Oxidative Degradation of l-Isoleucine by Au3+ Complexes in Weakly Acid Medium: A Kinetic and Mechanistic Investigation

Article abstract of DOI:10.1002/kin.21081

Oxidative degradation of l-isoleucine (Ileu) by Au³⁺ complexes has been studied spectrophotometrically in weakly acid medium (acetic acid–sodium acetate buffer, pH range 3.72–4.80) in the temperature range 288–308 K. The reaction is first order with respect to Au^III but complex order (<1) with respect to isoleucine. Ionic strength has no significant effect on the reaction kinetics. Both H⁺ and Cl^? ions have been found to show inhibiting effect on the reaction rate. Decreasing solvent polarity exerts an adverse effect on the reaction. Au³⁺ complexes react with the zwitterion form of isoleucine in a one-step two-electron transfer redox process. The reaction passes through intermediate formation of iminic cation, which hydrolyzes to produce 2-methyl butanal, identified by ¹H NMR. The activation parameters ΔH^≠ and ΔS^≠ related to the rate-limiting step of the reaction are evaluated. The derived rate law is in excellent agreement with the experimental results. The kinetic and activation parameters of this investigation have been compared and analyzed with those of the oxidation of l-leucine by gold(III).

Full text of DOI:10.1002/kin.21081

Oxidative Degradation of
L-Isoleucine by Au3+
Complexes in Weakly Acid
Medium: A Kinetic and
Mechanistic Investigation
PRATIK K. SEN,1 KRISHNENDU MAITI,1 BISWAJIT PAL2
1Department of Chemistry, Jadavpur University, Kolkata, 700 032, India
2Department of Chemistry, St. Paul’s C. M. College, Kolkata, 700 009, India
Received 8 November 2016; revised 27 January 2017; accepted 27 January 2017
DOI 10.1002/kin.21081
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Oxidative degradation of L-isoleucine (Ileu) by Au3+ complexes has been studied
spectrophotometrically in weakly acid medium (acetic acid–sodium acetate buffer, pH range
3.72–4.80) in the temperature range 288–308 K. The reaction is first order with respect to AuIII
but complex order (<1) with respect to isoleucine. Ionic strength has no significant effect on
the reaction kinetics. Both H+ and Cl− ions have been found to show inhibiting effect on
the reaction rate. Decreasing solvent polarity exerts an adverse effect on the reaction. Au3+
complexes react with the zwitterion form of isoleucine in a one-step two-electron transfer
redox process. The reaction passes through intermediate formation of iminic cation, which
hydrolyzes to produce 2-methyl butanal, identified by 1H NMR. The activation parameters
ࣔ
ࣔ
ꢀH and ꢀS related to the rate-limiting step of the reaction are evaluated. The derived
rate law is in excellent agreement with the experimental results. The kinetic and activation
parameters of this investiꢀCgation have been compared and analyzed with those of the oxidation
of L-leucine by gold(III).
2017 Wiley Periodicals, Inc. Int J Chem Kinet 1–12, 2017
ber of metabolic reactions [1]. Isoleucine is one of
INTRODUCTION
the essential amino acids which play vital roles in
a living system. It is both a glucogenic and a keto-
genic amino acid. It is needed for hemoglobin forma-
tion and also stabilizes and regulates blood sugar and
energy levels. Isoleucine has been found to stimulate
insulin independent glucose uptake in muscle cells [2].
Thus the mechanism of the degradation of this par-
ticular amino acid will be of immense importance in
the perspective of the involvement of isoleucine in so
Amino acids comprise the major component of bio-
logical cells and tissues in the form of proteins. Amino
acids perform critical roles in processes such as neuro-
transmission, biosynthesis of proteins, and in a num-
Correspondence to: P. K. Sen; e-mail: senpkju@gmail.com;
B. Pal; e-mail: palbiswajit@yahoo.com.
ꢀC
2017 Wiley Periodicals, Inc.

2
SEN, MAITI, AND PAL
many significant functions in the biological processes.
Metal ions [3–5] have been found to play important
roles in the oxidative decarboxylation of amino acids.
Only a few reports have been found in the literature
involving the mechanistic studies on the oxidation of
isoleucine by different oxidants such as alkaline diperi-
odatoargentate(III) [6], alkaline permanganate [7,8],
benzyltrimethylammonium tribromide [9], and sodium
N-bromobenzenesulfonamide [10].
and to the C-4 position in leucine. Since in isoleucine
the branching is closer to the reaction center, i.e.,
the amino acid moiety [─CH(NH2)COOH] compared
to leucine, it may be expected that they will have
different steric as well as electronic effects on the
mechanism of the oxidation of these amino acids by
gold(III). Thus it will be interesting to see how the
steric and electronic factors arising out of a change
in the nature of the substituents at the α-carbon in-
fluence the change in reactivity and activation param-
eters. In this context, we have investigated system-
atically the oxidative degradation of L-isoleucine by
gold(III) under various experimental conditions and
made an attempt to compare the results with those
of the oxidation of L-leucine [21] by gold(III) in the
light of reactivity of the substrates due to the pres-
ence of different substituents at the α-carbon and pos-
sible influence of the substituents on the activation
parameters.
Gold(III) is isoelectronic (d8) with platinum(II)
and forms square planar (dsp2) complexes [11]. Like
several platinum(II) complexes, they may be poten-
tial candidates as anticancer drugs [12,13]. Among
the antitumor drugs, gold(I) and gold(III) compounds
have nowadays gained increasing importance owing
to their strong inhibitory effects on tumor cell growth
[13,14]. Several gold(III)-dithiocarbamato derivatives
have been designed as potential anticancer agents
[15,16]. There are some recent reports of Au(I)
and Au(III) complexes, viz., [Au(dppe)2]Cl (dppe =
1,2-bis(diphenylphosphanyl)ethane) and [Au(TPP)]Cl
(H2TPP = 5,10,15,20-tetraphenylporphyrin), which
have been found to exhibit anticancer activities [17].
Gold(I) compounds, viz., gold(I)-thiolate and gold(I)-
phosphine complexes, are used for the treatment of
rheumatoid arthritis [18]. However, their application is
limited due to toxic side effects involved. The mech-
anism of action and the molecular basis of their side
effects are not well established. Zou et al [18] reported
that in inflammatory situation, strong oxidants such
as H2O2 and ClO− are potentially available in vivo,
and these probably oxidize gold(I) to gold(III). This
gold(III) has a possibility to undergo a redox reaction
with the peptides and amino acids in the biological
system accounting for the toxic side effects. Au(III)
complexes have been found to inhibit the activities of
thiol-containing enzymes [17]. Hence the interaction
of gold(III) compounds with proteins and amino acids
is of immense importance.
The reaction of gold(III) by sulfur-containing amino
acids such as cysteine [19] and methionine [20]
has been reported earlier. Gold-induced oxidation of
glycine and identification of the reaction intermediates
as well as the final products via multinuclear NMR
spectroscopy were carried out by Zou et al [18]. Re-
cently, the electron transfer redox reaction between
gold(III) complexes and L-leucine [21] was studied
by our research group in aqueous and microhetero-
geneous environments. There seems to be no report on
the kinetic and mechanistic studies on the oxidation
of isoleucine by gold(III). Isoleucine is actually an
isomer of leucine having the same molecular formula.
However, they are structurally different in that a methyl
group is attached to the C-3 position in isoleucine
EXPERIMENTAL
Materials
L-Isoleucine (extrapure, SRL, Mumbai, India), NaClO4
monohydrate (GR, Loba, Mumbai, India), hydrochlo-
ric acid (EMPARTA, Merck, Mumbai, India), NaCl
(ExcelaR, Qualigens, Mumbai, India), NaOH (EM-
PARTA, Merck, Mumbai, India), and acetic acid
glacial (extrapure, AR, SRL, Mumbai, India) were
used without purification. 1,4-Dioxan (extrapure AR,
SRL, Mumbai, India) was treated with Mohr salt and
kept overnight at room temperature, followed by fil-
tration and distillation. The standard gold(III) solution
was prepared by dissolving tetrachloroauric acid (SRL,
Mumbai, India) in 0.01 M HCl. The strength of the
gold(III) solution was determined by measuring the
absorbance (A) of the solution at the absorbance max-
imum (λ = 313 nm, є = 4860 M−1 cm−1) of AuCl4−
in the UV region [22,23]. Milli-Q water (Merck Mil-
lipore, Darmstadt, Germany) was used throughout the
work.
Kinetic Measurements
All the kinetic studies were performed under pseudo–
first-order condition where [Ileu] ꢀ [AuIII]. Constant
pH of the reaction medium was maintained by using
sodium acetate–acetic acid buffer. The progress of
the reaction was followed in a Shimadzu UV–visible
spectrophotometer (UV-1800, model-Tcc-240A,
Kyoto, Japan) at a constant temperature main-
tained within a Peltier controlled thermostated cell
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

OXIDATIVE DEGRADATION OF L-ISOLEUCINE BY Au3+ COMPLEXES IN WEAKLY ACID MEDIUM
3
was mixed with gold(III) chloride and allowed to stand
for 2 h in a stoppered round bottom flask, and then it
was distilled. With one part of the distillate, the fuch-
sine reagent test was performed. A pink color appeared,
indicating the presence of a –CHO group [25] in the
reaction product. The second part of the distillate was
treated with 2,4-dinitrophenylhydrazine (2,4-DNP) so-
lution maintaining the acidity with sulfuric acid, which
gave an orange precipitate of the 2,4-DNP derivative
of the oxidation product. The precipitate was purified,
and the melting point of the derivative was found to
be 127°C [26]. The melting point resembles that of the
2,4-DNP derivative of 2-methylbutanal. The 2,4-DNP
1
derivative was also analyzed by H NMR (300 MHz,
Brucker DPX-300, CDCl3) spectroscopy (Fig. 2). The
1H NMR spectral data furnished the following char-
acteristic signals. A broad singlet at δ 10.99 appeared
due to the ─NH moiety of the hydrazone, which was
intramolecularly hydrogen bonded to the ─NO2 group
at the ortho position. The aromatic ─H located between
two ─NO2 groups appeared as a doublet at δ 9.12 with J
= 2.5 Hz. A doublet of doublet appeared at δ 8.30 with
J1 = 9.6 Hz and J2 = 2.2 Hz due to aromatic ─H ortho
with respect to one ─NO2 -group and para with respect
to other ─NO2 group. A doublet at δ 7.93 with J =
9.6 Hz was present due to aromatic ─H, which was
meta with respect to both the ─NO2 groups. The
presence of 2,4-dinitrophenylhydrazone moiety in the
product was substantiated with the aforesaid signals.
The presence of ─CH═N─ moiety was evident from
the doublet at δ 7.44 with J = 5.7 Hz. A quintet at δ
2.49 with J = 6.7 Hz appeared due to the methine H
of the aliphatic side chain. A multiplet around δ 1.47–
1.68 was due to the >CH2 moiety. The methyl group
nearer to the >C═N─ moiety appeared as a doublet at
δ 1.19 with J = 6.8 Hz. A triplet at δ 1.00 with J = 7.4
Hz was found corresponding to the remaining methyl
group. From the aforesaid spectral features, the occur-
rence of 2,4-DNP derivative of 2-methylbutanal was
confirmed. Therefore, it is conclusively proved that 2-
methylbutanal was formed as the reaction product of
isoleucine (Scheme 1).
Figure 1 Time-resolved spectra of the reaction mixture.
[Ileu] = 3.05 × 10−2 M, [Au3+] = 2.47 × 10−4 M, [H+] =
9.12 × 10−5 M, [Cl−] = 0.04 M, temperature = 298 K, and
scanning time interval 1 min. [Color figure can be viewed at
wileyonlinelibrary.com]
compartment. Requisite quantities of acetate buffer
solution, sodium chloride, and isoleucine were placed
in the cuvette within the cell compartment, and the
mixture was kept for 10 min for thorough mixing
and temperature equilibration. The reaction was
then initiated by adding appropriate volume of AuIII
solution. The time-resolved spectra (scanning time
interval = 1 min) of the reaction mixture exhibit
maximum absorbance at 313 nm (Fig. 1). The
absorbance value at the maximum decreases with
the progress of the reaction. The kinetic run for the
reaction was monitored by following the absorbance
of the reaction mixture in the visible region at λ =
400 nm although the absorbance maximum was at
313 nm. The reason is that in the presence of UV
light there might be formation of gold particles, which
may interfere in the rate measurements [21,24]. The
observed pseudo–first-order rate constants (kobs) were
evaluated from the slopes of the linear plots of ln A(A
= absorbance) versus “time,” and the results were
reproducible to within 5%.
The overall stoichiometry of the reaction may there-
fore be represented as
Product Analysis
The reaction product of oxidation of L-isoleucine was
1
identified by spot tests [25] as well as by H NMR
spectroscopy. The principal product is the correspond-
ing carbonyl compound (aldehyde). Evolved ammo-
nia gas from the reaction mixture was confirmed by
Nessler’s reagent test, and the presence of CO2 as a
product was identified by the lime water test. In an-
other experiment, isoleucine (excess) in acetate buffer
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

4
SEN, MAITI, AND PAL
Figure 2 NMR spectra of 2,4-DNP derivative of the oxidation product of L-isoleucine.
perature constant at 4.04 × 10−2, 9.12 × 10−5, 4.0 ×
10−2 M, and 298 K, respectively. When log A was plot-
ted against time, parallel straight lines were obtained
for different initial [AuIII]. The rate constant (kobs) was
found to be (9.83 0.05) × 10−4 s−1 (Table I), which
indicates that the reaction is of first order with respect
to AuIII.
The effect of variation of [Ileu] on the reaction rate
was studied at five different temperature (288–308 K)
keeping the concentrations of other reactants constant
(Table II). Representative pseudo–first-order plots of
ln A versus “timeȍ at various [Ileu] at a constant tem-
perature of 298 K are presented in Fig. 3. The pseudo–
first-order rate constant (kobs) was found to increase
with increasing isoleucine concentration (Table II).
Plots of log kobs versus log [Ileu] at different temper-
atures produced straight lines having slopes ranging
within 0.78–0.88, which indicated that the reaction is
of complex order with respect to isoleucine. A double
Scheme 1 2,4-DNP derivative of the oxidation product of
L-isoleucine. [Color figure can be viewed at wileyonlineli-
brary.com]
RESULTS
Effect of Change in Reactant
Concentrations
−1
reciprocal plot of k against [Ileu]−1 (Fig. 4) at each
obs
temperature, however, produced a straight line (R2 =
0.97–0.99) with a positive slope and positive intercept
on the Y-axis. This type of plot indicated the formation
of an intermediate complex between isoleucine and
gold(III) in a fast preequilibrium step [21,27,28].
To determine the order of the reaction with respect
to [AuIII] , the reaction was studied under pseudo–
first-order condition by varying [AuIII] [(0.8–3.2) ×
10−3 M] while keeping [Ileu], [H+], [Cl−], and tem-
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

OXIDATIVE DEGRADATION OF L-ISOLEUCINE BY Au3+ COMPLEXES IN WEAKLY ACID MEDIUM
5
Table I Influence of Changing [AuIII], Ionic Strength (μ), [H+] and [Cl−] on the Rate of Reduction of Gold(III) by
L-Isoleucine at 298 K
103 [AuIII] (M)
102 [Ileu] (M)
μ (M)
105 [H+] (M)
102 [Cl−] (M)
104 kobs (s−1
)
0.8
1.5
2.6
3.2
4.04
4.04
4.04
4.04
0.08
0.08
0.08
0.08
9.12
9.12
9.12
9.12
4.0
4.0
4.0
4.0
9.85
9.80
9.86
9.78
2.5
2.5
2.5
2.5
2.5
3.05
3.05
3.05
3.05
3.05
0.13
0.18
0.28
0.43
0.18
9.12
9.12
9.12
9.12
1.05
4.0
4.0
4.0
4.0
4.0
8.98
8.96
8.46
8.75
15.1
2.5
2.5
2.5
2.5
3.05
3.05
3.05
3.05
0.14
0.10
0.08
0.06
2.34
5.37
9.12
9.12
4.0
4.0
4.0
2.0
12.4
10.1
8.50
13.6
8.89
6.74
3.83
2.00
2.5
2.5
2.5
2.5
3.05
3.05
3.05
3.05
0.08
0.14
0.24
0.44
9.12
9.12
9.12
9.12
4.0
10.0
20.0
40.0
that mechanistic steps involving different species of
Au3+ will be influenced by the change of [Cl−]. In
the reaction mixture, [Cl−] was varied from 0.02 to
0.4 M by the addition of NaCl, maintaining [AuIII],
Effect of Ionic Strength
The effect of ionic strength on the reaction rate was
studied by changing the concentration of NaClO4 (0–
0.35 M) in the reaction mixture maintaining [AuIII],
[Ileu], [H+], [Cl−], and temperature constant at 2.47
× 10−3, 3.05 × 10−2, 9.12 × 10−5, 4.0 × 10−2 M,
and 298 K, respectively. The pseudo–first-order rate
[Ileu], [H+], and temperature constant at 2.47 × 10−3
,
3.05 × 10−2, 9.12 × 10−5 M, and 298 K, respectively.
The chloride ion produced a retarding effect on the
reaction rate (Table I). The kobs showed a nonlinear
variation w−it1h [Cl−]. Therefore, an attempt was made
to plot kobs against [Cl−] and a straight line (R2 =
0.99) was obtained with a positive slope and a positive
intercept (Fig. 6).
constant ((8.79 0.33) × 10−4
s
−1) was found to be
unaffected, reflecting that the variation of ionic strength
did not influence the reaction rate (Table I).
Effect of pH
The influence of H+ on the reaction rate was inves-
tigated at different pH (3.72–4.80) of the reaction
medium keeping [AuIII], [Ileu], [Cl−], and temperature
constant. No attempt was made to keep ionic strength
constant as it had no influence on the pseudo–first-
order rate constant. The reaction rate was found to
decrease with an increase in [H+] (Table I). A similar
observation was obtained from earlier investigation in-
volving the oxidation of leucine [21]. The pseudo–first-
order rate constant, kobs, did not show linear variation
with the H+ ion concentration. However, a linear plot
(Fig. 5) of 1/kobs against [H+] (R2 = 0.99) was ob-
tained, predicting an inhibiting role of H+ ion in the
mechanistic path during the oxidation of L-isoleucine.
Effect of Changing Dielectric Constant
The dielectric constant of the reaction medium was al-
tered by adding different amounts of 1,4- dioxane (0–
35% v/v), whereas other parameters such as [AuIII],
[Ileu], [H+], [Cl−], and temperature were kept con-
stant. Addition of dioxane showed an inhibiting effect
on the reaction rate. The pseudo–first-order rate con-
stant diminished with decreasing dielectric constant (ε)
of the reaction mixture (Table III).
Test for Free Radicals
To ascertain whether the present reaction involves any
free radical intermediate, the reaction was carried out in
the presence of 10% (v/v) acrylonitrile, while [AuIII],
[Ileu], [H+], [Cl−], and temperature were maintained
at 2.47 × 10−3, 3.05 × 10−2, 9.12 × 10−5, 4.0 ×
Effect of Chloride Ion on kobs
The equilibrium of HAuCl4 dissociation is controlled
by the Cl− concentration [29]. Therefore, it is obvious
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

6
SEN, MAITI, AND PAL
Table II Effect of [Ileu] on the Rate of Reduction of Gold(III) by L-Isoleucine at Different Temperatures. [AuIII] = 2.47
× 10−3 M, [H+] = 9.12 × 10−5 M, [Cl−] = 4 × 10−2
Temperature(K)
M
102 [Ileu] (M)
104 kobs (s−1
)
103 k (s−1
)
104 K6 (M)
K 7 (M−1
)
288
0.838
1.52
2.29
3.89
5.64
7.85
2.05
3.35
4.55
7.67
9.22
11.6
2.40
–
−
293
298
303
308
0.838
1.52
2.29
3.89
5.34
7.77
2.30
3.80
5.73
8.22
10.5
13.1
3.03
3.8
−
0.563
−
−
25.08
−
0.838
1.68
2.36
4.04
5.72
7.62
2.79
4.94
6.81
9.83
14.4
17.1
0.762
1.52
2.44
3.96
5.34
7.62
2.93
5.31
7.65
12.4
16.5
21.5
5.3
0.838
1.52
2.29
3.81
5.56
7.62
4.11
6.82
9.43
14.4
21.9
27.0
6.41
−
−
10−2 M, and 298 K, respectively. However neither any
suspension nor any precipitate was observed within
30 min. Therefore, it may be concluded that the present
reaction did not involve the one-electron transfer pro-
cess [4].
K2
AuCl−4 + H2O ꢀ AuCl3 (OH2) + Cl−;
K2 = 9.5 × 10−6 M (298 K)
(3)
(4)
K3
AuCl3 (OH2) ꢀ AuCl3 (OH)− + H+;
DISCUSSION
K3 = 25 × 10−2 M (298 K)
Gold(III) complexes are isostructural and isoelectronic
with platinum(II) complexes. Such gold(III) com-
plexes are reported to be predominantly square pla-
nar in geometry. Tetrachloroauric(III) acid readily un-
dergoes hydrolysis producing three gold(III) complex
ions, viz., AuCl4−, AuCl3(OH2), AuCl3(OH)− accord-
ing to the following equilibria [22,29]:
Although four different gold(III) species are in-
volved in the hydrolysis equilibria of chloroauric acid,
the relative concentration of each of these species will
be controlled by the pH, [Cl−], and the respective equi-
librium constant values. Thus, the effective oxidizing
species of gold(III) that are predominant in the re-
action system appear to be AuCl4− and AuCl3(OH)−
under the present reaction conditions (pH 3.72–4.80,
[Cl−] = 0.04 M).
K1
HAuCl4 ꢀ AuCl−4 + H+; K1 = 1.0 M (298 K) (2)
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

OXIDATIVE DEGRADATION OF L-ISOLEUCINE BY Au3+ COMPLEXES IN WEAKLY ACID MEDIUM
7
Figure 3 Typical pseudo–first-order plots for the reaction between L-isoleucine and gold(III) at 298 K. Plots of ln A versus
“time.” [Au3+] = 2.47 × 10−3 M, [H+] = 9.12 × 10−5 M, [Cl−] = 0.04 M. [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 4 Dependence of the pseudo–first-order rate constant on L-isoleucine concentration. Plots of kobs versus [Ileu]−1 at
−1
five different temperatures. [AuIII] = 2.47 × 10−3 M, [H+] = 9.12 × 10−5 M, [Cl−] = 4.0 × 10−2 M. [Color figure can be
viewed at wileyonlinelibrary.com]
An amino acid generally exists in a zwitterionic
K4
COOH
COO–
+
H+
(5)
state when the –COOH group is deprotonated, and
the –NH2 group is protonated. The pH of the solution
controls the state of protonation. Different forms of
isoleucine are involved in the following equilibria ((5)–
(6)):
+
+
NH3
NH3
(HA)
(H2A+)
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

8
SEN, MAITI, AND PAL
K5
COO-
COO-
NH2
+
H+ (6)
+
NH3
(HA)
(A-)
The values of ionization constants, K4 and K5, are
4.79 × 10−3 and 1.74 × 10−10 M, respectively, at
298 K [30]. The present reaction was investigated in
the pH range 3.72–4.80, where the ratio of concen-
trations of the zwitterionic and the cationic species
of isoleucine, i.e., [HA]/[H2A+], varies from 25:1 to
302:1. Again most of the reactions were carried out
at pH 4.04, and at this pH [HA]/[H2A+] ࣈ 53:1. The
value of K5 indicates that the anionic form (A−) would
be present in a negligible concentration within this pH
range. Thus it is evident that the zwitterionic species,
HA, is the predominant reactive form of isoleucine that
reacts with AuCl4− and AuCl3(OH)− under the reac-
tion conditions. The linear plot of kobs−1 versus [Ileu]−1
(Fig. 4) suggested that the reaction involved the forma-
tion of an intermediate complex via a preequilibrium
between gold(III) and zwitterionic species, HA. The
formation of such a type of the intermediate gold(III)
complex has already been observed in a number of
studies [18,21,27]. The intermediate complex subse-
quently decomposed in a slow rate-determining step to
give the reaction products.
Figure 5 Dependence of−t1he pseudo–first-order rate con-
stant on [H+]. Plot of kobs versus [H+] at 298 K. [AuIII
]
= 2.47 × 10−3 M, [Ileu] = 3.05 × 10−2 M, [Cl−] = 4.0 ×
10−2 M.
K6
HA + AuCl−4 ꢀ (AuCl3 · · · A)− + H+ + Cl− (7)
fast
K7
HA + AuC3 (OH)− ꢀ (AuCl3 · · · A)− + H2O (8)
fast
k
(AuCl3 · · · A)− → Products
(9)
slow
Figure 6 Influence of the chloride ion concentration on the
pseudo–first-order rate constant. Plot of kobs−1 versus [Cl−]
at 298 K. [AuIII] = 2.47 × 10−3 M, [Ileu] = 3.05 × 10−2 M,
[H+] = 9.12 × 10−5 M.
In Eqs. (7) and (8), K6 and K7 are the equilibrium
constants corresponding to the reactions of AuCl4−
and AuCl3(OH)−, respectively, with HA. Both the
gold(III) species produced the same intermediate com-
plex, (AuCl3···A)−. Based on the reaction steps ((7)–
(9)), the rate of consumption of gold(III) can be ex-
pressed as
Table III Influence of Solvent Composition on the
Pseudo–First-Order Rate Constant at 298 K. [AuIII] =
2.47 × 10−3 M, [Ileu] = 3.05 × 10−2 M, [H+] = 9.12 ×
10−5 M, and [Cl−] = 0.04 M
ꢀ
ꢁ
d AuIII
ꢀ
ꢁ
Dioxane (%, v/v)
ε
104 kobs (s−1
)
T
v = −
= k (AuCl3 · · · A)−
(10)
dt
0
5
10
20
30
35
78.6
74.2
69.7
60.8
51.9
47.5
8.68
7.44
6.26
3.69
2.53
2.27
where
ꢀ
ꢁ
(AuCl3 · · · A)−
ꢀ
ꢁ
ꢂꢀ
ꢁ
ꢀ
ꢁꢃ
= AuIII
−
AuCl−4 + AuCl3 (OH)−
(11)
T
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

OXIDATIVE DEGRADATION OF L-ISOLEUCINE BY Au3+ COMPLEXES IN WEAKLY ACID MEDIUM
9
Table IV Comparison of the Values of k, K6, and K7 at 298 K from Experiments of [Ileu], [H+], and [Cl−] Variation
Different Effects
103 k (s−1
)
104 K6 (M)
K 7 (M−1
)
[Ileu] variation
[H+] variation
[Cl−] variation
3.8 0.6
3.6 0.5
3.5 0.4
0.56 0.05
0.60 0.04
0.87 0.09
24
25
37
2
2
2
From Eqs. (7) and (8), the expression for the equi-
librium constants becomes
value of rate constant k for the slow rate-determining
step was determined from the intercept of the straight
line on the ordinate and presented in Table II. The k
values were found to increase with an increase in tem-
perature in the range 288–308 K. The values of K6 and
K7 at 298 K (Table II) were evaluated from the slope
of such plot at that temperature. However, we could
not calculate the values of K6 and K7 at other temper-
atures as the values of K2 and K3 at those tempera-
tures were not available in the literature. Equation (18)
predicts linear plots of 1/kobs versus [H+] and 1/kobs
versus [Cl−], and these have been corroborated by
the experimental plots (Figs. 5 and 6). The values
of k and K6 at 298 K were also calculated from
the slope and intercept of these plots and compared
(Table IV). The values from different effects are found
to be in good agreement with each other. It is evi-
dent that the rate of the reaction decreases with an
increase in the concentration of H+. The retarding ef-
fect of H+ on the reaction rate may be accounted for
by the decreasing concentration of the more reactive
species AuCl3(OH)− [31] as an increase of [H+] dis-
places the equilibrium in Eq. (4) toward left. Moreover,
an increase of [H+] also diminishes the concentration
of the intermediate complex, (AuCl3···A)−, resulting
in the inhibition of the rate. Similarly, an increase in
[Cl−] will decrease the concentrations of AuCl3(OH)−
(Eq. (3)) and the intermediate complex (Eq. (8)) and
thus inhibits the reaction rate.
ꢀ
ꢁ ꢀ ꢁ ꢀ
ꢁ
(AuCl3 · · · A)− H+ Cl−
ꢀ
ꢁ
K6 =
(12)
(13)
AuCl−4 [HA]
ꢀ
ꢁ
(AuCl3 · · · A)−
ꢀ
ꢁ
K7 =
AuCl3 (OH)− [HA]
Considering equilibria ((3), (4)) and using Eqs. (11)
and (12) one can get
ꢀ
ꢁ
ꢀ
ꢁ
K6 AuIII HA
K2K3 + [H+] Cl + K6 [HA]
[
]
(AuCl3 · · · A)−
=
T
ꢀ
−ꢁ
(14)
Also it can be shown that
K6 = K2K3K7
(15)
Thus, the pseudo–first-order rate constant comes out
to be
ꢀ
ꢁ
d AuIII
1
AuIII
T
ꢀ
ꢁ
−
= kobs
dt
T
kK6 [HA]
ꢀ
−ꢁ
=
(16)
K2K3 + [H+] Cl + K6 [HA]
The enthalpy of activation (ꢀH#) for the slow rate-
determining step has been calculated from the Eyring
plot of log(k/T) versus 1/T (Fig. 7) using the different
k values obtained from the effect of variation of [Ileu]
in the temperature range 288–308 K, and the value
The above equation may also be rearranged as
ꢀ
ꢁ ꢀ
ꢁ
K2K3 + H+ Cl−
k K6
1
kobs
1
1
=
+
(17)
(18)
k
[IIeu]
was found to be (35
2) kJ mol−1. The entropy of
activation (ꢀS#) was then evaluated and found to be
ꢀ
ꢁ ꢀ
ꢁ
ꢄ
ꢅ
H+ Cl−
(−174 12) J K−1 mol−1
.
1
kobs
1
K2K3
=
+
+
Mechanistically, it can be viewed that a nucleophilic
attack by the carboxylate group of the zwitterion
into the coordination sphere of square planar (dsp2)
gold(III) complex, AuCl4− or AuCl3(OH)−, thereby
replacing either Cl− or HO− produces the same in-
termediate complex [18,21,27]. Considering only one
species of gold(III), viz., AuCl3(OH)−, a detailed
k
k K6 [IIeu]
k K6 [IIeu]
Here, [Ileu] stands for [HA], because the zwit-
terionic species is the predominant reactive form of
isoleucine as already −di1scussed. The linear double re-
ciprocal plots of kobs against [Ileu]−1 (Fig. 4) are
therefore justified by Eq. (17). From such plots, the
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

10
SEN, MAITI, AND PAL
Figure 7 Variation of the rate constant for the slow step with temperature. Plot of log(k /T) versus T−1
.
Scheme 2 Mechanistic steps of the reaction. [Color figure can be viewed at wileyonlinelibrary.com]
mechanism is shown in Scheme 2. The intermedi-
ate complex undergoes decomposition in a slow rate-
determining step to produce an iminic cation [4,23] and
the latter, through fast hydrolysis, yields the reaction
product, 2-methylbutanal, which was confirmed by 1H
NMR spectroscopy (discussed in the Experimental sec-
tion). Like Pt(IV) [32], in this case Au(III) behaves as
a two-electron transfer oxidant and is reduced to Au(I).
It is important to note that in this intermediate com-
plex (as shown in Scheme 2), the –NH2 group is at-
tached to the α-carbon which is electron deficient due
to the presence of adjacent electron-withdrawing car-
boxyl group. That is why deamination cannot take
place prior to decarboxylation, and in such a process
electron transfer between amino acid and gold(III) will
not be feasible. There are a number of reports [4,5,33]
on the oxidation of α-amino acids where a similar
type of reaction mechanism involving the initial nu-
cleophilic attack by ─COO─ of the amino acid with
the formation of an intermediate complex has been
proposed. The intermediate complex ultimately leads
to the formation of the corresponding aldehyde through
decarboxylation prior to deamination.
Although the oxidative degradations of leucine [21]
and isoleucine (present study) by gold(III) follow the
similar mechanistic path toward the formation of reac-
tion products, but three-dimensional structures of the
intermediate complexes are different from the perspec-
tive of steric crowding. In case of leucine that has an
isobutyl group, the side chain methyl group is away
from the reaction center. But in case of isoleucine,
there is a sec-butyl [─CH(Me)Et] group and thus the
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

OXIDATIVE DEGRADATION OF L-ISOLEUCINE BY Au3+ COMPLEXES IN WEAKLY ACID MEDIUM
11
Table V Comparison of the Activation Parameters and the Rate Constant at Two Different Temperatures for L-Leucine
and L-Isoleucine
Amino acid
103 k (s−1) at 298 K
103 k (s−1) at 303 K
ꢀH #(kJ mol−1
)
ꢀS#(JK−1 mol−1
−13
)
Reference
L-Leucine
L-Isoleucine
2.33
3.80
3.72
5.30
70
35
2
2
7
[21]
Presentwork
−174 12
side chain ─CH3 group is adjacent to the reaction cen-
ter thereby producing a greater steric crowding near
the reaction center. Owing to this greater steric strain,
the entropy of activation in case of isoleucine be-
comes much more negative compared to that of leucine
(Table V). Nevertheless, more energy will be needed
for the formation of the intermediate complex in the
ground state but at the same time there is also release of
steric strain where much crowded intermediate com-
plex decomposes fast to give the products. As a result,
difference in activation energy between ground state
and excited state will be less for isoleucine than that
for leucine. This type of steric assistance [34,35] of
kinetics of oxidation may explain why the enthalpy of
activation for isoleucine is lower than that for leucine,
and the oxidation of isoleucine is faster than leucine
as evident from Table V. An electronic factor may
also account for the lower ꢀH# and a higher reaction
rate of isoleucine. The sec-butyl group exerts stronger
+I effect than the isobutyl group of leucine, resulting
in a much polar transition state (TS) for isoleucine.
Thus, the TS will be more stabilized by solvation in
the medium and corresponding value of ꢀH# will be
lowered. Also +I effect of the sec-butyl group attached
to the α-carbon of isoleucine may assist in the forma-
tion of iminic cation by stabilizing it through the +I
effect, thereby increasing the rate. Also the TS on de-
composition leads to a number of oppositely charged
ions, and naturally it will be highly polar attracting ap-
preciable solvent molecules. This electrostriction [36]
phenomenon is another factor responsible for highly
negative ꢀS#. In several earlier reports [33,37,38] in-
volving the oxidation of amino acids such highly neg-
ative ꢀS# were also noted.
Thus all the kinetic results of the effects of variation
of substrate, chloride, pH, ionic strength, and dielec-
tric constant and also the product studies and obtained
activation parameters for the oxidation of isoleucine
justify the proposed mechanism and rate law. A com-
parison of the activation parameters for the oxidation
of both L-leucine and L-isoleucine indicates that the
oxidation reactions are enthalpy controlled.
CONCLUSION
The oxidative degradation of L-isoleucine by gold(III)
complexes has been proposed to take place through
the one-step two-electron transfer process between
AuCl4−/AuCl3(OH)− and the zwitterionic form of
isoleucine. The reaction involves the formation of an
intermediate complex followed by its decomposition
to form an iminic cation, which undergoes fast hy-
drolysis to produce 2-methylbutanal, confirmed by 1H
NMR. The activation parameters for the oxidation of
L-isoleucine predict that the reaction is enthalpy con-
trolled rather than entropy controlled one. Compared
to the oxidation of L-leucine, L-isoleucine underwent
oxidation at a faster rate with lower enthalpy of acti-
vation and higher negative entropy of activation. The
different steric and electronic factors due to a change in
the nature of the substituents at the α-carbon of these
two amino acids account for the difference in reactivity
and activation parameters.
Financial assistance from DST, Govt. of India, through the
PURSE program (Phase II) and from UGC, New Delhi, India,
through the CAS program (Phase II) to the Department of
Chemistry, Jadavpur University, is gratefully acknowledged.
Dr. Biswajit Pal would like to thank UGC, New Delhi, India,
for providing financial support to Minor Research Project.
Authors also thank Dr. Sanjoy Bhar, Professor, Department
of Chemistry, Jadavpur University, for helpful discussions.
The reaction rate was found to decrease with a
decrease in the dielectric constant of the medium
(Table III). The equilibria (2) and (4) involve the
reaction between the oppositely charged ions and
hence lowering of the dielectric constant will decrease
the concentrations of the oxidant ions AuCl4− and
AuCl3(OH)−, thereby diminishing the reaction rate.
Also a lowering of the polarity of the medium will
disfavor the highly polar TS, and this fact too might
be another important factor for the inhibition of the
reaction rate.
BIBLIOGRAPHY
1. Wilson, W. E.; Koeppa, R. E. J Biol Chem 1961, 236,
365–369.
International Journal of Chemical Kinetics DOI 10.1002/kin.21081

12
SEN, MAITI, AND PAL
2. Doi, M.; Yamaoka, I.; Nakayama, M.; Mochizuki,
S.; Sugahara, K.; Yoshizawa, F. J Nutr 2005, 135, 2103–
2108.
3. Laloo, D.; Mahanti, M. K. J Phys Org Chem 1990, 3,
799–802.
4. Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Int J Chem
Kinet 2009, 41, 473–482.
5. Sherigara, B. S. ; Bhat, K. I.; Pinto, I.; Gowda, N. M. M.
Int J Chem Kinet 1995, 27, 675–690.
6. Kulkarni, S. D.; Nandibewoor, S. T. Transition Met
Chem 2008, 33, 23–28.
7. Kini, A. K.; Farokhi, S. A.; Nandibewoor, S. T. Transi-
tion Met Chem 2002, 27, 532–540.
8. Bilehal, D.; Kulkarni, R.; Nandibewoor, S. T. J Mol
Catal: A Chem 2005, 232, 21–28.
9. Garg, D.; Kothari, S. Indian J Chem, Sect B: Org Chem
Incl Med Chem 2005, 34, 1909–1914.
20. Isab, A. A.; Sadler, P. J. Biochim Biophys Acta 1979,
492, 322–330.
21. Sen, P. K.; Gani, N.; Pal, B. Ind Eng Chem Res 2013,
52, 2803–2813.
22. Maritz, B. S.; Van Eldik, R. Inorg Chim Acta 1976, 17,
21–28.
23. Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Transition Met
Chem 2008, 33, 229–236.
24. Balzani, V.; Carassiti, V. Photochemistry of Coordina-
tion Compounds; Academic Press: London, 1970, p 273.
25. Feigl, F. Spot Tests in Organic Analysis; Elsevier: New
York, 1975, p. 195.
26. Clarke, H. T. A Handbook of Organic Analysis;
Edward Arnold: London, 1960, p. 127.
27. Sen Gupta, K. K.; Pal, B; Sen, P. K. Int J Chem Kinet
1999, 31, 873–882.
28. Pal, B.; Sen Gupta, K. K. Bull Chem Soc Jpn 2000, 73,
553–560.
29. Bekker, P.; van Z.; Robb, W. Inorg Nucl Chem Lett 1972,
8 ed.; CRC Press: Boca Raton, FL, 849–854.
10. Hemming, S.; Yathirajan Raju, C. R.; Mohana,
K. N.; Shashikanth, S.; Nagaraja, P. Turk J Chem 2003,
27, 571–580.
11. Kostova, I. Anticancer Agents Med Chem 2006, 6, 19–
32.
12. Nobili, S.; Mini, E.; Landini, I.; Gabbiani, C.; Casini,
A.; Messori, L. Med Chem Res 2010, 30, 550–580.
13. Gabbiani, C.; Casini, A.; Messori, L. Gold Bull 2007,
40, 73–88.
30. Weast, R. C. CRC Handbook of Chemistry &
Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986–
1987; p 699.
31. Sen, P. K.; Sanyal, A.; Sen Gupta, K. K. Bull Chem Soc
Jpn 1996, 69, 1543–1548.
32. Pal, B.; Sen Gupta, K. K.; Sen, P. K. Transition Met
Chem 2005, 30, 593–600.
33. Puttaswamy; Vaz, N. J Chem Sci 2001, 113, 325–332.
34. Fry, J. L.; Engler, E. M.; Schleyer, P. v. R. J Am Chem
Soc 1972, 94, 4628–4634.
14. Lima, J. C.; Rodriguez, L. Anticancer Agents Med Chem
2011, 11, 921–928.
15. Marzano, C.; Ronconi, L.; Chiara, F.; Giron,
M. C.; Faustinelli, I.; Cristofori, P.; Trevisan, A.; Freg-
ona, D. Int J Cancer 2011, 129, 487–496.
16. Ronconi, L.; Fregona, D. Dalton Trans 2009, 10670–
10680.
35. Eliel, E. L. Stereochemistry of Carbon Compounds;
ed.; Tata McGraw-Hill: New Delhi, India, 1993, p. 224–
225.
17. Zou, T.; Lum, C. T.; Lok, C.-N.; Zhanga, J.-J.; Che,
C.-M. Chem Soc Rev 2015, 44, 8786–8801.
18. Zou, J.; Guo, Z.; Parkinson, J. A.; Chen, Y.; Sadler, P. J.
Chem Commun 1999, 1359–1360.
36. Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper
Row: New York, 1987, p. 195.
37. Sen, P. K.; Gani, N.; Midya, J. K.; Pal, B. Int J Chem
Kinet 2012, 44, 482–493.
19. Witkiewicz, P. L.; Shaw, C. F. J Chem Soc, Chem Com-
mun 1981, 1111–1114.
38. Shukla, R., Sharma, P. K., Banerji, K. K. J Chem Sci
2004, 116, 101–106.
International Journal of Chemical Kinetics DOI 10.1002/kin.21081