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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Influence of Microheterogeneous Environments of Sodium Dodecyl Sulfate on the Kinetics of Oxidation of L‑Serine by Chloro and Chlorohydroxo Complexes of Gold(III) Krishnendu Maiti,† Pratik K. Sen,*,† Anil K. Barik,‡ 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

‡

ABSTRACT: The oxidation of L-serine by chloro and chlorohydroxo complexes of gold(III) was spectrophotometrically investigated in acidic buffer media in the absence and presence of the anionic surfactant sodium dodecyl sulfate (SDS). The oxidation rate decreases with increase in either [H+] or [Cl−]. Gold(III) complex species react with the zwitterionic form of serine to yield acetaldehyde (principal reaction product) through oxidative decarboxylation and subsequent deamination processes. A reaction pathway involving one electron transfer from serine to Au(III) followed by homolytic cleavage of α-C−C bond with the concomitant formation of iminic cation intermediate has been proposed where Au(III) is initially reduced to Au(II). The surfactant in the submicellar region exhibits a catalytic effect on the reaction rate at [SDS] ≤ 4 mM; however, in the postmicellar region an inhibitory effect was prominent at [SDS] ≥ 4 mM. The catalytic effect below the critical micelle concentration (cmc) may be attributable to the electrostatic attraction between serine and SDS that, in turn, enhances the nucleophilicity of the carboxylate ion of the amino acid. The inhibition effect beyond cmc has been explained by considering the distribution of the reactant species between the aqueous and the micellar pseudophases that restricts the close association of the reactant species. The thermodynamic parameters ΔH0 and ΔS0 associated with the binding between serine and SDS micelle were calculated to be −14.4 ± 2 kJ mol−1 and −6.3 ± 0.5 J K−1 mol−1, respectively. Water structure rearrangement and micelle−substrate binding play instrumental roles during the transfer of the reactant species from aqueous to micellar pseudophase.

1. INTRODUCTION In the pool of amino acids, although L-serine is labeled as a nonessential amino acid, metabolically it plays an instrumental role in a number of cellular processes.1,2 In vivo L-serine acts as an important precursor for transferring one carbon unit in methylation during several biosyntheses.3 The catalytic function of many enzymes2,4 cannot be discussed without highlighting the active role of L-serine in the bio processes. Thus, the study on the oxidation of serine has recently received intense attention to understand the complicated biochemical reactions at the molecular level. Several kinetic studies have already reported the oxidation of serine by different oxidants such as Ce(IV),5 bis(hydrogen periodato)argentite(III),6 Mn(III),7 Cr(VI),8 and periodate9 in different reaction environments producing different reaction products. However, the literature lacks in kinetic data on the oxidation of the same amino acid by Au(III) species. Gold complexes in the sea of metal ion oxidants deserve special attention because of their acclaimed potential utility as metallodrugs.10−13 Since some gold(III) complexes show cytotoxic effects on different types of tumor cells, they are used as antitumor drugs.14 So long as the anticancer properties are concerned between the isoelectronic (d8) and isostructural (square-planar) Pt(II) and gold(III) species, due to the added advantage of less toxicity of gold(III) compounds in biological © XXXX American Chemical Society

systems, uses of the gold(III) compounds eclipse those of the Pt(II) compounds.15 Particularly the importance of dithiocarbamato12,13,16 and porphyrin10 derivatives of gold(III) as promising anticancer agents has recently been reflected in a number of publications. The activities of thiol-containing enzymes are proven to be inhibited by gold(III) complexes17 via ligand exchange reaction because of the softness of the gold(III) ion. Under a physiological environment which is reducing in nature, gold(III) compounds via interactions with biomolecules such as proteins and amino acids are expected to be reduced to thermodynamically stable gold(I) species;16 those are to some extent toxic18 to human health. In this context the investigation on the oxidation of proteins and amino acids by gold(III) compounds draws special attention. As per a literature survey, the kinetic data related to gold(III) mediated amino acid oxidations are scantily documented. Recently, the oxidation of a few neutral amino acids19−22 by gold(III) chloride in acetate buffer medium has been investigated in our laboratory where gold(III) and an amino acid interact to form an iminic cation as the intermediate that concomitantly decomposes to the respective aldehyde as the Received: March 12, 2018 Revised: May 3, 2018

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DOI: 10.1021/acs.jpca.8b02409 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

SRL, India) was used to find the effect of the dielectric constant on the reaction rate. Solutions of Au(III) were prepared in 0.01 M HCl and the concentrations were verified spectrophotometrically as described earlier.26 Other organic and inorganic chemicals were of the highest available purity. The solutions were prepared in deionized Milli-Q water (Merck Millipore, Darmstadt, Germany). Absorption spectra were recorded in a UV−visible spectrophotometer (Shimadzu, UV-1800, Model-Tcc-240A, Kyoto, Japan). A digital pH meter (Systronic, Model 361, Ahmedabad, India) was utilized to determine the pH of the reaction mixture. 1H NMR spectra were recorded on a Bruker DPX spectrometer (Billerica, MA, USA) operating at 300 MHz using CDCl3 as an internal reference. Measurement of surface tension of the surfactant solutions was performed by a du Nuoy tensiometer (Jencon, Kolkata, India) using the ring detachment technique. Absorbance measurements during kinetic experiments were carried out in a Shimadzu UV−visible spectrophotometer equipped with a Peltier controlled thermostated cell compartment to maintain the constant and uniform temperature of the reaction mixture. Before mixing, gold(III) solution and all other reactant species except gold(III) were kept in two separate quartz cuvettes within the spectrophotometer cell compartment for thermal equilibration. After thermal equilibration, the requisite quantity of gold(III) solution was injected into the reaction cuvette using a micropipet with thorough mixing. Throughout the experiments, pseudo-first-order conditions (concentration of serine in large excess over gold(III) concentration) were maintained; however, the gold(III) concentration was adjusted following Beer’s law. The reactions were followed by measuring the absorbance of gold(III) complexes as a function of time. The reactive process was analyzed by taking the kinetic scan (scanning interval 60 s) spectra of the reaction mixture as shown in Figure 1. The time dependent spectra reveal that the maximum absorbance26 (at 313 nm wavelength) decreases with the advancement of the reaction without shifting of the position of the λmax. This probably implies that there is no kind of intermediate

reaction product with gold(III) being reduced to gold(I). Soni et al.23 reported the oxidation of histidine by Au(III) in strong acid medium that passes through a one electron transfer mechanistic path. The oxidative deamination and subsequent decarboxylation of isotopically labeled glycine and alanine by gold(III) species has been carried out by Zou et al.,24 where an NMR study was followed to establish the reaction products. Vujačić et al.25 studied the kinetics of the reaction between tetrachloroaurate(III) and L-methionine in HClO4 medium and reported the formation of a short-lived complex, Au(III)−(Lmethionine), that subsequently turns into Au(I)−(L-methionine) via the replacement of a Cl− ligand. Serine is a neutral amino acid containing one primary −OH group on the carbon chain, for which its carbon skeleton is prone to easy oxidation in comparison with other amino acids except the sulfur-containing ones. This prompts us to make an attempt to investigate the kinetics of oxidation of serine by gold(III) with a view to elucidate a suitable mechanism and a fitting rate law. In aqueous solution, the long chain amphiphilic surfactant molecules agglomerate to form organized dynamic assemblies called “micelles” which owing to their anisotropic interfacial regions between polar aqueous and nonpolar hydrocarbon phases generate an inhomogeneous microreaction environment.26,27 As a consequence, the organized assemblies influence the physical and chemical properties of a system by enveloping the reactant molecules through the noncovalent-like hydrophobic and electrostatic interactions.26,28 A micelle catalyzed reaction is known to be similar to an enzyme catalyzed reaction in many aspects for which a micelle catalyzed reaction model may be employed to study the complex reactions occurring in biological systems.27,29,30 There are a number of surfactantmediated reactions where micelles influence the reaction rates.31−35 Micelles of different anionic and cationic surfactants are also found to accelerate or inhibit the oxidation rates of different amino acids.21,26,36−41 In the present study, since the reactants hold either charged groups and/or polar hydroxyl groups, it may be expected that cationic or anionic surfactants may interact with the reactant species through electrostatic or H-bonding that in turn may play an instrumental role in the reaction kinetics. The present work aims to investigate the kinetic and mechanistic aspects of oxidative behavior of chloro and chlorohydroxo complexes of gold(III) toward serine under various reaction conditions and also to study spectrophotometrically the effect of pre- and postmicellar aggregates of sodium dodecyl sulfate (SDS) on the rate process. Efforts have also been made to evaluate the kinetic parameters, binding constant, and thermodynamic parameters associated with substrate−micelle binding in order to support the proposed reaction mechanism and the theoretical rate law.

2. EXPERIMENTAL SECTION 2.1. Reagents, Equipment, and Kinetic Methods. LSerine (AR, SRL, Mumbai, India), chloroauric acid trihydrate (HAuCl4·3H2O, AR, SRL, Mumbai, India), NaCl (Loba, Mumbai, India), NaClO4 (Loba, Mumbai, India), CH2Cl2 (Merck, Mumbai, India), piperidine (Merck, Mumbai, India), sodium nitroprusside dihydrate (Merck, Mumbai, India), and SDS (Loba, Mumbai, India) were used as received. Rhodanine and methyl viologen were purchased from Sigma-Aldrich (Hamburg, Germany). Acetate buffer was used to maintain the pH of the reaction mixture. 1,4-Dioxane (extra pure, AR,

Figure 1. Time resolved spectra of reaction mixture for Au(III) oxidation of serine. [Ser] = 0.035 M, [Au(III)] = 0.24 mM, [H+] = 0.089 mM, [Cl−] = 0.04 M, temperature = 298 K, and scanning time interval = 1 min. B


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H NMR (300 MHz, Bruker DPX-300, CDCl3): δ 9.12(1H, d, J = 2.5 Hz, H1); δ 8.30 (1H, dd, J1 = 10.1 Hz, J2 = 2.5 Hz, H2); δ 7.94(1H, d, J = 10.0 Hz, H3); δ 11.04 (1H, br s, N−H); δ 7.56 (1H, q, J = 5.4 Hz, =CH−CH3); pair of doublets at δ 2.14 (J = 5.4 Hz) and δ 2.08 (J = 5.5 Hz) for = CH−CH3. [The quartet at δ 7.56 (J = 5.4 Hz) is due to =CH−CH3, and its corresponding doublet at δ 2.14 (J = 5.4 Hz) is owing to =CH−CH3 in trans configuration; the doublet at δ 2.08 (J = 5.5 Hz) assigned as =CH−CH3 is probably of cis configuration, for which the counter quartet due to =CH−CH3 (supposed to appear in the near vicinity of δ 7.56) is obscured in the NMR spectrum, which may be owing to the high population of the trans configuration (more stable diastereomer) over the cis one.] The spectral data clearly indicates that acetaldehyde was formed as the oxidation product of serine. The formation of acetaldehyde is further justified by determination of the melting point of the 2,4-dinitrophenyl hydrazone derivative (observed mp 146 °C; literature mp 148 °C45). During the reaction, the formation of ammonia gas was confirmed by Nessler’s reagent test, whereas the evolved CO2 was identified by the limewater test as described in our earlier communication.26 2.3. Measurements of Surface Tension and Critical Micelle Concentration (cmc). The surface tension values of the surfactant in aqueous solution, in acetate buffer of pH 4.05, in serine acetate buffer media (0.04 M serine, pH 4.05), and also in gold(III) acetate buffer media (1.2 mM Au(III), pH 4.05), were determined using a Jencon tensiometer by the platinum ring detachment method. The critical micelle concentration (cmc) of SDS has been determined from the plot of surface tension (γ) vs log(SDS). The γ value decreases linearly with an increase in SDS concentration up to a certain limiting value (determined as cmc), beyond which it remains almost unchanged although the SDS concentration is increased (Figure 4). The cmc values were found to be 6.45, 4.29, 4.70, and 4.21 mM at 298 K for the above respective media. 2.4. Detection of Free Radicals. The possibility of the intervention of free radicals in the reaction was examined at room temperature by the addition of 15% (v/v) acrylonitrile to the reaction mixture (1.2 mM [Au(III)], 0.04 M [Ser], 0.09 mM [H+], and 0.04 M [Cl−]) in nitrogen atmosphere. A white suspension of polyacrylonitrile46 was obtained on standing (∼20 min). A similar observation (white polymeric product) was also made when the reaction mixture was treated with 0.5 M acrylamide solution in an inert atmosphere. However, acrylonitrile or acrylamide failed to produce any precipitate or suspension due to polymerization when either gold(III) or serine was separately mixed with acrylonitrile or acrylamide. These induced polymerizations clearly indicated the intervention of free radical in the oxidation of serine by gold(III) in weakly acidic medium. Such a type of induced polymerization was also reported in the oxidation of serine by Mn(III).47 1

complexation between serine and gold(III) species during the reaction. As mentioned in an earlier communication,37 the kinetic runs were followed by noting the absorbance in the visible region at λ = 400 nm.42 The pseudo-first-order rate constants [kobs (or kψ), s−1] were computed from the initial linear part (up to two half-lives) of the plots of ln(absorbance) versus “time”. Representative pseudo-first-order plots at different concentrations of serine are shown in Figure 2. The observed rate constant values were reproducible within ±5%.

Figure 2. Pseudo-first-order plots for variation of serine concentrations in Au(III) oxidation of serine at 303 K. [Au(III)] = 1.2 mM, [H+] = 0.089 mM, and [Cl−] = 0.04 M.

2.2. Analysis of Reaction Products. Two sets of reaction mixtures with serine concentration in excess over Au(III) concentration were prepared and kept for 60 min at room temperature. In one set, 10% NaHCO3 solution was added to the reaction mixture and the pH of the solution was adjusted at 7.5−8.0. The resulting alkaline solution was then extracted with CH2Cl2. The aqueous part of the organic layer was removed by using anhydrous Na2SO4. The organic solution so obtained was then evaporated to dryness on a water bath. The addition of methyl viologen (1,1′-dimethyl-4,4′-bipyridinium dichloride) in acetic acid medium to a part of the residue and subsequent addition of diphenylamine failed to give any green coloration,43 which indicates that glycolaldehyde was not produced as the oxidation product of serine. To another part of the residue, a mixture of 20% piperidine and 5% sodium nitroprusside solution in equal volume was added and the solution was treated with aqueous Na2CO3 to make it alkaline. Appearance of a blue color44 implied the presence of acetaldehyde. The other set of reaction mixture was distilled. The distillate was treated with 2,4-dinitrophenyl hydrazine solution in 4 N H2SO4 when a yellow mass separated out. It was filtered off, washed, and dried. The yellow mass was confirmed as the 2,4dinitrophenyl hydrazone derivative of acetaldehyde by 1H NMR spectral data analysis (Figure 3). The structure of the 2,4dinitrophenyl hydrazone derivative is represented below followed by the spectral data (in ppm).

3. RESULTS AND DISCUSSION 3.1. Effects of [Au(III)], [Ser], Temperature, and μ on kobs. The effect of [Au(III)] on the rate of oxidation of serine was studied by varying its concentrations from 1.0 to 4.5 mM at 0.035 M [Ser], 0.089 mM [H+], and 0.04 M [Cl−] at 298 K. The pseudo-first-order plots of log(Abs) versus “time” were made for different initial concentrations of Au(III). The plots were parallel to each other indicating the first-order dependence of the reaction rate on [Au(III)]. The values of 104kobs (s−1) were found to be 8.78, 8.88, 8.98, 8.87, and 9.04 at 1.0, C


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Figure 3. 1H NMR spectrum of 2,4-DNP derivative of the oxidized product.

To investigate the effect of changing serine concentration on the reaction rate, different sets of reactions were carried out by taking serine in the concentration window of 8.0−80 mM at fixed concentrations of Au(III), H+, and Cl− at 1.2 mM, 0.089 mM, and 0.04 M, respectively. At a fixed temperature, the plot of kobs versus [Ser] yielded a straight line with zero intercept (Figure 5), indicating the first-order dependence with respect to serine. Thus, it may be stated that, since the reaction order is not less than 1, no intermediate complex is formed between gold(III) and the amino acid during the course of the reaction.21,22,48 A fitting empirical rate expression may be proposed now as follows: kobs = −

d[Au(III)] 1 = k[Ser] [Au(III)] dt

(1)

where k is the second-order rate constant and [Ser] is the concentration of serine. To study the effect of temperature on the rate parameters, the reactions were investigated under different serine concentrations at four different temperatures, viz., 298, 303, 308, and 313 K. The second-order rate constant (k) values that were calculated at different temperatures were found to increase with increase in temperature (Table 1). The enthalpy of activation (Δ‡H0) and the entropy of activation (Δ‡S0) parameters were evaluated from the linear plot of log(k/ T) versus 1/T (Figure 6) using the Eyring equation (eq 2). The values of Δ‡G0, Δ‡H0, and Δ‡S0 are depicted in Table 1.

Figure 4. Tensiometric plots of γ versus log [SDS] in aqueous solution, in acetate buffer of pH 4.05, in serine acetate buffer media (0.04 M serine, pH 4.05), and in gold(III) acetate buffer media (1.2 mM Au(III), pH 4.05) at 298 K.

2.0, 3.0, 4.0, and 4.5 mM [Au(III)] respectively, which means that kobs is independent of Au(III) concentration. The average value of the rate constant (kobs) comes out to be (8.91 ± 0.13) × 10−4 s−1. D


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Figure 6. Dependence of k on temperature. Eyring plot of log(k/T) versus 1/T.

Figure 5. Dependence of kobs on [Ser] at different temperatures. [Au(III)] = 1.2 mM, [H+] = 0.089 mM, and [Cl−] = 0.04 M.

log(k /T ) = [log(kB/h) + (Δ‡S 0/2.303R )] − (Δ‡H 0/2.303RT )

(2)

The dependence of the reaction rate on ionic strength was examined at 298 K by varying the concentration of NaClO4 (0.02−0.28 M) in the reaction medium although the concentrations of other reactants ([Au(III)] = 1.2 mM, [Ser] = 0.04 M, [H+] = 0.089 mM, and [Cl−] = 0.04 M) were kept constant. It was observed that the rate constant (8.82 ± 0.05 s−1) remained unaffected with the change of concentration of NaClO4. Thus the redox reaction, under discussion, is ionic strength independent. 3.2. Effects of [H+] and [Cl−] on kobs. To investigate the influence of H+ on the reaction mechanism, the reactions were performed at varying pH in the range 3.72−4.78 keeping other reaction parameters constant. Since ionic strength has no effect on the reaction rate, no attempt was made to keep its value constant. At a fixed temperature, the second-order rate constant (k) was found to decrease slowly in a nonlinear manner with increasing [H+] (Figure 7). The second-order rate constant (k) values under different pHs at two different Cl− concentrations, viz., 0.04 and 0.20 M, respectively, are mentioned in Table 2. Since the variation was nonlinear, an attempt was made to explain the kinetic behavior and to fit the data into an equation through a curve-fitting procedure using different nonlinear equations. The best fitting was found with the kinetic equation (eq 3) for the dependence of k on [H+] with the empirical constants (k1, k2, and k3).

k=

k1 + k 2[H+] k 3 + [H+]

Figure 7. Dependence of k on [H+]. [Au(III)] = 1.2 mM and [Ser] = 0.04 M.

Table 2. Dependence of Second-Order Rate Constant (k/ M−1 s−1) on [H+] and [Cl−]a 102k (M−1 s−1)

(3) a

The values of these constants were determined at two different Cl− concentrations from Figure 7 employing eq 3. The different

−

102k (M−1 s−1) −

2

−

pH

0.04 M [Cl ]

0.20 M [Cl ]

10 [Cl ] (M)

pH 4.05

pH 4.45

3.72 4.05 4.27 4.45 4.63 4.78

1.94 2.16 2.32 2.44 2.56 2.65

1.67 1.81 1.91 2.13 2.34 2.57

1.0 2.0 4.0 6.0 8.0 10.0

2.76 2.49 2.20 2.07 1.95 1.88

3.55 3.31 2.93 2.70 2.56 2.41

[Au(III)] = 1.2 mM, [Ser] = 0.04 M, and temperature = 298 K.

Table 1. Values of Temperature Dependent Second-Order Rate Constants (k/M−1 s−1) and Related Activation Parametersa temp (K) 102k (M−1 s−1)

298 2.15 ± 0.16

303 3.15 ± 0.21

308 4.19 ± 0.25

313 5.80 ± 0.31

[Au(III)] = 1.2 mM, [Ser] = 8−80 mM, [H+] = 0.089 mM, and [Cl−] = 0.04 M. Δ‡G0 = 83.4 ± 1 kJ mol−1, Δ‡H0 = 48.2 ± 3 kJ mol−1, and Δ‡S0 = −115.2 ± 6 J K−1 mol−1 a

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The Journal of Physical Chemistry A values of each empirical constant at two different Cl − concentrations of 0.06 and 0.20 M are as follows: 106k1/s−1, 1.87 ± 0.08 and 0.40 ± 0.09; 102k2/M−1 s−1, 1.62 ± 0.02 and 1.54 ± 0.04; and 105k3/M, 6.0 ± 0.29 and 0.91 ± 0.36, respectively, at 298 K. The values indicate that k1 and k3 are dependent on [Cl−], while k2 is independent of [Cl−]. The dependence of [Cl−] on kobs was followed by varying its concentration in the range 0.01−0.1 M at a fixed but different concentrations of Au(III), serine, and H+ each at 298 K (Table 2). The plot of k versus [Cl−] (Figure 8) produced a nonlinear

Table 3. Influence of Solvent Polarity (ε) on the PseudoFirst-Order Rate Constant at 298 Ka % 1,4-dioxane (v/v) ε 104kobs (s−1)

0

5

10

20

30

78.6 6.80

74.2 6.35

69.7 5.94

60.8 5.28

51.9 4.68

[Au(III)] = 1.2 mM, [Ser] = 0.032 M, [H+] = 0.089 mM, and [Cl−] = 0.04 M. a

the participation of the effective species of the reductant and the oxidant in the rate-determining step. The concentration of the different ionic species depends on the concentrations of hydrogen ion (for both serine and gold(III)) as well as chloride ion (for gold(III) only). In aqueous solution, serine exists47 in dipolar ions (HA) in equilibrium with cationic (H2A+) and anionic (A−) forms (Scheme 1) depending upon the pH of the Scheme 1

media. Thus, considering the dissociation constant values51 of serine (Ka1 = 6.50 × 10−3; Ka2 = 6.18 × 10−10 at 298 K) and the low experimental acidity (in the order of 10−5 M), it is suggested that serine does exist predominantly in its dipolar form (HA) in the reaction medium. In addition, owing to the presence of the active nucleophilic group, −COO−, the dipolar form serves as the effective reducing agent. Gold(III) species present in aqueous solution of tetrachloroauric(III) acid are AuCl4−, AuCl3(OH2), and AuCl3(OH)− with the involvement of the equlibria26,52 illustrated in Scheme 2. As most of the reactions were carried

Figure 8. Nonlinear dependence of k on [Cl−] in the oxidation process. [Au(III)] = 1.2 mM and [Ser] = 0.04 M.

curve where the rate of the reaction decreased with increase in [Cl−]. Based on the above observation, a similar nonlinear curve-fitting procedure (as in the case of [H+] variation) was employed and the following rate expression (eq 4) was found to fit well with the variation of k with [Cl−]. k=

k4 + k5[Cl−] k6 + [Cl−]

Scheme 2

(4)

out at pH 4.05 and 0.04 M [Cl−], considering the literature K values (Khy = 9.5 × 10−6, Ka3 = 0.25 at 298 K)52 corresponding to the two equilibria (hydrolysis followed by dissociation) in Scheme 2, the ratio of the concentrations of different Au(III) species under this experimental condition are [AuCl4−]: [AuCl3(OH2)] = 4210:1, [AuCl3(OH)−]:[AuCl3(OH2)] = 2809:1, and [AuCl4−]:[AuCl3(OH)−] = 1.5:1. Thus, it is quite evident that, among the different Au 3+ species, tetrachloaurate(III) and trichlorohydroxoaurate(III) are the prime oxidizing species.23,25,37 Thus, two anionic gold(III) species, namely, AuCl4− and AuCl3(OH)−, are expected to participate in the reaction process simultaneously via two parallel pathways (Scheme 3). A literature survey reveals that in aqueous acidic solution both serine6,47 and gold(III) chloride53 exist in different ionic forms that render the oxidation kinetics and the reaction mechanism complicated. Again, the generation of free radical intermediate in the oxidation of serine by gold(III) in weakly acidic medium (as discussed in the Experimental Section) indicates that the reaction occurs through a one electron transfer process where gold(III) is reduced to gold(II). Thus, considering first-order

The empirical constants, k4, k5, and k6, obtained from the curve-fitting procedure, at two different pHs of 4.05 and 4.45 are as follows: 106k4/s−1, 0.9 ± 0.10 and 2.18 ± 0.26; 102k5/ M−1 s−1, 1.51 ± 0.04 and 1.58 ± 0.10; and 105k6/M, 2.82 ± 0.36 and 5.58 ± 0.73, respectively, at 298 K. From the results, it is found that the constants k4 and k6 are acid dependent while k5 is an acid independent constant. 3.3. Effect of Dielectric Constant on kobs. The dependence of kobs on ε (dielectric constant) was studied by varying the percentage of 1,4-dioxane in the range 5−30% (v/ v) at fixed [Au(III)], [Ser], [Cl−], and [H+] at 298 K. The values of ε for different percentages of 1,4-dioxane have been used from an earlier literature report.49 The kobs values with the corresponding percentage of solvent mixture and ε are listed in Table 3. The data clearly indicate that increasing the solvent polarity of the medium (with increasing ε) enhances the oxidation rate,50 which suggests the involvement of dipole− dipole or ion−dipole interaction during the reaction. 3.4. Reaction Steps and Rate Law. The experimental results suggest that the oxidation of serine by Au(III) involves the first-order kinetics on both serine and gold(III) indicating F


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each set of reactions is an indication of greater reactivity of AuCl3(OH)− than AuCl4− so long as the oxidizing property is concerned. Similar observations are also recorded in the literature.20,23 The inverse dependence of the pseudo-first-order rate constant on the concentration of either H+ or Cl− (as discussed in section 3.2) stands for the reliability of the theoretical rate law (eq 7). This retarding effect of H+ and/or Cl− on the reaction rate may be attributed to the fact that the concentration of the more reactive oxidizing species, i.e., AuCl3(OH)−, gets diminished as soon as the concentration of H+ and/or Cl− is increased (as is evident from Scheme 2), thereby slowing down the reaction. 3.5. Proposed Reaction Mechanism. The nature of a substrate in combination with the experimental conditions in a reaction controls the behavior of gold(III) to serve as either a one or two electron transfer oxidant.54,55 Since the reaction mixture upon contact with acrylonitrile or acrylamide afforded a polymeric white suspension,56 the reaction obviously passes through the free radical intermediate. This observation consequently rules out the conversion of gold(III) to gold(I) in a single-step two electron reduction process. This proposition is quite in agreement with a number of gold(III)mediated redox reactions.54,55 Again, the existence of transient Au(II) species has also been affirmed by Soni et al.23 in the kinetic studies of oxidation of L-histidine by tetrachloroaurate(III). A detailed mechanism showing different reaction steps is described in Scheme 4 considering AuCl4− as the representative Au(III) complex.

Scheme 3

kinetics with respect to Au(III) and serine each, the reaction steps in Scheme 3 can be proposed. The rate law, formulated from the experimental observations and Scheme 3, is expressed by eq 5, which transforms to eq 6 by substituting the values of [AuCl4−] and [AuCl3(OH)−] in terms of the hydrolysis constant (Khy) of AuCl4− and acid dissociation constant (Ka3) of AuCl3(OH2) utilizing the equlibria in Scheme 2. Equation 6 is finally rearranged to find the second-order rate constant, k, in eq 7. v=−

d[Au(III)] = {k1(rds)[AuCl4 −] dt

+ k 2(rds)[AuCl3(OH)− ]}[Ser] −

(5)

d[Au(III)] 1 = kobs [Au(III)] dt =

k=

k1(rds)[H+][Cl−] + k 2(rds)KhyK a3 KhyK a3 + [H+][Cl−]

kobs = [Ser]

+

[Ser] (6)

Scheme 4

−

k1(rds)[H ][Cl ] + k 2(rds)KhyK a3 KhyK a3 + [H+][Cl−]

(7)

From eq 7, a plot of kobs versus [Ser] should be a straight line passing through the origin, which has also been experimentally verified (Figure 5) to indicate that the reaction is first-order with respect to [Ser]. Upon comparison between the theoretical rate law (eq 7) and the empirical ones (eqs 3 and 4), the expressions so obtained for different k’s show that k1 and k3 are functions of [Cl−] and k4 and k6 are [H+] dependent constants, whereas k2 and k5 are independent of both [Cl−] and [H+]. This has actually been observed experimentally (as stated in section 3.2) to verify the proposition. The rate constant values of k1(rds) and k2(rds) in Scheme 3 were evaluated using the empirical constants obtained from the variation of [H+] and [Cl−] on the reaction rate by varying one with other keeping constant. The collection of k1(rds) and k2(rds) values for different experimental sets are presented in Table 4, which shows that either k1(rds) or k2(rds) does not change appreciably with change in varying parameters, i.e., [H+] and [Cl−]. In view of the above findings the derived theoretical rate law (eq 7) may be claimed justified. Moreover, the higher value of k2(rds) over k1(rds) for

Initially, the carboxylate ion (−COO−) of the dipolar form of serine will be the better nucleophile compared to the cationic site (−NH3+) and therefore the former will attack the gold(III) center in a one electron transfer process in the slow step to generate cationic free radical, I. There are a number of reports available in the literature involving the oxidation of α-amino acids where the initial nucleophilic attack takes place by −COO− of the amino acid on the metal center.26,38,39 The free radical, I, subsequently decarboxylates to form another cationic free radical intermediate,38,39 II. This decarboxylation process is facilitated due to the presence of the strong electron withdrawing group (NH3+) at the α-carbon center. The existence of an analogous kind of radical cation (I and II) has already been cited57 in the oxidation of L-citrulline (an amino acid) by permanganate in acid medium. Wisniowski et al.58 also identified the decarboxylated radical (NH2−•C(CH3)2) of the anion of α-methylalanine using time-resolved ESR and steady-

Table 4. Values of Rate Constants for Slow RateDetermining Steps Involving Oxidant Species AuCl4− and AuCl3(OH)−. 102k1(rds) (M−1 s−1)

experiment −

pH variation at 0.04 M [Cl ] pH variation at 0.20 M [Cl−] chloride variation at pH 4.05 chloride variation at pH 4.45

1.62 1.54 1.51 1.58

± ± ± ±

0.02 0.04 0.04 0.10

102k2(rds) (M−1 s−1) 3.15 3.39 3.38 3.26

± ± ± ±

0.13 0.76 0.38 0.39 G


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from that of the earlier studies involving the oxidation of neutral amino acids such as glycine19 and alanine20 by Au(III) in weakly acidic medium, where Au(III) was reduced to Au(I) in a one-step two electron transfer mechanism although the kinetic rate expressions are nearly same. It is noteworthy that, unlike in the earlier oxidation studies of the above amino acids19,20 by Au(III) where the reaction path mainly circled around the amino acid center (−CH(NH3+)COO−) leading to the formation of the corresponding aldehyde, in the present work, apart from the main amino acid reaction center, the role of the side chain (HO−CH2−) cannot be ignored in the reaction to form acetaldehyde as the oxidized product of serine. From the mechanistic point of view, it is apparent that the transition state (TS) in the rate-determining step being quite polar in nature invites a number of polar water molecules to surround it leading to electrostriction, thus lowering the disorder of the system which may account for the moderately high negative value63 of the entropy of activation (−115.2 J K−1 mol−1). The moderate value (48.2 kJ mol−1) of the enthalpy of activation may be attributed to the intervention of free radical intermediate in the reaction path. It may be noted that in determining the Gibbs free energy of activation the role of the entropy of activation is quite significant and its contribution is slightly less than that of the enthalpy of activation. However, it is very difficult to give a meaningful explanation of the activation parameters in terms of the reaction mechanism since the second-order rate constant, k (=kobs/[Ser]), does not represent a single elementary step. In the rate-determining step the TS being ionic in nature is favored to be stabilized in polar solvent medium. Dioxane (a nonpolar aprotic solvent) decreases the polarity of the medium; as a result solvent−solute interaction decreases, which is reflected through the lowering in the rate constant20,64 with an increase in the percentage amount of dioxane in the solvent mixture. Hence all the observed kinetic phenomena accord to the proposed reaction mechanism and the derived rate law. 3.6. Micellar Effects on the Electron Transfer Reaction. An effort has been made to investigate the effects of cetyltrimethylammonium bromide (CTAB), a representative cationic surfactant, and SDS, a representative anionic surfactant, on the redox reaction between serine and gold(III) in acetate buffer. The effect of variation of CTAB concentration (0.3−1.0 mM) on the reaction rate was studied at fixed concentrations of Au(III), serine, H+, and Cl− at 1.2 mM, 40 mM, 0.089 mM and 0.04 M, respectively. In every attempt the reaction ended up with an immediate light yellow precipitation that was also observed just by the addition of CTAB on gold(III) solution (even in the absence of serine, H+, and Cl−). This observation prompted us not to carry out the reaction furthermore with CTAB. The appearance of light yellow precipitate in the solution is possibly due to the formation of water-insoluble CTA+---AuCl4− ion pair complex between AuCl4− and CTAB.65 This observation is also in the same line as an earlier report.66

state spin-trapping. It is to be stated that the O−H bond (111 kcal/mol) is stronger than the C−O bond59 (80 kcal/mol). Consequently the C−O bond is homolytically cleaved in order to quench the free radical so generated in II at the suitable center leading to the formation of stable CC bond in III which, in turn, converts into iminium cation, IV, via rapid proton exchange. This iminium cation ultimately undergoes fast hydrolysis (deamination) to form acetaldehyde as the primary oxidation product of serine. A similar observation has been reported by a number of authors in the oxidation of amino acids by metal ion oxidants,38,39,60,61 where the intermediate iminium cation quickly hydrolyzed to the corresponding aldehyde. The formation of acetaldehyde has already been confirmed by the melting point measurement and 1H NMR studies of its 2,4-dinitrophenyl hydrazone derivative (discussed in section 2.2). In the reaction the inorganic intermediate species Au(II) [obtained due to one-step one electron reduction of Au(III)] being highly unstable is rapidly disproportionate23,54,62 to Au(III) and Au(I) as illustrated in Scheme 5. Scheme 5

Au(I) was identified by the addition of rhodanine as the complexing agent to a part of the reaction mixture that produced a brown colored solution which when analyzed spectrophotochemically registered a characteristic hump at 500 nm. This finding actually corroborates with the earlier reports.26,55 The oxidation of L-serine by different oxidants such as manganese(III)47 and silver(I) catalyzed cerium(IV)5 in sulfuric acid medium has been reported to end up in the formation of glycolaldehyde as the primary reaction product proceeding via a free radical mechanistic route. However, in the present study that employs Au(III) as an oxidant despite the involvement of free radical, the product has been identified as acetaldehyde (not glycolaldehyde), which may partly be owing to the participation of −CH2OH group in the reaction. Shi et al.6 in an earlier communication reported the formation of glyoxalic acid and formaldehyde when L-serine was oxidized by bis(hydrogen periodato)argentate(III) in alkaline medium via a two electron transfer reduction process at the silver(III) center. Thus, the formation of different reaction products appearing from different studies suggests that the oxidation of serine followed different mechanistic paths that may be due to the different natures of the oxidants as well as the different experimental conditions. In the present study, the reaction mechanism, where Au(III) is reduced via a free radical path, has been found to be different

Table 5. Dependence of Pseudo-First-Order Rate Constant on [SDS] at Different Temperaturesa [SDS] (mM) 104kψ (s−1) at 298 K 104kψ (s−1) at 303 K 104kψ (s−1) at 308 K a

0.0

1.0

2.0

3.0

4.0

5.0

7.0

10.0

20.0

8.70 13.0 16.9

9.05 13.3 17.4

9.35 13.6 17.9

9.65 13.8 18.3

10.0 14.4 19.1

9.59 13.6 18.0

9.25 13.4 17.4

9.13 13.0 16.8

8.67 12.5 16.4

[Au(III)] = 1.2 mM, [Ser] = 0.04 M, [H+] = 0.089 mM, and [Cl−] = 0.04 M. H


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dipolar form of serine gets associated with the surfactant monomers through electrostatic interaction between the positively charged amino group (−NH3+) of serine and the anionic headgroup of SDS. This association possibly increases the nucleophilicity of the carboxylate anion (−COO−) of serine to facilitate the electron transfer from serine to gold(III), thereby enhancing the reaction rate in the premicellar region. Such a type of premicellar association between the amino acid and the surfactant followed by the interaction between the carboxylate ion of the amino acid and the metal center was reported earlier in the oxidation of leucine21 and phenylalanine.39 The formation of catalytic premicelles before the cmc has also been evidenced from the electron transfer reaction between Ru(II) complexes and S2O82− in different microheterogeneous systems.70,71 The nature of the kψ−[SDS] profile in Figure 9 indicates that the reaction rate decreases after the kinetic cmc. It is wellknown that the micelle is a porous cluster with deep water-filled cavities.35,72,73 Therefore, it is very difficult, if not impossible, to locate the exact position of the reactant species within the micellar core.74 However, in the present situation it may be suggested that, due to the electrostatic attraction between the negatively charged headgroup of SDS and the positively charged −NH3+ group of serine, the latter remains in the inner side of the Stern layer in close association with the headgroup of SDS (Scheme 6). On the other hand, AuCl4−/

The influence of SDS on the oxidation kinetics was studied by varying its concentration (0−20 mM) where each set of experiments was conducted at three different temperatures (Table 5). A plot of kψ versus [SDS] (Figure 9) indicates that,

Figure 9. Influence of [SDS] on kψ at three different temperatures. [Au(III)] = 1.2 mM, [Ser] = 40 mM, [H+] = 0.089 mM, and [Cl−] = 0.04 M.

Scheme 6

as the concentration of SDS increased, the kψ was found to increase gradually, reaching a maximum value, and then it decreased. This implies that SDS in its low concentration region (up to ∼4 mM) shows an accelerating effect following which a retarding effect turns out. The present observation also accords with earlier reports21,39 that also dealt with the oxidation of some other amino acids. The concentration of SDS corresponding to the maximum kψ value at a particular temperature is known as the cmc of the surfactant under kinetic conditions. The average kinetic cmc value was found to be near 4.0 mM, which is somewhat nearer to 4.21 mM in Au(III) acetate buffer, 4.29 mM in acetate buffer only, and 4.70 mM in a mixture of serine and acetate buffer but much lower than 6.45 mM in aqueous medium only obtained at 298 K employing the tensiometric method. It is well-known that the cmc value of a surfactant decreases in the presence of an electrolyte. In the present study, the decrease in the cmc value of SDS in the presence of Au(III) acetate buffer media compared with its normal cmc value (pure water) might be owing to the electrostatic repulsion between AcO−/AuCl4−/AuCl3(OH)− and anionic SDS monomers. However, in the medium the concentration of AcO− (from acetate buffer) is much higher than that of AuCl 4 − /AuCl 3 (OH) − . That is why the contribution of AcO− in lowering the cmc value is much more significant than that of Au(III), although the contribution of Au(III) anions cannot be totally ignored. It is established that a surfactant exists as monomers when the concentration is appreciably low.30 But after a certain critical concentration the solution begins to form micellar aggregates which remain in dynamic equilibrium with free monomers as well as the monomers adsorbed as a film at the interface.67,68 In the premicellar region small aggregates of surfactant monomers may exist and these aggregates may interact physically with reactant molecules forming active species.67−69 In our present study, the catalytic effect in the premicellar region may be accounted for by the fact that the

AuCl3(OH)− resides at the outer periphery of the Stern layer due to electrostatic repulsion by the similarly charged headgroup of the free SDS monomers (which are not bound to serine) within the micellar core (Scheme 6). Owing to this repulsion the gold(III) complexes become spatially oriented away from the serine molecules, leading to a decrease in reaction rate. It is worthwhile to mention here that the polarities of micelle−water interfaces are lower than that of bulk water and hence the concentration of anionic gold(III) species in the Stern layer will be much less than that in the aqueous layer, and this lowering in local concentration of gold(III) may be another reason for a decreased reaction rate. Also possible inclusion of serine molecules within the micellar I


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The Journal of Physical Chemistry A core results a lower concentration of serine in the aqueous layer leading to a fall in reaction rate. The inhibition effect in the postmicellar region may be quantitatively explained using the pseudophase kinetic model proposed by Berezin.75 According to this kinetic model both reactants are distributed between the aqueous and micellar pseudophases (Scheme 7). Scheme 7

In Scheme 7, Dn represents the micellar aggregates of SDS and [Dn] = [D] − cmc, which means the concentration of micellized surfactant. KSer and KAu(III) are the binding constants for serine and gold(III), respectively, with the micelles. kW and kM are the rate constants for the reaction in the aqueous phase and the micellar pseudophase, respectively. Based on Scheme 7, the quantitative expression for the pseudo-first-order rate constant (kψ) for such bimolecular micellar catalyzed reactions is expressed in eq 8. It is to be noted that the concentration of SDS used in this experiment is substantially low so that [Dn]2 will be very low (∼10−5 M), and thus in the numerator of eq 8, kW + kMKSerKAu(III)[Dn]2 may be approximated to kW resulting in eq 9. Again, since [Dn]2 is too small to recognize, the terms containing [Dn]2 in the denominator of eq 9 may be neglected and it reforms to eq 10. kψ =

kψ =

Figure 10. Plots of kψ−1 versus [Dn] at temperatures of 298, 303, and 308 K. [Au(III)] = 1.2 mM, [serine] = 0.04 M, [H+] = 0.089 mM, and [Cl−] = 0.04 M.

Table 6. Temperature Dependent Rate Constants (kW), Substrate−Micelle Association Constants, and Related Change of Enthalpy and Entropya temp (K) 4

10 kW (s ) KSer + KAu(III) a

k W + kMKSerKAu(III)[Dn ]2 {1 + KSer[Dn ]}{1 + KAu(III)[Dn ]}

KSer + KAu(III) 1 1 [Dn ] = + kψ kW kW

303

308

9.51 6.17

13.6 5.74

17.7 5.11

ΔH0 = −14.4 ± 2 kJ mol−1; ΔS0 = −6.3 ± 0.5 J K−1 mol−1.

The small negative value of ΔH0 may be owing to the slightly greater energy release due to serine−micelle binding (when it enters the Stern layer) than the absorption of energy due to the water structure breaking around serine when it goes from aqueous phase to micellar pseudophase. Also during the transfer of serine from aqueous to micellar pseudophase, the desolvation process increases the entropy while the binding of serine with SDS micelle increases the structuring (decrease of entropy). These two opposing effects almost balance each other, resulting in a very low value of ΔS0 (−6.3 ± 0.5 J K−1 mol−1). Therefore, it may be concluded that the redox reaction between gold(III) and serine in the presence of SDS micelle is moderated by the substrate−micelle interaction associated with the restructuring of surrounding water molecules in conjunction with the distribution of ionic gold(III) species as well as serine between the two pseudophases.

(8)

kW 1 + {KSer + KAu(III)}[Dn ] + KSerKAu(III)[Dn ]2

−1

298

(9)

(10)

−1

Thus, a plot of kψ vs [Dn] will produce a straight line with positive slope and positive intercept (Figure 10), which is in agreement with the rate law. The intercept of the above plot gives the values of kW at three different temperatures (Table 6), and such values corroborate with the pseudo-first-order rate constants in the absence of surfactant (Table 5). From the slope of such plots the values of the binding constants (KSer + KAu(III)) were also determined at three different temperatures (Table 6). It is evident that the value of a binding constant decreases with increasing temperature indicating the exothermic nature of the binding process. From a plot of log(KSer + KAu(III)) versus 1/T, the enthalpy change (ΔH0) for the binding process was found to be −14.4 ± 2 kJ mol−1. As shown in Scheme 6, the electrostatic repulsion between the negatively charged headgroup of SDS and anionic gold(III) species makes the binding weak and thereby KAu(III) will be very small. On the other hand, electrostatic attraction between SDS and serine makes the binding more favorable, leading to a greater value of KSer. Hence it may be assumed that (KSer + KAu(III)) ≈ KSer. Thus, the value of ΔH0 mentioned above is possibly the enthalpy change due to binding of serine with SDS micelle.76

4. CONCLUSION In the redox reaction between gold(III) and serine the zwitterionic form of the reductant is oxidized to give CH3CHO. Between the chloro and the chlorohydroxo gold(III) complexes the latter is found to be more reactive. The reaction pathway involves free radicals where one electron transfer takes place from serine to gold(III) in the ratedetermining step. In the low concentration of SDS (premicellar region) the electrostatic attraction between SDS and serine increases the nucleophilicity of the carboxylate ion of serine, thereby increasing the reaction rate. In the postmicellar region, electrostatic repulsion between the surfactant and oxidant species, electrostatic attraction between surfactant and subJ


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strate, and the distribution of the reactants between the two pseudophases account for the inhibition of the reaction rate. The present study underscores that, unlike the earlier oxidation products of L-serine by using the oxidants other than Au(III) species where in major cases glycolaldehyde was the ultimate reaction product, so far as our knowledge goes this work may be claimed as the first report of acetaldehyde as the oxidation product of L-serine.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-33-23503682 (B.P.). ORCID

Biswajit Pal: 0000-0002-3297-5238 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial assistance from the Department of Chemistry, Jadavpur University, to Prof. Pratik K. Sen is gratefully acknowledged. Dr. Biswajit Pal would like to thank the UGC, New Delhi, for providing financial support. The authors thank Prof. Sanjoy Bhar, Department of Chemistry, Jadavpur University, for providing the NMR instrumental support and also for fruitful discussion with him during analysis of the reaction products. The authors are also grateful to Dr. Kalyan Kumar Mandal, Department of Chemistry, St. Paul’s C. M. College, Kolkata, for his valuable and thoughtfully appreciable comments; those were really beneficial for analyzing the 1H NMR spectral data.

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REFERENCES

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