Influence of Microheterogeneous Environments of Sodium Dodecyl Sulfate on the Kinetics of Oxidation of l -Serine by Chloro and Chlorohydroxo Complexes of Gold(III)

Article abstract of DOI:10.1021/acs.jpca.8b02409

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.

Full text of DOI:10.1021/acs.jpca.8b02409

Article
pubs.acs.org/JPCA
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.
systems, uses of the gold(III) compounds eclipse those of the
Pt(II) compounds.15 Particularly the importance of dithiocar-
bamato12,13,16 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
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
Received: March 12, 2018
Revised: May 3, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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.
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 spectrophoto-
metrically 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).
̌ ́
Vujacic
et al.25 studied the kinetics of the reaction between
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
tetrachloroaurate(III) and L-methionine in HClO4 medium and
reported the formation of a short-lived complex, Au(III)−(L-
methionine), that subsequently turns into Au(I)−(L-methio-
nine) via the replacement of a Cl− ligand.
1
reaction mixture. H NMR spectra were recorded on a Bruker
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.
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 compart-
ment 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
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 environ-
ment.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 hydro-
phobic 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 surfactant-
mediated 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 spectrophoto-
metrically 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. L-
Serine (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
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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%.
1H 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 inter-
vention 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
Figure 2. Pseudo-first-order plots for variation of serine concen-
trations 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 2,4-
dinitrophenyl hydrazone derivative of acetaldehyde by 1H
NMR spectral data analysis (Figure 3). The structure of the 2,4-
dinitrophenyl 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 depend-
ence 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
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
1
Figure 3. H 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:
d[Au(III)]
dt
1
kobs = −
= k[Ser]
[Au(III)]
(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
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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 S0/2.303R)]
‡
− (Δ H0/2.303RT)
(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).
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/
a
M−1 s−1) on [H+] and [Cl−]
102k (M−1 s−1
)
102k (M−1 s−1
)
pH
0.04 M [Cl−] 0.20 M [Cl−] 102[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
2.76
2.49
2.20
2.07
1.95
1.88
3.55
3.31
2.93
2.70
2.56
2.41
4.0
6.0
k1 + k2[H+]
k =
8.0
k3 + [H+]
10.0
(3)
a
[Au(III)] = 1.2 mM, [Ser] = 0.04 M, and temperature = 298 K.
The values of these constants were determined at two different
Cl− concentrations from Figure 7 employing eq 3. The different
a
Table 1. Values of Temperature Dependent Second-Order Rate Constants (k/M−1 s−1) and Related Activation Parameters
temp (K)
102k (M−1 s−1
298
303
308
313
)
2.15 0.16
3.15 0.21
4.19 0.25
5.80 0.31
a
[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
E
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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
Table 3. Influence of Solvent Polarity (ε) on the Pseudo-
First-Order Rate Constant at 298 K
a
% 1,4-dioxane (v/v)
1.54
0.04; and 105k3/M, 6.0
0.29 and 0.91
0.36,
0
5
10
20
30
respectively, at 298 K. The values indicate that k1 and k3 are
dependent on [Cl−], while k2 is independent of [Cl−].
ε
78.6
6.80
74.2
6.35
69.7
5.94
60.8
5.28
51.9
4.68
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
104kobs (s−1
)
a
[Au(III)] = 1.2 mM, [Ser] = 0.032 M, [H+] = 0.089 mM, and [Cl−]
= 0.04 M.
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.
Figure 8. Nonlinear dependence of k on [Cl−] in the oxidation
process. [Au(III)] = 1.2 mM and [Ser] = 0.04 M.
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
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−].
Scheme 2
k4 + k5[Cl−]
k =
k6 + [Cl−]
(4)
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.
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
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.
quite evident that, among the different Au3+ 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
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
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
each set of reactions is an indication of greater reactivity of
Scheme 3
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.
kinetics with respect to Au(III) and serine each, the reaction
steps in Scheme 3 can be proposed.
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.
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.
d[Au(III)]
−
v = −
= {k1(rds)[AuCl4 ]
dt
+ k2(rds)[AuCl3(OH)−]}[Ser]
(5)
d[Au(III)]
1
−
= kobs
[Au(III)]
dt
k
1(rds)[H+][Cl−] + k2(rds)KhyK
KhyKa3 + [H+][Cl−]
=
a3 [Ser]
(6)
(7)
Scheme 4
k
1(rds)[H+][Cl−] + k2(rds)KhyKa3
KhyKa3 + [H+][Cl−]
kobs
[Ser]
k =
=
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
Table 4. Values of Rate Constants for Slow Rate-
−
Determining Steps Involving Oxidant Species AuCl4 and
AuCl3(OH)−
.
+
withdrawing group (NH3 ) at the α-carbon center. The
experiment
102k1(rds) (M−1 s−1
)
102k2(rds) (M−1 s−1
)
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-
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 0.02
1.54 0.04
1.51 0.04
1.58 0.10
3.15 0.13
3.39 0.76
3.38 0.38
3.26 0.39
G
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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
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
1
confirmed by the melting point measurement and H 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
a
Table 5. Dependence of Pseudo-First-Order Rate Constant on [SDS] at Different Temperatures
[SDS] (mM)
0.0
1.0
2.0
3.0
4.0
5.0
7.0
10.0
20.0
104kψ (s−1) at 298 K
104kψ (s−1) at 303 K
104kψ (s−1) at 308 K
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
a
[Au(III)] = 1.2 mM, [Ser] = 0.04 M, [H+] = 0.089 mM, and [Cl−] = 0.04 M.
H
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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,
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 phenyl-
alanine.39 The formation of catalytic premicelles before the cmc
has also been evidenced from the electron transfer reaction
2−
between Ru(II) complexes and S2O8 in different micro-
heterogeneous 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 well-
known 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
+
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.
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 /
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 AuCl4−/AuCl3(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
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
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.
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
a
Change of Enthalpy and Entropy
temp (K)
298
303
308
104kW (s−1
KSer + KAu(III)
)
9.51
6.17
13.6
5.74
17.7
5.11
a
ΔH0 = −14.4 2 kJ mol−1; ΔS0 = −6.3 0.5 J K−1 mol−1.
kW + kMKSer
{1 + KSer[D ]}{1 + KAu(III)[D ]}
K
Au(III)[D ]2
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 con-
junction with the distribution of ionic gold(III) species as well
as serine between the two pseudophases.
n
kψ =
n
n
(8)
(9)
kW
kψ =
1 + {KSer + KAu(III)}[D ] + KSer
K
Au(III)[D ]2
n
n
KSer + K
1
kψ
1
kW
=
+
Au(III) [D ]
n
kW
(10)
Thus, a plot of kψ−1 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
+
K
Au(III)) were also determined at three different temperatures
4. CONCLUSION
(Table 6). It is evident that the value of a binding constant
decreases with increasing temperature indicating the exother-
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 rate-
determining 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 sub-
mic nature of the binding process. From a plot of log(KSer
+
K
Au(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
J
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
(11) Ott, I. On the Medicinal Chemistry of Gold Complexes as
Anticancer Drugs. Coord. Chem. Rev. 2009, 253, 1670−1681.
(12) Kouodom, M. N.; Boscutti, G.; Celegato, M.; Crisma, M.;
Sitran, S.; Aldinucci, D.; Formaggio, F.; Ronconi, L.; Fregona, D.
Rational Design of Gold(III)-Dithiocarbamato Peptidomimetics for
the Targeted Anticancer Chemotherapy. J. Inorg. Biochem. 2012, 117,
248−260.
(13) Marzano, C.; Ronconi, L.; Chiara, F.; Giron, M. C.; Faustinelli,
I.; Cristofori, P.; Trevisan, A.; Fregona, D. Gold(III) Dithiocarbamato
Anticancer Agents: Activity, Toxicology and Histopathological Studies
in Rodents. Int. J. Cancer 2011, 129, 487−496.
(14) Cattaruzza, L.; Fregona, D.; Mongiat, M.; Ronconi, L.; Fassina,
A.; Colombatti, A.; Aldinucci, D. Antitumor Activity of Gold(III)
Dithiocarbomato Derivatives on Prostate Cancer Cells and Xenoqrafts.
Int. J. Cancer 2011, 128, 206−215.
(15) Nardon, C.; Boscutti, G.; Fregona, D. Beyond Platinums: Gold
Complexes as Anticancer Agents. Anticancer Res. 2014, 34, 487−492.
(16) Ronconi, L.; Marzano, C.; Zanello, P.; Corsini, M.; Miolo, G.;
Macca, C.; Trevisan, A.; Fregona, D. Gold(III) Dithiocarbamate
Derivatives for the Treatment of Cancer: Solution Chemistry, DNA
Binding, and Hemolytic Properties. J. Med. Chem. 2006, 49, 1648−
1657.
(17) Zou, T.; Lum, C. T.; Lok, C.-N.; Zhang, J.-J.; Che, C.-M.
Chemical Biology of Anticancer Gold(III) and Gold(I) Complexes.
Chem. Soc. Rev. 2015, 44, 8786−8801.
(18) Best, S. L.; Sadler, P. J. Gold Drugs: Mechanism of Action and
Toxicity. Gold Bull. 1996, 29, 87−93.
(19) Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Kinetics and
Mechanism of the Reduction of Gold(III) by Glycine in Acetate
Buffer. Transition Met. Chem. 2008, 33, 229−236.
(20) Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Mechanism of
Oxidation of Alanine by Chloroaurate(III) Complexes in Acid
Medium: Kinetics of the Rate Processes. Int. J. Chem. Kinet. 2009,
41, 473−482.
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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bp.cem@spcmc.ac.in. Tel.: +91-33-23503682 (B.P.).
ORCID
Biswajit Pal: 0000-0002-3297-5238
Notes
The authors declare no competing financial interest.
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
1
comments; those were really beneficial for analyzing the H
NMR spectral data.
(21) Sen, P. K.; Gani, N.; Pal, B. Effects of Microheterogeneous
Environments of SDS, TX-100 and Tween 20 on the Electron
Transfer Reaction between L-Leucine and AuCl4− /AuCl3(OH)−. Ind.
Eng. Chem. Res. 2013, 52, 2803−2813.
(22) Sen, P. K.; Maiti, K.; Pal, B. Oxidative Degradation of L-
Isoleucine by Au3+ Complexes in Weakly Acid Medium: A Kinetic and
Mechanistic Investigation. Int. J. Chem. Kinet. 2017, 49, 363−374.
(23) Soni, V.; Sindal, R. S.; Mehrotra, R. N. Kinetics of Oxidation of
L-Histidine by Tetrachloroaurate(III) Ion in Perchloric Acid Solution.
Polyhedron 2005, 24, 1167−1174.
REFERENCES
■
(1) Metcalf, J. S.; Dunlop, R. A.; Powell, J. T.; Banack, S. A.; Cox, P.
A. L-Serine: A Naturally-Occurring Amino Acid with Therapeutic
Potential. Neurotoxic. Res. 2018, 33 (1), 213−221.
(2) Kalhan, S. C.; Hanson, R. W. Resurgence of Serine: An often
Neglected but Indispensable Amino Acid. J. Biol. Chem. 2012, 287,
19786−19791.
(3) Amelio, I.; Cutruzzola, F.; Antonov, A.; Agostini, M.; Melino, G.
Serine and Glycine Metabolism in Cancer. Trends Biochem. Sci. 2014,
39 (4), 191−198.
(24) Zou, J.; Parkinson, J. A.; Sadler, P. J. Gold(III)-induced
Oxidation of Amino Acids and Malonic Acid: reaction Pathways
Studied by NMR Spectroscopy. J. Chin. Chem. Soc. 2002, 49, 499−504.
(4) Antflick, J. E.; Baker, G. B.; Hampson, D. R. The Effects of a Low
Protein Diet on Amino Acids and Enzymes in the Serine Synthesis
Pathway in Mice. Amino Acids 2010, 39, 145−153.
(25) Vujaci
̌
c,
́
A. V.; Savic,
́ ́ ́ ́ ́
J. Z.; Sovilj, S. P.; Meszaros Szecsenyi, K.;
̌
Todorovic, N.; Petkovic,
́
́
M. Z.; Vasic, V. M. Mechanism of Complex
́
(5) Sumathi, T.; Shanmugasundaram, P.; Chandramohan, G. A
Kinetic and Mechanistic Study on the Silver(I) Catalyzed Oxidation of
L-Serine by Cerium(IV) in Sulfuric Acid Medium. J. Saudi Chem. Soc.
2013, 17, 227−235.
Formation between [AuCl4]− and L-Methionine. Polyhedron 2009, 28,
593−599.
(26) Maiti, K.; Sen, P. K.; Pal, B. Influence of Premicelles and
Micellar Aggregates of Ionic and Nonionic Surfactants in the Oxidative
Decarboxylation of L-lysine by Gold(III) Complexes. J. Mol. Liq. 2018,
251, 238−248.
(27) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and
Macromolecular Systems; Academic Press: New York, 1975.
(28) Sen, P. K.; Talukder, S.; Pal, B. Specific Interactions of Anions
and Pre-micelles in the Alkaline Fading of Crystal Violet Carbocation.
Colloids Surf. A: Physicochem. Eng. Asp. 2015, 467, 259−269.
(29) Sawada, K.; Ueda, M. Characteristic of Aqueous Microenviron-
ments in Non-Ionic Surfactant Reverse Micelles and their Use for
Enzyme Reactions in Non-Aqueous Media. J. Chem. Technol.
Biotechnol. 2004, 79, 369−375.
(6) Shi, H.; Shen, S.; Sun, H.; Liu, Z.; Li, L. Oxidation of L-Serine
and L-Threonine by Bis(hydrogen periodato)argentite(III) Complex
Anion: A Mechanistic Study. J. Inorg. Biochem. 2007, 101, 165−172.
(7) Chandraju, S.; Rangappa, K. S.; Made Gowda, N. M.
Manganese(III) Oxidation of L-Serine in Aqueous Sulfuric Acid
Medium: Kinetics and Mechanism. Int. J. Chem. Kinet. 1999, 31, 525−
530.
(8) Khan, Z.; Kabir-ud-Din. Kinetics and Mechanism of Oxidative
Degradation of DL-Serine by Chromium(VI). Indian J. Chem. 1999,
38A, 361−365.
(9) Pascual, R.; Herraez, M. A. The Mechanism of the Oxidative
Deamination of Serine and Threonine by Periodate in Acid Medium.
Can. J. Chem. 1985, 63, 2349−2353.
(10) Lammer, A. D.; Cook, M. E.; Sessler, J. L. Synthesis and Anti-
Cancer Activities of a Water Soluble Gold(III) Porphyrin. J. Porphyrins
Phthalocyanines 2015, 19, 398−403.
(30) Sen, P. K.; Chatterjee, P.; Pal, B. Evidence of Co-operativity in
the Pre-micellar Region in the Hydrolytic Cleavage of Phenyl
Salicylate in the Presence of Cationic Surfactants of CTAB, TTAB
and CPC. J. Mol. Catal. A: Chem. 2015, 396, 23−30.
K
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
(31) Aydinoglu, S.; Biver, T.; Secco, F.; Venturini, M. The
mechanism of the reaction between Au(III) and PADA in
Sodiumdodecylsulphate. Colloids Surf. A: Physicochem. Eng. Asp.
2016, 498, 81−87.
(32) Majumdar, T.; Bhowmik, D.; Kundu, A.; Dasmandal, S.;
Mahapatra, A. The Effect of Serum Albumin, Surfactant and their
Mixture on the Reduction of a Cobalt(III) Complex by Ascorbic Acid.
Colloids Surf. A: Physicochem. Eng. Asp. 2013, 436, 185−192.
(33) Brandariz, I.; Iglesias, E. Micellar Effects on Aromatic Esters
Hydrolysis. Colloids Surf. A: Physicochem. Eng. Asp. 2014, 454, 180−
188.
(34) Sen, P. K.; Midya, J. K.; Bysakh, S.; Pal, B. Kinetics and
Mechanistic Studies on the Oxidation of D-Glucose by MnO2
Nanoparticles. Effect of Microheterogeneous Environments of
CTAB, Triton X-100 and Tween 20. Molecular Catalysis. 2017, 440,
75−86.
(35) Chakraborty, G.; Chowdhury, M. P.; Hassan, P. A.; Tsuchiya,
K.; Torigoe, K.; Saha, S. K. Interaction of Tyrosine Analogues with
Quaternary Amonium Head Groups at the Micelle/water Interface
and Contrasting Effect of Molecular Folding on the Hydrophobic
Outcome and End-Cap Geometry. J. Phys. Chem. B 2018, 122, 2355−
2367.
(49) Akerlof, G.; Short, O. A. The Dielectric Constant of Dioxane-
Water Mixtures between 0 and 80°. J. Am. Chem. Soc. 1936, 58, 1241−
1243.
(50) Pal, B.; Sen Gupta, K. K. Kinetics and Mechanism of the Inner-
Sphere Reduction of Hexachloroplatinate(IV) by Dithionite in Acetate
Buffer. Transition Met. Chem. 2012, 37, 671−678.
(51) CRC Handbook of Chemistry & Physics, 67th ed.; Weast, R. C.,
Ed.; CRC Press: Boca Raton, FL, 1986−87; p 699.
(52) Van, P.; Bekker, Z.; Robb, W. Kinetics of Hydrolysis of the
Tetrachloroaurate(III) Anion in Aqueous Perchloric Acid. Inorg. Nucl.
Chem. Lett. 1972, 8, 849−854.
(53) Sen, P. K.; Sarkar, A.; Maiti, K.; Pal, B. Reduction of Gold(III)
by Malonate in Acetate Buffer: Mechanism of the Rate Processes. J.
Indian Chem. Soc. 2016, 93, 321−326.
(54) Sen Gupta, K. K.; Pal, B.; Begum, B. A. Reactivity of Some
Sugars and Sugar Phosphates Towards Gold(III) in Sodium Acetate-
Acetic Acid Buffer Medium. Carbohydr. Res. 2001, 330, 115−123.
(55) Streszewski, B.; Jaworski, W.; Szacilowski, K.; Paclawski, K.
Kinetics and Mechanism of Redox Reaction between
Tetrachloroaurate(III) Ions and Hydrazine. Int. J. Chem. Kinet. 2014,
46, 328−337.
(56) Bera, A. K.; Pal, B.; Sen Gupta, K. K. Outer-Sphere reduction of
Hexacyanoferrate(III) by Enolizable and nonenolizable Aldehydes in
Alkaline Medium. Int. J. Chem. Kinet. 2012, 44, 494−505.
(57) Althagafi, I. I.; Fawzy, A. Kinetics and Mechanistic Study of
Permanganate Oxidation of L-Citrulline in Acidic and Basic Media.
Am. J. Phys. Chem. 2016, 5, 99−107.
(58) Wisniowski, P.; Carmichael, I.; Fessenden, R. W.; Hug, G. L.
Evidence for β Scission in the Oxidation of Amino Acids. J. Phys.
Chem. A 2002, 106, 4573−4580.
(59) Finar, I. L. Organic Chemistry, the Fundamental Principles, 6th
ed.; ELBS, Longman: 1973; Vol. 1, p 36.
(60) Perez-Benito, J. Permanganate Oxidation of α-Amino Acids:
Kinetic Correlations for the Nonautocatalytic and Autocatalytic
Reaction Pathways. J. Phys. Chem. A 2011, 115, 9876−9885.
(61) Malik, M. A.; Sheikh, R. A.; Al-Thabaiti, S. A.; Obaid, Y.; Khan,
Z. Micelles-Assisted MnO4− Oxidation of Isoleucine: A Kinetic Study.
J. Dispersion Sci. Technol. 2011, 32, 1173−1178.
(36) Altaf, M.; Jaganyi, D. Oxidation of Methionine by Colloidal
MnO2 in Aqueous and Micellar Media: A Kinetic Study. J. Dispersion
Sci. Technol. 2013, 34, 1481−1487.
(37) Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Effects of Nonionic
Micellar Aggregates on the Electron Transfer Reaction between L-
Glutamic Acid and Gold(III) Complexes. Int. J. Chem. Kinet. 2012, 44,
482−493.
(38) Sheikh, R. A.; Al-Nowaiser, F. M.; Malik, M. A.; Al-Youbi, A. O.;
Khan, Z. Effect of Cationic Micelles of Cetyltrimethylammonium
−
Bromide on the MnO4 Oxidation of Valine. Colloids Surf. A:
Physicochem. Eng. Asp. 2010, 366, 129−134.
(39) Zaheer, Z.; Rafiuddin. Sub- and Post-Micellar Catalytic and
Inhibitory effects of Cetyltrimethylammonium Bromide in the
Permanganate Oxidation of Phenylalanine. Colloids Surf., B 2009, 69,
251−256.
(62) Pal, B.; Sen, P. K.; Sen Gupta, K. K. Reactivity of Alkanols and
Aryl Alcohols Towards Tetrachloroaurate(III) in Sodium Acetate-
Acetic Acid Buffer Medium. J. Phys. Org. Chem. 2001, 14, 284−294.
(63) Pal, B.; Dhar, B. B.; Sen Gupta, K. K. Reactivity of Dichloro-
(1,10-phenanthroline)Gold(III) Chloride towards Sulfite in Phen-
PhenH+ Buffer Medium. J. Indian Chem. Soc. 2017, 94, 983−991.
(64) Pal, B. Mechanistic Investigation of the Oxidative Cleavage of
the Carbon-Carbon Double Bond in α,β-Unsaturated Compounds by
Hexachloroiridate(IV) in Acetate Buffer. Int. J. Chem. Kinet. 2014, 46,
31−40.
(40) Pandey, E.; Upadhyay, S. K. Effect of Micellar Aggregates on the
Kinetics of Oxidation of α-Aminoacids by Chloramine-T in Perchloric
Acid Medium. Colloids Surf. A: Physicochem. Eng. Asp. 2005, 269, 7−15.
(41) Malik, M. A.; Basahel, S. N.; Obaid, A. Y.; Khan, Z. Oxidation of
Tyrosine by Permanganate in Presence of Cetyltrimethylammonium
Bromide. Colloids Surf., B 2010, 76, 346−353.
(42) Pal, B.; Sen, P. K.; Sen Gupta, K. K. Electron Transfer Reactions
Between Chloroaurate(III) Complexes and Various Inorganic and
Organic Reducing Agents in Acid Medium- A Brief Review. J. Indian
Chem. Soc. 2006, 83, 762−776.
(43) Dische, Z.; Borenfreund, E. A Specific Color Reaction of
Glycolic Aldehyde. J. Biol. Chem. 1949, 180, 1297−1300.
(44) Huntress, E. H.; Mulliken, S. P. Identification of Pure Organic
Compounds; John Wiley: New York, 1953; p 43.
(45) Sen Gupta, K. K.; Pal, B.; Sen, P. K. Kinetics and Mechanism of
the Oxidation of Some α-Hydroxy Acids by Tetrachloroaurate(III) in
Acetic Acid Sodium Acetate Buffer Medium. Int. J. Chem. Kinet. 1999,
31, 873−882.
(46) Sen Gupta, K. K.; Pal, B.; Bhattacharjee, N.; Ghosh, S. Kinetics
and Mechanism of Tris(pyridine-2-carboxylato)manganese(III) Re-
duction by Azide Ion in Sodium Pyridine-2-carboxylate-Pyridine-2-
Carboxylic Acid Buffer Media. Bull. Chem. Soc. Jpn. 1999, 72, 1769−
1774.
(47) Chandraju, S.; Mahadevappa, D. S.; Rangappa, K. S.; Made
Godwa, N. M. Anodically Generated Manganese (III) Oxidation of L-
Serine and Threonine in Aqueous Acetic Acid. Synth. React. Inorg. Met.-
Org. Chem. 1997, 27, 1329−1346.
(48) Chimatadar, S. A.; Madawale, S. V.; Nandibewoor, S. T.
Mechanistic Study of Iodide Catalysed Oxidation of L-Glutamic Acid
by Cerium(IV). Transition Met. Chem. 2007, 32, 634−641.
(65) Khan, Z.; Singh, T.; Hussain, J. I.; Hashmi, A. A. Au(III)-CTAB
Reduction by Ascorbic Acid: Preparation and Characterization of Gold
Nanoparticles. Colloids Surf., B 2013, 104, 11−17.
(66) Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth
Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth
Method. Chem. Mater. 2003, 15, 1957−1962.
(67) Andrabi, S. M. Z.; Malik, M. A.; Khan, Z. Permanganate
Partitioning in Cationic Micelles of Cetyltrimethylammonium Bro-
mide: A Kinetic Study of D-Fructose Oxidation. Colloids Surf. A:
Physicochem. Eng. Asp. 2007, 299, 58−64.
(68) Brinchi, L.; Di Profio, P.; Germani, R.; Giacomini, V.; Savelli, G.;
Bunton, C. A. Surfactant Effects on Decarboxylation of Alkoxyni-
trobenzisoxazole-3-carboxylate Ions. Acceleration by Premicelles.
Langmuir 2000, 16, 222−226.
(69) Sen, P. K.; Mukherjee, P.; Pal, B. Effects of Pre-micelles of
Anionic Surfactant SDS on the Electron Transfer Reaction Between
Methylene Blue and Ascorbic Acid. J. Mol. Liq. 2016, 224, 472−479.
(70) Lop
́ ́
ez-Cornejo, P.; Mozo, J. D.; Roldan, E.; Domínguez, M.;
2‑
San
́
chez, F. Kinetic Study of the Reaction *[Ru(bpy)3]2+ + S2O8 in
Solutions of Brij-35 at Premicellar and Micellar Concentrations. Chem.
Phys. Lett. 2002, 352, 33−38.
L
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
́ ́ ́
(71) Lopez-Cornejo, P.; Perez, P.; García, F.; de la Vega, R.; Sanchez,
F. Use of the Pseudophase Model in the Interpretation of Reactivity
under Restricted Geometry Conditions. An Application to the Study
2‑
of the [Ru(NH3)5pz]2+ + S2O8 Electron-Transfer Reaction in
Different Microheterogeneous Systems. J. Am. Chem. Soc. 2002, 124,
5154−5164.
(72) Onel, L.; Buurma, N. J. The Nature of the Sodium
Dodecylsulfate Micellar Pseudophase as Studied by Reaction Kinetics.
J. Phys. Chem. B 2011, 115, 13199−13211.
(73) Mao, S.; Chen, Z.; fan, D.; An, X.; Shen, W. Spectrometric Study
of AOT-Hydrolysis Reaction in Water/AOT/Isooctane Microemul-
sions Using Phenolphthalein as a Chemical Probe. J. Phys. Chem. A
2012, 116, 158−165.
(74) Cavasino, F. P.; cervellati, R.; Lombardo, R.; Turco Liveri, M. L.
Micellar Effects on the Kinetics of cerium(IV) Oxidation and the
Cerium(IV)-Catalyzed Belousov-Zhabotinsky Reaction with Methyl-,
Ethyl-, or Benzylmalonic Acid. J. Phys. Chem. B 1999, 103, 4285−4291.
(75) Berezin, I. V.; Martinek, K.; Yatsimirskii, A. K. Physicochemical
foundations of Micellar Catalysis. Russ. Chem. Rev. 1973, 42, 787−802
(Engl. Transl.).
(76) Sen, P. K.; Samaddar, P. R.; Das, K. Kinetics and Mechanism of
the Oxidation of Propan-1-ol and Propan-2-ol by permanganate in the
absence and presence of Tween-20 in Perchloric Acid Medium.
Transition Met. Chem. 2005, 30, 907−914.
M
DOI: 10.1021/acs.jpca.8b02409
J. Phys. Chem. A XXXX, XXX, XXX−XXX