmicellar kinetics has been explained in the light of Berezin’s model where both the oxidant and the substrate are solubilized in the micellar pseudophase and then react. The leucine molecules occupy the hydrophobic core of the micelle with the polar amino and acid groups projected in the Stern/palisade layer among the surfactant head-groups and the oxidant species. The binding constants for substrate−micelle association and the corresponding enthalpy changes have been evaluated. The hydrophobic interaction of leucine with SDS micelles is greater than those with Tween 20 and TX-100 micelles. The compensation between substrate−water interaction and substrate−micelle interaction determines the enthalpy change for the substrate−micelle association. Entropy change for (Leu)W → (Leu)M controls substrate−micelle binding.

1. INTRODUCTION Micelles are biologically important dynamic aggregates that are formed in aqueous solutions by amphiphilic surfactant molecules and possess anisotropic interfacial regions forming the boundary between the highly polar aqueous and the nonpolar hydrocarbon phases.1−3 Such organized assemblies are well-known to affect the physical as well as chemical properties of a system by compartmentalizing the reactant molecules by means of electrostatic and hydrophobic interactions. Enhancement/inhibition of reaction rates of organic molecules in the presence of micelles/microenvironments has received wide attention in recent times since micelles may be taken as model for enzyme catalyzed reactions and their hydrophobic interactions in biochemical systems may be compared to the complex reactions occurring in biological assemblies.4,5 Liposomes are lipid−water systems, which have come into widespread use as a simplified model of biological membranes and delivery systems.6 Study of the processes involved in liposome−surfactant solubilization are also of great value as this can provide useful information to better understand complex phenomenon7−9 within biological systems. The oxidations of a few neutral amino acids have been reported in micellar media,10−12 where the surfactants modified the reaction rate although the mechanism remains unchanged. Thus it may be interesting to study the oxidation of amino acids in different micellar media. In the recent past, gold compounds were widely used as drug materials.13−17 However, they were found to be potentially toxic and the toxicity may be related to the reactions of gold(III) in the biological system.18,19 Thus, it is important to © 2013 American Chemical Society

understand the chemistry of gold(III)-biomolecule interactions. Reactions of amino acids with gold(III) will naturally be an important aspect for investigation due to the degradation of these compounds in biological systems. A number of studies have been reported exploring the reduction of gold(III) by amino acids such as cystine,20 methionine,21,22 glycine,23,24 alanine25 and glutamic acid.26 Leucine is a vital amino acid and plays important roles in the functioning of living system. This amino acid is present in amino acid pools and in active site cavities of several enzymes. So, oxidation of this amino acid may help in understanding some aspects of enzyme kinetics. There are reports in the literature for the oxidation of L-leucine by different oxidants such as Mn(VII),27−29 Ag(III),30,31 tetra butyl ammonium tribromide,32 N-bromoacetamide33 and chloramine-T.34 However, no such investigations are available in the literature on the oxidation of the titled amino acid by gold(III) chloride complexes as well as the effect of surfactants on such reactions. Leucine, a neutral amino acid contains an extra hydrocarbon chain over glycine and alanine and this may result in an extra effect on the reaction center. Apart from that there is a possibility that the extra hydrocarbon chain will have hydrophobic interactions with surfactant molecules. In view of these, we have taken up systematic kinetic studies of the oxidation of L-leucine by gold(III) complexes in aqueous acid and anionic (SDS) and nonionic (TX-100 and Tween 20) Received: Revised: Accepted: Published: 2803

September 28, 2012 January 31, 2013 January 31, 2013 January 31, 2013 dx.doi.org/10.1021/ie302656d | Ind. Eng. Chem. Res. 2013, 52, 2803−2813

Industrial & Engineering Chemistry Research

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micellar media in order to find out the mechanistic aspects and evaluate different kinetic and thermodynamic parameters for the reaction. The reaction could not be studied in the presence of CTAB (cationic surfactant) since anionic gold complexes form a precipitate with the cationic surfactant.

that of the DNP derivative of isovaleraldehyde or 3methylbutanal. The DNP derivative was further analyzed by 1 H NMR (300 MHz, Bruker DPX 300, CDCl3) spectroscopy (Figure S1, Supporting Information). The spectral data reveal characteristic signals, e.g., a broad singlet at δ 11.03 corresponds to the presence of −NH moiety of the hydrazone which is intramolecularly hydrogen bonded with the −NO2 group at the ortho position. A doublet with J = 2.6 Hz at δ 9.12 was assigned to the aromatic H flanked between two −NO2 groups. A characteristic doublet of doublet at δ 8.29 with J1 = 9.6 Hz and J2 = 2.6 Hz appeared for the aromatic −H ortho with respect to one NO2 group and para with respect to the other NO2 group. The remaining aromatic −H which is meta with respect to both the NO2 groups appeared as a doublet at δ 7.93 with J = 9.6 Hz. All of these signals corroborated with the presence of 2,4-dinitrophenyl hydrazone moiety in the product. A triplet at δ 7.53 with J = 6 Hz has been assigned to the −CHN proton which is significant for an imine-type linkage. Triplets at δ 2.32 (J = 6.0 Hz) was due to −CH2 moiety. A septet at δ 1.98 with J = 6.7 Hz (due to (CH3)2 CH−) and doublet at δ 1.03 with J = 6.7 Hz (due to (CH3)2 CH− moiety) altogether attest the isopropyl moiety. Thus, from the 1H NMR spectral analysis, the 2,4-DNP derivative isolated from the reaction mixture has been proved to be obtained from isovaleraldehyde (Me 2 CHCH 2 CHO), which obviously must have been produced from leucine (see Scheme 1).

2. MATERIALS AND METHODS 2.1. Materials. Aqueous solutions of L-leucine (extrapure CHR, SRL, India) were prepared by dissolving the appropriate amount of sample in double distilled water. NaClO4 (Loba, India), TritonX-100 (SRL, India), Tween 20 (SRL, India), and SDS (SRL, India) were used without purification. 1,4-Dioxane (extrapure, AR, SRL, India) was purified by treatment with Mohr salt followed by filtration and distillation. Gold(III) solution was prepared in 0.01 mol dm−3 HCl by dissolving chloroauric acid trihydrate (GR, SRL, India). The absorption spectra of gold(III) solutions in the concentration range (0.30 − 2.37) × 10−4 mol dm−3 in 0.01 mol dm−3 hydrochloric acid were recorded in the UV region on a Shimadzu 1700 model UV−vis spectrophotometer and the spectral pattern was found to remain unaltered with changes in the concentration of gold(III). An absorption maximum at 313 nm (ε = 4860 dm3 mol−1 cm−1) was obtained.35 The concentration of the gold(III) solution was estimated as mentioned earlier.25 A calibrated Jencon (India) tensiometer was used to measure the surface tension at the air/solution interface of the surfactant solutions by the du Nuoy ring detachment method. 2.2. Kinetic Measurements. The pseudo-first-order conditions ([L-leucine] ≫ [Au(III)]) were maintained during all of the kinetic runs. Sodium acetate−acetic acid buffer was used to maintain the constancy of pH. The kinetic advancement of the reaction was monitored by noting the absorbance (A) at 400 nm, although the maximum absorbance of Au(III) occurs at 313 nm. This is due to the fact that there is a possibility of the formation of colloidal gold in the presence of UV light (especially where reducing species are present in the reaction system), which might complicate the rate measurements.36 All of the kinetic investigations were carried out in an Elico (India) UV−visible spectrophotometer (model BL-198) with a peltier controlled thermostatted cell compartment. The plots of log A (A = absorbance) versus time were linear at least up to two half-lives and the pseudo-first-order rate constants (kobs), evaluated from such plots, were reproducible to within ±7%. 2.3. Product Analysis. The main product of the oxidation of leucine is the corresponding carbonyl compound. The reaction product was identified by spot tests37 and then by 1H NMR spectroscopy. The presence of ammonia in the reaction mixture was detected by the Nessler’s reagent test. The liberated CO2 was qualitatively detected by limewater test. In a typical experiment, the reaction mixture containing leucine and gold(III) chloride was allowed to stand for 2 h in a closed vessel and then distilled. The distillate was then subjected to the following tests. A drop of fuchsin reagent was taken in a spot plate and treated with a drop of sulphurous acid when it turned colorless and then it was treated with two drops of the distillate. A pink color appeared37 within 5 min, which confirmed the presence of an aldehyde (−CHO) group in the reaction product. Another portion of the distillate was then treated with 2,4-dinitrophenyl-hydrazine (DNP) solution in 4 N H2SO4 which led to the precipitation of an orange DNP derivative. After purification, the melting point of the DNP derivative was found to be 122 °C,38 which was identical with

Scheme 1. 2,4-DNP Derivative of the Oxidation Product of L-Leucine

Thus the stoichiometry of the reaction may be written as RCH(NH 2)COOH + Au 3 + + H 2O → RCHO + Au+ + CO2 + NH4 + + H+

(1)

where R = −CH2CH(CH3)2.

3. RESULTS 3.1. Effect of [AuIII] and [Leu] on kobs. The effect of gold(III) on the reaction rate was studied at different [AuIII] in the region (0.80−4.80) × 10−3 mol dm−3 but at constant [Leu], [H+], and [Cl−] and temperature of 4.85 × 10−2, 1.00 × 10−4, and 0.04 mol dm−3 and 298 K, respectively. An exponential decrease of gold(III) concentration was observed with time. Thus, the plots of log A versus time were drawn for different initial [AuIII] and parallel straight lines were found. The value of kobs obtained from the slopes of such plots was found be (1.04 ± 0.05) × 10−3 s−1, which indicates a simple first order dependence on [AuIII] (Table 1). The concentration of leucine was also varied ((0.815−8.15) × 10−2 mol dm−3) at fixed concentrations of other reactants at four different temperatures. An increase in reaction rate was observed with an increase in 2804

dx.doi.org/10.1021/ie302656d | Ind. Eng. Chem. Res. 2013, 52, 2803−2813


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Table 1. Effect of [AuIII], [Leu], Ionic Strength (μ), [H+], and [Cl−] on the Rate of Oxidation of Leucine at 298 K 103 [AuIII] (mol dm−3)

102 [Leu] (mol dm−3)

μ (mol dm−3)

105 [H+] (mol dm−3)

10 [Cl−] (mol dm−3)

104 kobs (s−1)

0.9 1.2 2.4 3.6 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

4.85 4.85 4.85 4.85 1.63 3.26 6.52 8.15 3.26 3.26 3.26 1.52 1.52 1.52 1.52 1.52 1.64 1.64 1.64 1.64

0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.18 0.28 0.38 0.16 0.12 0.10 0.07 0.06 0.14 0.24 0.34 0.44

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 1.58 3.55 5.37 14.1 19.1 10.0 10.0 10.0 10.0

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 1.0 2.0 3.0 4.0

10.4 10.5 10.7 10.5 5.02 8.40 12.8 14.2 8.53 8.47 8.51 9.31 7.97 6.69 4.26 3.65 3.26 2.15 1.53 1.12

3.2. Effect of Ionic Strength. Keeping [AuIII], [Leu], [H+], [Cl−], and temperature constant at 1.20 × 10−3, 3.26 × 10−2, 1.0 × 10−4, and 0.04 mol dm−3 and 298 K, respectively, the effect of changing ionic strength on the pseudo-first-order rate constant was studied by adding NaClO4 (0−0.30 mol dm−3). The pseudo-first-order rate constant was found to remain unaltered (8.53 ± 0.08 s−1) indicating that variation of ionic strength has no influence on the reaction rate (Table 1). 3.3. Effect of Variation of [H+]. The pseudo-first-order rate constant (kobs) was found to be dependent on [H+] and the effect was studied by a variation of pH of the reaction mixture from 3.72 to 4.80, keeping other parameters unaltered. Ionic strength was not maintained constant since it has no influence on the reaction rate. The rate was found to increase with a decrease in [H+] (Table 1). The plot of kobs versus [H+] was not linear. Instead a plot of kobs−1 versus [H+] produced a straight line with a positive slope and a positive intercept (Figure 2). 3.4. Effect of Variation of [Cl−]. Chloride ion takes part in governing the equilibria involving the dissociation of chloroaurate(III) complexes. Thus, it may be expected that Cl− ion will have an important role in controlling the different mechanistic steps in the present reaction. [Cl−] was varied from 0.02 to 0.40 mol dm−3 by the addition of NaCl, keeping [AuIII], [Leu], [H+], and temperature constant at 1.20 × 10−3, 1.64 × 10−2, and 1.0 × 10−4 mol dm−3 and 298 K, respectively, and the influence of this variation on kobs was analyzed. The reaction rate was found to diminish with increasing [Cl−] (Table 1). The plot of kobs versus [Cl−] showed a curvature. However, a plot of kobs−1 versus [Cl−] showed a good linearity (R2 ≈ 0.98) with a positive slope and a positive intercept (Figure 3). 3.5. Effect of Changing Dielectric Constant. Varying amounts of 1,4-dioxane (0−40% v/v) were added to the reaction mixture in order to find out if there was any influence of solvent parameters on the reaction rate, where other variables such as [AuIII], [Leu], pH, [Cl−], and temperature were kept unaltered. Addition of dioxane was found to have an adverse effect on the reaction rate. The pseudo-first-order rate

[Leu] (Table 1). A plot of kobs (pseudo-first-order rate constant) versus [Leu] at a definite temperature did not produce a straight line passing through the origin indicating that the reaction was not simple first order with respect to [Leu]. Also a plot of log kobs versus log [Leu] at different temperatures gave straight lines with slopes ranging from 0.69 to 0.87, which indicated that the reaction was of complex order with respect to leucine. However, when a Lineweaver and Burk plot of kobs−1 versus [Leu]−1 (Figure 1) was drawn, a straight

Figure 1. Variation of reaction rate with leucine concentration. Plots of kobs−1 versus [Leu]−1 at four different temperatures. [AuIII] = 1.20 × 10−3 mol dm−3, [H+] = 1.0 × 10−4 mol dm−3, and [Cl−] = 4.0 × 10−2 mol dm−3.

line (R2 ≈ 0.99) was obtained at each temperature with a positive slope and a positive intercept on the ordinate. Such a plot indicates that a Michaelis−Menten type of intermediate complex is formed between gold(III) and leucine in a fast preequilibrium step.39−42 2805



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experiments were carried out at pH 4.0 in the presence of leucine (5.0 × 10−2 mol dm−3) and gold(III) (1.2 × 10−3 mol dm−3) separately, but little change was noted in the cmc value. The cmc values of Tween 20 and SDS were also measured tensiometrically in acetate buffer medium (pH 4.0) and were found to be 2.71 × 10−5 and 6.5 × 10−3 mol dm−3, respectively. 3.8. Oxidation in the Presence of Surfactant. The oxidation of leucine by Au(III) was also studied in the presence of two nonionic surfactants (TX-100 and Tween 20) and an ionic surfactant (SDS). Influence of these three surfactants on the rate of the reaction was studied at different temperatures by changing the surfactant concentrations while keeping other reactant concentrations, viz. [AuIII], [Leu], [H+], [Cl−] unchanged. The kΨ value first increases, passes through a maximum and then decreases gradually (Figures 4a−c). The kinetic cmc’s of the surfactants (where kΨ becomes maximum) in the presence of acetate buffer (pH 4.0) and leucine were determined from the above plots. The values are 2.8 × 10−4, 3.0 × 10−5, and 7.0 × 10−3 mol dm−3 for TX-100, Tween 20 and SDS respectively. The cmc values of the three surfactants in the presence of acetate buffer (pH 4.0), already determined by the tensiometric method (section 3.7), are found to be very close to the respective kinetic cmc’s obtained from the plots of Figures 4a−c.

Figure 2. Influence of [H+] on the pseudo-first-order rate constant. Plot of kobs−1 versus [H+] at 298 K. [AuIII] = 1.20 × 10−3 mol dm−3, [Leu] = 1.52 × 10−2 mol dm−3, and [Cl−] = 4.0 × 10−2 mol dm−3.

4. DISCUSSION The standard reduction potential of Au3+/Au+ is highly dependent on the ligands attached to the metal with different oxidation states. The value for the AuCl4−/AuCl2− system43 is +0.93 V. Tetrachloroauric(III) acid dissociates to form the square planner AuCl4−, which in turn undergoes fast hydrolysis producing two other gold(III) species, viz, AuCl3(OH2) and AuCl3(OH)−. Thus four gold(III) species, viz., HAuCl4, AuCl4−, AuCl3(OH2), and AuCl3(OH)−, coexist in a solution of chloroauric acid and make the system complicated. K1

HAuCl4 ⇌ H+ + AuCl4 −

(2)

K2

AuCl4 − + H 2O ⇌ AuCl3(OH 2) + Cl−

(3)

K3

AuCl3(OH 2) ⇌ AuCl3(OH)− + H+

Figure 3. Variation of pseudo-first-order rate constant with chloride ion concentration. Plot of kobs−1 versus [Cl−] at 298 K. [AuIII] = 1.20 × 10−3 mol dm−3, [Leu] = 1.64 × 10−2 mol dm−3, and [H+] = 1.0 × 10−4 mol dm−3.

(4)

The corresponding equilibrium constant values are K1 = 1.0, K2 = 9.5 × 10−6, and K3 = 0.25 at 298 K.44 It is to be noted that most of the experiments were carried out at an [H+] = 1.0 × 10−4 mol dm−3 and hence [HAuCl4] ≪ [AuCl4−]. Further the kinetic experiments were mostly performed at [Cl−] = 0.04 mol dm−3. Thus under the experimental conditions, the concentrations of different Au(III) species are found to be in the ratio [AuCl4−]/[AuCl3(OH2)] ≈ 4200:1 and [AuCl3(OH)−]/ [AuCl3(OH2)] ≈ 2500:1. Therefore, it is evident that AuCl4 − and AuCl3(OH)− are the predominant oxidizing species in the present reaction. Amino acids are known to exist as cations (H2A+), zwitterions (HA), and anions (A−) in aqueous solutions and their relative concentrations are pH dependent.

constant decreases with decreasing dielectric constant of the medium (Table S1, Supporting Information). 3.6. Test for Free Radicals. The intervention of free radicals, if any, was investigated with the addition of acrylonitrile in the concentration range 5−20% (v/v) to the reaction mixture where gold(III), leucine and chloride concentrations were maintained at 1.2 × 10−3, 4.85 × 10−2, and 0.04 mol dm−3, respectively in acetate buffer (pH 4.0) at 298 K. No suspension or precipitate was observed. This indicates that the reduction of gold(III) to gold(I) does not involve a one-electron transfer process. 3.7. Measurement of Critical Micelle Concentration (CMC). Tensiometric experiments were carried out to find out the cmc values of TX-100 solutions in aqueous medium as well as in acetate buffer (pH 4.0) and the plot of surface tension versus log [TX-100] led to the values of cmc to be 2.2 × 10−4 and 1.2 × 10−4 mol dm−3, respectively, at 298 K. Control

+

K4

+

RCH(NH3)COOH ⇌ RCH(NH3)COO− + H+ (H 2A+) +

(HA) K5

RCH(NH3)COO− ⇌ RCH(NH 2)COO− + H+ (HA)

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(5)

(A−)

(6)



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viz, AuCl4− and AuCl3(OH)−. Lineweaver and Burk type of plot of kobs−1 versus [Leu]−1 indicates the existence of a preequilibrium involving an intermediate complex formation (AuCl3·A)− between the above-mentioned reacting species. The intermediate complex (AuCl3·A)− decomposes in a slow step to give the products. Scheme 2. Reaction Steps Involved in the Oxidation of Leucine

On the basis of the reaction in Scheme 2, applying the steady state approximation to the intermediate (AuCl3·A)−, the rate of the reaction is −

d[Au III] = k[(AuCl3·A)− ] dt

(7)

If C0 = [AuIII]0, x = [(AuCl3·A)−], y = [AuCl4−], and z = [AuCl3(OH)−], then C0 = x + y + z

(8)

Therefore, from eqs 3 and 4, one can get y = [AuCl−4 ] =

(C0 − x)[H+][Cl−] K 2K3 + [H+][Cl−]

(9)

Now, from Scheme 2, we get K6 =

[(AuCl3·A)− ][H+][Cl−] [AuCl−4 ][HA]

(10)

[(AuCl3·A)− ] [AuCl3(OH)− ][HA]

(11)

and K7 =

Combining eqs 9 and 10 and on simplification, we get [(AuCl3·A)− ] =

Figure 4. (a) Effect of addition of surfactants on the pseudo-first-order rate constant (kψ). Plots of (a) kψ versus [TX-100], (b) kψ versus [Tween 20], and (c) kψ versus [SDS] at three different temperatures. [AuIII] = 1.20 × 10−3 mol dm−3, [Leu] = 1.67 × 10−2 mol dm−3, [H+] = 1.0 × 10−4 mol dm−3, and [Cl−] = 4.0 × 10−2 mol dm−3.

K 6C0[HA] K 2K3 + [H+][Cl−] + K 6[HA]

(12)

Again, combining eqs 9 and 11, we get [(AuCl3·A)− ] =

where R = −CH2CH(CH3)2. The values of K4 and K5 are 4.68 × 10−3 and 1.82 × 10−10, respectively, at 298 K.45 Under the present reaction conditions (pH 3.72−4.80), the zwitterionic species, HA, is in predominant form ([HA]/[H2A+] ≈ 47:1 at pH 4.00 and 132:1 at pH 4.45). Thus, parallel reactions occur between the zwitterion (HA) form of leucine and the two gold(III) species,

K 2K3K 7C0[HA] K 2K3 + [H+][Cl−] + K 2K3K 7[HA] (13)

It is evident from eqs 12 and 13 that K 6 = K 2K3K 7 , or, K 6/K 7 = K 2K3

(14)

Thus utilizing eq 12 in eq 7, we get 2807



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Equation 18 corroborates the linear plot (Figure 2) of kobs−1 versus [H+], while eq 19 accounts for the linear plot (Figure 3) of kobs−1 versus [Cl−]. From the slopes and intercepts of these plots (Figures 2 and 3), the values of k and K6 at 298 K have been evaluated. The values of k and K6 from substrate effect, acid effect and chloride effect have been found to be in agreement with each other (Table 2). Using the different “k” values (obtained from substrate effect) at four different temperatures, a plot of ln(k/T) versus T−1 was drawn and the enthalpy of activation (ΔH⧧) for the rate determining step was evaluated using Eyring equation, followed by the determination of the entropy of activation (ΔS⧧). The values of ΔH⧧ and ΔS⧧ have been found to be (70.0 ± 2.2) kJ mol−1 and (−13.0 ± 7) J K−1 mol−1, respectively. It has already been reported25,35,46 that AuCl3(OH)− is more reactive than AuCl4−, since OH− is more easily displaced than Cl−. It is evident from eq 4 that an increase of H+ shifts the equilibrium toward left, thereby diminishing the concentration of the more reactive species AuCl3 (OH)− and thus decreasing the reaction rate . Also Scheme 2 predicts that increasing [H+] will decrease the concentration of (AuCl3·A)− and hence will inhibit the reaction rate. The retarding effect of [H+] on the reaction rate is thus explained. In a similar manner, eq 3 and the first equation of Scheme 2 indicate that an increase in [Cl−] will inhibit the reaction rate. Thus, the theoretical rate laws (eqs 17−19) explained all the kinetic results for the oxidation of leucine. A detailed mechanism of the oxidation reaction is depicted in Scheme 3, where the zwitterionic form of the substrate enters into the coordination sphere of AuCl4−/AuCl3(OH)− through the nucleophilic attack by −COO−, thereby forming the intermediate gold(III) complex.The intermediate complex decomposes in a slow step to produce an iminic cation, which finally produces the corresponding aldehyde and NH4+ through fast hydrolysis.25,47,48 The existence of similar type of gold(III) intermediate complex has already been reported in a number of studies.23,26,49 The formation of isovaleraldehyde in the reaction product has been confirmed by 1H NMR spectroscopy as discussed in the product analysis part (section 2.3). A decrease in solvent polarity decreases the reaction rate. Considering equilibria 2 and 4, it is evident that the backward reaction will be favored by a lowering of the dielectric constant (ε) of the medium, since both of these involve reaction between oppositely charged ions. Thus, a lowering of “ε” diminishes the concentrations of the effective oxidant species, viz., AuCl4− and AuCl3(OH)−, thereby decreasing the reaction rate. Moreover, the intermediate complex (AuCl3·A)− disproportionates to a number of oppositely charged ions, which

1 d[Au III] − = kobs [Au III] dt =

kK 6[HA] K 2K3 + [H+][Cl−] + K 6[HA] (15)

or kobs =

kK 6[Leu]T K 2K3 + [H+][Cl−] + K 6[Leu]T

(16)

The above equation may be rearranged to 1 kobs

=

K K + [H+][Cl−] 1 1 + 2 3 k kK 6 [Leu]T

(17)

or 1 kobs

⎛1 K 2K3 ⎞ [Cl−] [H+] =⎜ + ⎟+ kK 6[Leu]T ⎠ kK 6[Leu]T ⎝k

(18)

⎛1 K 2K3 ⎞ [H+] [Cl−] =⎜ + ⎟+ kK 6[Leu]T ⎠ kK 6[Leu]T ⎝k

(19)

or 1 kobs

−1

Thus, eq 17 predicts a linear plot of kobs versus [Leu]−1 with a positive slope and a positive intercept on the ordinate which is actually obtained experimentally. From the intercept of the plot, the value of k (disproportionation constant of the intermediate complex, (AuCl3·A)−) was evaluated at each temperature. The values of k are 2.33 × 10−3, 3.72 × 10−3, 5.59 × 10−3, and 9.17 × 10−3 (s−1) at 298, 303, 308, and 313 K, respectively. The formation constant K6 (at 298 K) of the intermediate complex was evaluated from the slope of the plot using the experimental value of k at 298 K and the literature values of K2 and K3 at the same temperature (Table 2). Table 2. Values of k (Rate Constant for the RateDetermining Step) and K6 and K7 (Equilibrium Constants for Intermediate Complex Formation) at 298 K for Different Sets of Experiment experiment

103 k (s−1)

104 K6 (mol dm−3)

K7 (mol−1 dm3)

substrate effect chloride effect acid effect

2.33 ± 0.19 2.23 ± 0.5 2.68 ± 0.23

1.09 ± 0.1 1.40 ± 0.4 1.01 ± 0.08

45.9 ± 4.2 56.8 ± 5.1 41.0 ± 3.4

Scheme 3. Overall Reaction Mechanism Showing the Oxidative Degradation of Leucine by Gold(III) Complexes

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monomers before the CMC are designated as premicelles. The formation of premicelles is sometimes induced by the substrate.53 In this respect, another observation of Brinchi et al.53 regarding the increase of reactivity in the premicellar region is to be noted: The substrates in premicellar complexes are more reactive than micellar bound substrates, and the rate maxima disappear as micelles form and dissolve the premicellar complexes. The rate of reaction was found to decrease with increasing surfactant concentration above the kinetic CMC value for each of the surfactants (Figures 4a−c). It may be worthwhile to mention here that the polarity of the micellar surface is known to be considerably less than that of the aqueous medium.61 Therefore, the decrease of the reaction rate may be either due to a decrease of the dielectric constant of the medium (as has been found in the presence of water-dioxane mixture) or due to the partition of the reactants between the aqueous and micellar pseudophases. However, in case of water-dioxane mixed solvent, the rate of decrease of pseudo-first-order rate constant (kobs) with the decrease of dielectric constant of the medium was found to be very high (Table S1, Supporting Information). However, in the case of a micellar solution, the rate of decrease was much slower. Hence, such a decrease of rate in the presence of micelle is possibly due to the partition of the reactants between the two pseudophases. Thus the inhibition of rates at higher concentration (above CMC) of surfactants may be explained following Berezin’s model62 in which both the reactants get solubilized in the micellar pseudophase and then react, in addition to the normal reaction occurring in the aqueous pseudophase (Scheme 4).

possibly result from a much polar transition state (T.S.). Thus, a lowering of “ε” will disfavor the T.S. and this might also play a key role in inhibiting the reaction rate. In this case the enthalpy of activation has been found to be slightly higher than that of glutamate.26 This may be explained by the fact that the isobutyl group attached to the α-carbon in the intermediate complex (AuCl3·A)− is much bulkier than the group in glutamate and hence producing some steric crowding toward the formation of the T.S. and thereby increasing ΔH⧧. Also, the positive inductive effect of the isobutyl group attached to the α-carbon of leucine may hinder the formation of the iminic cation while the −COOH group (negative inductive effect) in glutamate might help it. There are reports involving the oxidation of leucine and glutamic acid by metal ion oxidants such as MnO4−,29,50 Mn(III),51 Ag(III),30,31 and Fe(CN)63−.52 In all of these cases, the oxidation takes place through one-electron transfer process with the formation of free radicals, while in our studies such oxidation reactions have been found to proceed via one-step two-electron transfer process. Hence it was not possible to compare the activation parameters of our studies with those found in the literature. Acceleration or inhibition of reaction rates in micellar solutions arises from different rates of reaction of the reactants in the micellar pseudophase and in the aqueous pseudophase and the distribution of the reactants in these two pseudophases.53 Basically, these acceleration or retardation of reaction rates can be attributed to electrostatic and/or hydrophobic interactions between the reactants and the surfactant aggregates and in some cases to alterations in the structure of the surrounding water molecules.1 The addition of a surfactant to a buffer solution generally does not alter the pH of the buffer significantly even after the cmc.54 However, the local pH at the micellar surface may slightly differ from that in the bulk depending on the nature of the surfactant as well as the buffer. Nevertheless, in our present study both ionic and nonionic surfactants were used and the same type of variation of reaction rate with the change of surfactant concentration was observed. This possibly indicates that the partition of the reactants between the two pseudophases is primarily responsible for the variation of rate (compared to that in the aqueous pseudophase) rather than the change of pH. The initial increase in kψ with increase in surfactant concentration is owing to a catalytic effect of the surfactant monomers present in the solution. A small number of such monomers may aggregate with a substrate molecule to form a catalytic micelle and possibly such an aggregation sterically or entropically favors the reaction and enhances the reaction rate. There are instances where the rates of reactions are found to be influenced by the detergents in the premicellar region.55,56 Piszkiewicz57 analyzed a large number of published data on the dependence of rate constants on the detergent concentration. In order to explain these effects in the premicellar region, he proposed that a molecule of a substrate and a small number, n (approximately, 1−6), of detergent monomers would aggregate to form a catalytic micelle which might then react to give the product. This number, n, is sometimes viewed as an index of positive co-operativity.10 Thus it shows that catalytic micelles are different from normal micelles (formed by the aggregation of a large number of surfactant molecules only) and sometimes can explain the influence of surfactants on the reaction rate in the premicellar region. Recently, Sánchez et al. have studied the effect of surfactants on the kinetics of a number of reactions58−60 where such small assemblies of substrate and

Scheme 4. Schematic Representation of the Oxidation Reaction Taking Place in Aqueous and Miceller Pseudophases

It is apparent that a change in temperature may influence the size and shape of the micelles which may affect the reaction rate. However, in this experiment, the temperature variation during the study of micellar effect was small (only 10 °C). Moreover, the above pseudophase model works with the idea that changes in size and shape of the micelles, caused by the temperature changes as well as the presence of reactants in the reaction media, are not important and that the observed reaction rate is mainly affected by the partition of the reactants between the aqueous and micellar pseudophases.60 On the basis of Scheme 4 the following expression (eq 20) for the pseudo-first-order rate constant has been derived (Appendix 1, Supporting Information): kψ =

k W + kMKSK O(Csurf − CMC)2 [1 + KS(Csurf − CMC)][1 + K O(Csurf − CMC)]

(20)

where Ks and Ko are the binding constants of leucine and gold(III) respectively with the respective surfactant, Csurf is the analytical concentration of the particular surfactant, and kW and kM are the pseudo-first order rate constants in aqueous and 2809



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micellar pseudophases respectively. Since the oxidant is a small anion, its electrostatic interaction with the surfactant appears to be small and Ko will be of low value. Also the substrate is a small molecule and its hydrophobic interactions with the surfactant appear not to be appreciably high and so Ks will not be very high. Since the analytical concentration of the surfactant (Csurf) is very small it will not be unreasonable to think that k W > >kMKSK O(Csurf − CMC)2

Therefore, the eq 20 reduces to kψ =

kW 1 + (KS + K O)(Csurf − CMC) + KSK O(Csurf − CMC)2 (21)

Further, it is to be noted that (Csurf − CMC) is very small and thus the terms containing (Csurf −CMC)2 may be neglected, and eq 21 takes the form K + KO 1 1 (Csurf − CMC) = + S kψ kW kW

(22)

If kΨ−1 is plotted against (Csurf − CMC), a straight line with a positive slope and a positive intercept is to be expected. Good straight line plots (Figures 5a−c) were obtained for these three surfactants at three different temperatures. The values of kW at different temperatures were determined from the intercepts in the above plots for each surfactant and are recorded in Table 3. The pseudo-first-order rate constants in absence of surfactants as obtained from substrate effects are found to be 5.02 × 10−4, 6.13 × 10−4, and 7.78 × 10−4 s−1 at 298, 303, and 308 K, respectively. Thus it is observed that kW values (Table 3) at different temperatures corroborate with the above-mentioned values as also with each other. The values of binding constants (Ks + Ko) for different surfactants at different temperatures were also determined from the slopes of straight lines in Figure 5a−c (Table 3). A plot of ln(Ks + Ko) vs T−1 was found to be linear, and the values of ΔH° were evaluated from the slope of such plots (Table 3). It is quite logical to think that the interaction of the ionic species of gold(III) toward micelle will be much less than that of leucine toward micelle owing to its moderate hydrophobic hydrocarbon tail. Therefore, it may be plausible that the hydrocarbon chain of the leucine molecules occupy the hydrophobic core of the micelle with the polar amino and acid groups projected in the Stern/palisade layer among the surfactant head-groups together with the oxidant species (Scheme 5). So, it may be assumed that Ko ≪ Ks and thus Ks + Ko ≈ Ks. Therefore, ΔH° (from the plot of ln(Ks + Ko) vs T−1) will represent the enthalpy change associated with the binding of leucine with the micelle. Leucine, having a polar structure, is expected to remain solvated by water molecules in aqueous medium. During the transfer of leucine from aqueous phase to the micellar phase, breaking of the water structures will require absorption of energy (positive ΔH). On the other hand, leucine-micelle binding through hydrophobic as well as electrostatic interactions will result in a release of energy (negative ΔH). Owing to the presence of a moderate hydrophobic tail in leucine, enthalpy change for the hydrophobic interaction predominates and the release of energy (negative ΔH) overbalances the absorption of energy (positive ΔH) so that ΔH° for the transfer: [(Leu)w → (Leu)M] becomes negative. It may be possible that the hydrophobic

Figure 5. (a) Variation of pseudo-first-order rate constant (kψ) with [surfactant] in the micellar media beyond CMC. Plots of kψ−1 versus (Csurf − CMC) for (a) TX-100, (B) Tween 20, and (c) SDS at different temperatures. [AuIII] = 1.20 × 10−3 mol dm−3, [Leu] = 1.67 × 10−2 mol dm−3, [H+] = 1.0 × 10−4 mol dm−3, and [Cl−] = 4.0 × 10−2 mol dm−3. 2810



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Table 3. Values of Rate Constants (kW) in the Absence of Surfactant, the Binding Constants (KS + K0) for Substrate−Micelle Association at Different Temperatures, and the Corresponding Enthalpy Changes (ΔH°) 104 kW (s−1)

KS + K0

surfactant

298 K

303 K

308 K

298 K

303 K

308 K

ΔH° (kJ mol−1)

TX-100 Tween 20 SDS

5.16 4.99 5.07

6.12 5.81 6.09

7.82 7.45 8.04

154 ± 18 332 ± 37 12.9 ± 0.6

145 ± 15 321 ± 15 10.6 ± 0.3

135 ± 17 347 ± 40 8.2 ± 0.3

−10.5 ± 0.8 −34.2 ± 3

concluded that although the compensation between substratewater interaction and substrate-micelle interaction determines the enthalpy change for the substrate-micelle association, the ultimate substrate-micelle binding is controlled by the entropy change associated with such transfer process [(Leu)W → (Leu)M].

Scheme 5. Schematic Representation of Interaction between Reactants and SDS Micelles with Leucine Partially Embedded in the Hydrophobic Core and Gold(III) Species in the Stern Layer

5. CONCLUSION The electron transfer reaction between L -leucine and chloroaurate (III) complexes is proposed to take place through the interaction between the zwitterionic form of leucine and the two forms AuCl4− and AuCl3(OH)− of the oxidant species. The reaction passes through a pre-equilibrium to form an intermediate complex between the oxidant and the substrate and involves a one step two-electron transfer process and ultimately produces (CH3)2CH.CH2.CHO as confirmed by 1H NMR. Influence of the ionic (SDS) and nonionic (TX-100 and Tween 20) surfactants on the reaction rate in the postmicellar region has been explained on the basis of Berezin’s model which involves solubilization of both the reactants in the micellar phase. During the transfer of leucine from aqueous phase to the micellar phase, highly structured water molecules are set free resulting in absorption of energy (positive ΔH), while association of leucine with the micelle through hydrophobic as well as electrostatic interactions involves release of energy (negative ΔH). In presence of SDS the latter overbalances the former. The presence of polyoxyethylene groups and −OH groups in Tween 20 and TX-100 prevents the release of water molecules from palisade layer and thereby influences the ΔH° and ΔS° for the transfer process [(Leu)W → (Leu)M]. The compensation between substrate−water interaction and substrate−micelle interaction determines the enthalpy change for the substrate-micelle association, but the substrate−micelle binding is controlled by the entropy change.

interaction of leucine with SDS micelles is greater than that with TX-100 micelles. Hence ΔH° is more negative for SDS than TX-100 (Table 3). For Tween-20 the two enthalpy changes possibly balance each other and the net ΔH° is nearly zero. In case of Tween-20 there are twenty polyoxyethylene groups and three −OH groups which are hydrophilic in nature and such groups may be associated with a large number of water molecules in the palisade layer through hydrogen bonding. As the leucine molecule associate with Tween-20 micelles, highly structured water molecules are set free. As a result the enthalpy change becomes less negative and at the same time the entropy change (ΔS°) due to association of leucine becomes highly positive for Tween-20. Thus in spite of a very low value of ΔH° (nearly zero), the high positive ΔS° term makes the standard Gibb’s energy change (ΔG°) highly negative resulting in a high Ks value (ΔG° = −RT ln Ks) for Tween-20. The TX-100 has lesser number of hydrophilic polyoxyethylene groups and having only one −OH group in the molecule, its association with leucine will release relatively lesser number of water molecules from the palisade layer resulting in a more negative ΔH° and less positive ΔS°. This makes ΔG° moderately negative with a moderate high value of Ks which is less than that for Tween-20. However, in SDS there is neither a hydrophilic polyoxyethylene group nor any −OH group and thus association of leucine with SDS micelles does not involve a large increase in entropy. Therefore ΔG° term is much less negative and the asssocition constant (Ks) is much lower compared to Tween-20 and TX-100. Thus it may be

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: NMR spectra of 2, 4-DNP derivative of the oxidation product of L-leucine. Table S1: influence of solvent composition on the pseudo-first order rate constant at 298 K. Appendix 1: Derivation of the rate equation (eq 20). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-33-2457-2329. Fax: +91-33-2414-6223. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2811



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ACKNOWLEDGMENTS Financial assistance from DST through PURSE program to the Department of Chemistry, Jadavpur University is gratefully acknowledged. Dr. Biswajit Pal would like to thank UGC, New Delhi for providing financial support to Minor Research Project. Authors also thank Dr. Sanjoy Bhar, Associate Professor, Department of Chemistry, Jadavpur University for helpful discussions.

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Effects of Microheterogeneous Environments of SDS, TX-100, and

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