Transition metal-catalyzed oxidation of l-asparagine by platinum(IV) in acid medium: a kinetic and mechanistic study

Article abstract of DOI:10.1007/s11243-015-9951-0

Abstract Kinetic investigations of Pt(IV) oxidation of l-asparagine catalyzed by transition metals with different valencies, namely Ag(I), Pd(II), Cr(III) and Zr(IV), were studied spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The reaction was first order in [Pt(IV)], while the orders with respect to the concentrations of Asn, sulfuric acid and transition metal were less than unity over the concentration range studied. The rate constants decreased with increasing ionic strength and dielectric constant, while increasing temperature enhanced the rate. The order of catalytic efficiency was: Ag(I) > Cr(III) > Pd(II) > Zr(IV). A mechanism involving the formation of a complex between the catalyst and substrate is proposed. Oxidation by Pt(IV) is suggested to take place by an inner-sphere mechanism in which Pt(IV) is reduced to Pt(II) on the catalyst-bound substrate in a one-step two-electron transfer process. The oxidation products of Asn were identified as α-formyl acetamide, ammonium ion and carbon dioxide. The rate law associated with the reaction mechanism was deduced. Activation parameters of the reactions were evaluated and discussed.

Full text of DOI:10.1007/s11243-015-9951-0

Transition Met Chem (2015) 40:587–594
DOI 10.1007/s11243-015-9951-0
Transition metal-catalyzed oxidation of L-asparagine
by platinum(IV) in acid medium: a kinetic and mechanistic study
1
1
1,2
Basim H. Asghar
•
Hatem M. Altass
•
Ahmed Fawzy
Received: 22 April 2015 / Accepted: 26 May 2015 / Published online: 2 June 2015
Ó Springer International Publishing Switzerland 2015
Abstract Kinetic investigations of Pt(IV) oxidation of
L-asparagine catalyzed by transition metals with different
valencies, namely Ag(I), Pd(II), Cr(III) and Zr(IV), were
studied spectrophotometrically in sulfuric acid medium at
constant ionic strength and temperature. The reaction was
first order in [Pt(IV)], while the orders with respect to the
concentrations of Asn, sulfuric acid and transition metal
were less than unity over the concentration range studied.
The rate constants decreased with increasing ionic strength
and dielectric constant, while increasing temperature
enhanced the rate. The order of catalytic efficiency was:
Ag(I) [ Cr(III) [ Pd(II) [ Zr(IV). A mechanism involv-
ing the formation of a complex between the catalyst and
substrate is proposed. Oxidation by Pt(IV) is suggested to
take place by an inner-sphere mechanism in which Pt(IV)
is reduced to Pt(II) on the catalyst-bound substrate in a one-
step two-electron transfer process. The oxidation products
of Asn were identified as a-formyl acetamide, ammonium
ion and carbon dioxide. The rate law associated with the
reaction mechanism was deduced. Activation parameters of
the reactions were evaluated and discussed.
Introduction
Many transition metal complexes are good oxidants in
acidic, basic and neutral media. The oxidation capacity
depends on the redox potential of the metal, which in turn,
depends on the pH of the medium. The chemistry of bio-
logically active Pt(IV) complexes has become increasingly
important over the last few decades due to their anticancer
properties [1, 2]. Such complexes are usually substitution
inert, requiring reduction to more labile Pt(II) species
before they can act as potential anticancer drugs. The
reduction of Pt(IV) complexes by various reductants gen-
erally proceeds via a free radical one-electron transfer
mechanism [3–7]. An alternative path, whereby Pt(IV)
undergoes a two-electron reduction process, has also been
reported [8]. The experimental conditions and choice of
reductant determined whether a two-electron reduction
process could occur.
2
-
Hexachloroplatinate(IV) (HCP), [PtCl6] , has been
used to oxidize a limited number of organic [9–16] and
inorganic [17–19] compounds in different media. Knowl-
edge of the reactivity of Pt(IV) compounds and their
reduction by potential bioreductants, such as amino acids,
is important for understanding the mechanism of antitumor
activity as well as for designing new compounds with
fewer side effects. It has been reported [20] that there is a
correlation between the rate of reduction and the anticancer
activity in a series of homologous Pt(IV) complexes.
However, no details about the reaction mechanism were
provided.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-015-9951-0) contains supplementary
material, which is available to authorized users.
&
Ahmed Fawzy
afsaad13@yahoo.com
Oxidation of amino acids is of interest both from a
chemical point of view and for its bearing on the mecha-
nism of amino acid metabolism. Amino acids can be oxi-
dized by a variety of reagents and often undergo oxidative
decarboxylation and deamination [14–16].
1
2
Chemistry Department, Faculty of Applied Sciences,
Umm Al-Qura University, Makkah, Saudi Arabia
Chemistry Department, Faculty of Science,
Assiut University, Assiut, Egypt
123

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Transition Met Chem (2015) 40:587–594
The rate of reduction by L-asparagine (Asn) depends on
Kinetic measurements
the nature of the oxidant and pH of the medium. Studies of
the reaction of Asn with the following systems have been
reported: sodium N-chloro-p-toluene sulfonamide cat-
alyzed by Cu(II) in alkaline media [21], alkaline diperi-
odatonickelate(IV) [22], peroxydisulfate in acid medium
Kinetic investigations of the reaction between HCP and an
excess of Asn were conducted under pseudo-first-order
conditions. The ionic strength, I, of the reaction mixture
-
3
was adjusted to 2.0 mol dm using sodium sulfate. The
reaction temperature (25.0 °C) was controlled within
± 0.1 °C using a thermostat, unless otherwise stated. The
reaction was initiated by a rapid addition of a known
amount of pre-equilibrated HCP to the reaction mixture
containing the required amounts of Asn, sulfuric acid,
transition metal salt, sodium sulfate and water. The solu-
tions were then mixed and transferred to a cell of path
length 1.0 cm. The course of the reaction was followed
spectrophotometrically by monitoring the decrease in the
absorbance of HCP at k = 262 nm, its absorption maxi-
mum, as a function of time. All spectra were recorded
using an automatic scanning double-beam spectropho-
tometer (Shimadzu UV-1800 PC). The cell compartment
was maintained at a constant temperature by circulating
temperature-controlled water using a thermostat.
[
23] and permanganate in both acid [24] and alkaline [25]
media. In most of these studies, the final oxidation products
of Asn were a-formyl acetamide, ammonium ion and
carbon dioxide. Fawzy [15] reported the kinetics of the
oxidation of Asn by hexachloroplatinate(IV) in sulfuric
acid medium in both the absence and presence of Cu(II).
Fawzy’s work suggested that Asn combines with Cu(II) to
form an intermediate complex. This complex then reacts in
a slow step with HCP to give the products and regenerates
the Cu(II) catalyst.
Transition metal ions, M(Y), where M is the metal and Y
is its valency, have been widely employed as homogenous
catalysts for oxidation of organic and inorganic substrates by
reaction pathways such as formation of complexes with the
reactants, oxidation of substrate or the formation of free
radicals [26, 27]. Kinetic investigations on the homoge-
neously catalyzed oxidation of amino acids in liquid phase
are limited [14–16, 28, 29]. Therefore, the mechanistic study
of such reactions is considered an important research field
due to the role played by metals in biological systems.
The present study deals with the redox reaction of HCP
with Asn in the presence of transition metals with different
valencies, namely Ag(I), Pd(II), Cr(III) and Zr(IV). These
metals were investigated as catalysts in order to explore the
kinetic and mechanistic aspects of the redox reactions in
strong acid media. Our goal was to determine the mecha-
nism of these redox reactions.
It has been reported that, in the absence of transition
metal ion catalysts, Pt(IV) oxidation of Asn in sulfuric
acid can be detected and measured [15]. The total rate
constant (k
of the pseudo-first-order rate constants of the catalyzed
(k ) and uncatalyzed (k ) reactions (k = k - k ).
Hence, during a calculation of k , the uncatalyzed rate
constant k must be taken into account and kinetic runs
) for these reactions was thus equal to the sum
T
C
U
C
T
U
C
U
under similar conditions in the absence of catalyst were
also carried out to allow for this. The reactions were
followed for more than three half-lives. The values of k
C
were obtained from the slopes of log(absorbance) versus
time plots. The average of at least three independent
kinetic runs was calculated. The runs were reproducible to
within ± 4 %.
Experimental
The effect of dissolved oxygen on the reaction rate was
also checked by preparing another batch of reaction mix-
ture then conducting the same set of reactions under a
nitrogen atmosphere. No significant differences in the
results were seen with and without atmospheric oxygen.
Materials and methods
All chemicals used were analytical reagent grade, and
water was double distilled. L-asparagine (Merck) was used
as received. Solutions of Asn were prepared fresh for each
experiment by dissolving a known amount of Asn in water.
Solutions of chloroplatinic acid (Johnson Matthey) were
freshly prepared before each experiment by dilution of a
stock solution and standardized spectrophotometrically
Results
Time-resolved spectra
[
30]. The stock solution was stored in a dark bottle to
protect it from light and re-standardized periodically.
Solutions of silver(I) nitrate, palladium(II) chloride, chro-
mium(III) sulfate and zirconium(IV) nitrate were prepared
by dissolving known amounts of each salt separately in a
suitable solvent, then diluting with water.
Time-resolved spectra are displayed in Fig. 1 for, as an
example, the Ag(I)-catalyzed oxidation of Asn by HCP in
sulfuric acid medium. The absorbance of HCP at its kmax
(262 nm) decreased gradually with time suggesting that
Pt(IV) was reduced to Pt(II) by Asn.
1
23

Transition Met Chem (2015) 40:587–594
589
1
0
0
0
0
0
.0
.8
.6
.4
.2
Reaction orders
HCP
Mixture
Reaction orders were determined from the slopes of plots
of log kC versus log (conc.) obtained by individually
varying the concentrations of the reductant, sulfuric acid
and transition metal ion in turn, while keeping the other
parameters constant.
AgI
The concentration of oxidant HCP was varied over the
-3
Asn
-5 -5
range of 4.0 9 10 –12.0 9 10 mol dm
,
while
keeping the concentration of other components constant.
The first-order plots were found to be linear, and k values
C
for different [HCP] did not vary (Table 1), indicating an
order of unity with respect to [HCP].
.0
2
00
225
250
275
300
325
350
Kinetic runs were carried out by varying the initial
concentration of Asn, while keeping the concentrations of
oxidant, transition metal ion and sodium sulfate constant.
The pH and temperature were also kept constant. Plots of
Wavelength,nm
Fig. 1 Time-resolved spectra for the Ag(I)-catalyzed oxidation of
L-asparagine by hexachloroplatinate(IV) in sulfuric acid medium.
-5
Asn] = 5.0 9 10 , [HCP] = 7.6 9 10 , [H ] = 1.0, [Ag(I)] = 5.0 9
-2
?
[
1
kC versus [Asn] in the presence of different transition metal
-5
0
-3
, and I = 2.0 mol dm at 25 °C. Scan time interval was 2 min
ions were linear with positive intercepts (figure not shown),
suggesting that the order with respect to [Asn] was less
than unity.
Reaction stoichiometry and product identification
The reaction rates were measured at constant [Asn],
[
HCP], [M(Y)], ionic strength and temperature but with
Reaction mixtures with different sets of reactant concen-
trations containing various amounts of HCP and Asn at a
fixed pH, ionic strength and temperature were allowed to
react for 24 h in an inert atmosphere. After completion of
each reaction, the unreacted HCP was assayed spec-
trophotometrically. The results indicated that one mole of
Asn consumed one mole of HCP, as represented in Eq. (1),
?
various [H ] (0.2–2.0 mol dm ). The rate of reaction
-3
?
increased as [H ] increased. Plots of k versus [H ] in the
?
C
presence of different transition metal ions were linear with
positive intercepts (figure not shown), confirming that the
?
order with respect to [H ] was less than unity.
To evaluate the reaction orders using different transition
metal ion catalysts, the reaction rate was measured with the
-3
following [M(Y)]: (1.0–9.0) 9 10 mol dm for Ag(I) and
-5
MðYÞ
IV
RCHðNH2ÞCOOH þ ½Pt CI6Š þ H2O À!
2À
-4
-3
Cr(III) and (1.0–9.0) 9 10 mol dm for Pd(II) and Zr(IV).
All other variables were kept constant. The reaction rates
increased as [M(Y)] increased (Table 1). The catalytic effi-
ciency increased in the order: Ag(I) [ Cr(III) [
Pd(II) [Zr(IV). The orders with respect to Ag(I), Cr(III),
Pd(II) and Zr(IV) were 0.89, 0.79, 0.38 and 0.19, respectively,
RCHO þ NH þ CO2 þ [Pt Cl4Š þ 2Cl þ Hþ
þ
II
2À
À
4
ð1Þ
where R = –CH (CO)NH and RCHO is the correspond-
2
2
ing aldehyde (a-formyl acetamide), which was identified
by a spot test [31]. Other oxidation products of the amino
acid were identified as the ammonium ion (Nessler’s
reagent) [32] and carbon dioxide (lime water test). The
corresponding aldehyde was also detected quantitatively as
its 2,4-dinitrophenylhydrazine derivative (2,4-dinitro-
phenylhydrazine test) [32]. The aldehyde does not undergo
further oxidation under the present experimental condi-
tions, since a test for carboxylic acid was negative. Similar
oxidation products obtained under different experimental
conditions have been reported previously [14–16, 23].
as determined from plots of log k versus log [M(Y)] (Fig. 2).
C
Effects of ionic strength and dielectric constant
By addition of sodium sulfate to the reaction medium at
constant [Asn], [HCP], [M(Y)], pH and temperature, it was
found that increasing the ionic strength of the medium
increased the rate of reaction. Debye–H u¨ ckel plots were
linear with negative slopes (Figure shown in supplementary
material).
II
2-
Formation of [Pt Cl ]
4
was confirmed [33] by the
In order to determine the effect of the dielectric con-
stant (D) of the medium on the rate, the oxidation of Asn
by HCP was studied at different solvent compositions
observation of a black precipitate [platinum(II) hydroxide]
upon addition of alkali to the reaction mixture, according to
Eq. (2).
(
vol%) of acetic acid and water. The dielectric constant of
2
À
À
À
½
PtCl4Š þ 2OH ¼ Pt(OH) þ 4Cl
ð2Þ
2
the medium at different compositions was calculated
123

5
90
Transition Met Chem (2015) 40:587–594
?
Table 1 Effect of [HCP], [Asn], [H ], [M(Y)] and ionic strength, I, on the first-order rate constant k
oxidation of L-asparagine by hexachloroplatinate(IV) in sulfuric acid medium at 25 °C
C
in the transition metal ion-catalyzed
5
0 [HCP]
2
10 [Asn]
-3
(mol dm
?
[H ]
(mol dm
x
10 [M(Y)]
-3 a
(mol dm )
-3
5
10 k
-1
)
1
(
I (mol dm
)
C
(s
-3
-3
)
mol dm
)
)
Ag(I)
Pd(II)
Cr(III)
Zr(IV)
4
6
7
1
1
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
.0
.0
.6
0.0
2.0
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
.6
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.2
0.5
1.0
1.5
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
5.0
5.0
5.0
5.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.5
3.0
3.5
4.0
538
534
545
557
549
156
340
545
721
909
271
402
545
662
749
128
319
545
730
912
545
516
480
451
425
59
57
58
58
59
16
37
58
79
101
27
42
58
65
81
35
48
58
71
82
58
54
51
47
45
263
364
260
258
256
78
35
39
37
36
37
12
25
37
50
64
18
28
37
43
51
29
33
37
41
44
37
35
33
30
28
177
260
339
425
123
195
260
311
366
77
169
260
351
439
260
246
231
218
205
Experimental error ± 4 %
a
x = 5 for Ag(I) and Cr(III), x = 4 for Pd(II) and Zr(IV); Y is the valence of M
using the dielectric constants of water (78.5) and acetic
acid (6.15) at 25 °C. The rate constant clearly increased
as the dielectric constant of the solvent decreased (i.e.,
Test for free radicals
To study the possible intervention of free radicals during the
oxidation reaction, a known amount of acrylonitrile was
added to the reaction mixture which was then kept for 6 h in
an inert atmosphere. On diluting the mixtures with methanol,
no white precipitate was formed, thus confirming the
absence of free radical intervention in these reactions.
increasing acetic acid content). Plots of log k versus 1/D
C
were linear with positive slopes (Figure shown in sup-
plementary material).
Effect of temperature
The effect of temperature on the reaction rate was studied
at 15, 20, 25, 30 and 35 °C while keeping other variables
constant. The first-order rate constants were found to
increase as the temperature increased. The activation
parameters for the second-order rate constant, k (k = k /
Discussion
2
-
In acid media, it is thought [34] that [PtCl6]
is the
principal form of platinum(IV) species. During the redox
2-
reaction, [PtCl6] is reduced as shown in Eq. (3),
2
2
C
[
Asn]), were calculated using Arrhenius, Fig. 3a, and
½PtCl6Š À þ 2e ¼ ½PtCl4Š þ 2ClÀ
2
À
2À
ð3Þ
Eyring, Fig. 3b, plots and are listed in Table 2.
1
23

Transition Met Chem (2015) 40:587–594
591
where K is the stepwise protonation constant of the amino
1
-
-
-
-
2.0
2.5
3.0
3.5
acid.
-
-
-
5
5
4
1
0
0
M Ag(I)
Since the reaction rates were observed to increase upon
?
increasing the acid concentration, the protonated Asn
1
M Cr(III)
10 M Pd(II)
form may be the reactive species in the rate-determining
step. It has also been reported [39] that amino acids form
adducts with some transition metal ions, owing to the
availability of an electron pair on the carboxylate oxygen
-
4
1
0 M Zr(IV)
[
40] under conditions where the amine group is protonated.
There are two suggested mechanisms for the oxidation of
amino acids by HCP. The first involves a simultaneous two-
electron transfer in a single step, while the second involves
two successive one-electron transfer steps. If the transition
states of the reductant and/or oxidant are unstable, a
simultaneous two-electron transfer mechanism occurs, as
-
5.2
-4.8
-4.4
-4.0
log [M(Y)]
-3.6
-3.2
-2.8
Fig. 2 Plots of log k
catalyzed oxidation of L-asparagine by hexachloroplatinate(IV) in
C
versus log [M(Y)] in the transition metal ion-
2
-
observed in the oxidation of uranium(IV) by [PtCl6] [41].
In the present study, addition of acrylonitrile to the reaction
mixture failed to give polymerized products. It is possible
that a free radical such as a Pt(III) species was too short-
lived to interact with acrylonitrile under our experimental
conditions. Nevertheless, the two-electron transfer mecha-
nism seems most likely in our study.
-2
-5
sulfuric acid medium. [Asn] = 5.0 9 10 , [HCP] = 7.6 9 10
-3
H ] = 1.0 and I = 2.0 mol dm at 25 °C
,
?
[
In this redox reaction, octahedral Pt(IV) is reduced to
-
square planar Pt(II) with the release of two Cl ions.
Therefore, this reaction can be classified as a reductive
elimination [35]. Since Pt(IV) complexes are generally
substitution inert [36], initial complex formation between
Pt(IV) and the reductant prior to the electron transfer can
be excluded in reductive elimination reactions [37].
On the other hand, Martell and Smith [38] reported that
in acid media, amino acids exist as zwitterions and tend to
protonate at high acid concentration according to the fol-
lowing equilibria:
Reaction mechanism
The reaction between HCP and Asn in the presence of small
amounts of transition metal catalysts had a stoichiometry of
1:1, with a first-order dependence on [HCP] and less than
?
unit order each with respect to [Asn], [H ] and [M(Y)]. The
rates of the reactions decreased as the ionic strength and
dielectric constant of the medium increased, suggesting
reactions between oppositely charged ions [42].
þ
À
þ
RCHðNH2ÞCOOH ꢀ RCHð NH3ÞCOO þ H
Amino acid
Zwitterion
ð4Þ
K1
ꢀ
þ
Protonated form
The observed less than unit order for both [Asn] and
RCHð NH ÞCOOH
3
[M(Y)] suggests the formation of a complex between the
(
a) -1
(b) -31
-2
-3
-4
-5
-6
-
32
Ag(I)
Ag(I)
Cr(III)
Pd(II)
Zr(IV)
-33
Cr(III)
Pd(II)
Zr(IV)
-
34
-35
3.25
3.30
3.35
3.40
3.45
3.50
3.25
3.30
3.35
3.40
3.45
3.50
3
-1
3
-1
1
0 (1/T) K
1
0 (1/T), K
?
-3
-5
[H ] = 1.0 and I = 2.0 mol dm . [M(Y)] = 5.0 9 10 mol dm
-3
Fig. 3 a Arrhenius plots and b Eyring plots for the transition metal
ion-catalyzed oxidation of L-asparagine by hexachloroplatinate(IV) in
-
4
-3
for Ag(I) and Cr(III); and 5.0 9 10 mol dm for Pd(II) and Zr(IV)
-2
-5
sulfuric acid medium. [Asn] = 5.0 9 10 , [HCP] = 7.6 9 10
,
123

5
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Transition Met Chem (2015) 40:587–594
Table 2 Activation parameters for the second-order rate constants k
hexachloroplatinate(IV) in sulfuric acid medium
2
in the transition metal ion-catalyzed oxidation of L-asparagine by
Transition metal ion
Parameter
=
-1 -1
K
=
-1
)
=
-1
)
= -1
a
E (kJ mol )
DS (J mol
)
DH (kJ mol
DG298 (kJ mol
Ag(I)
-152.98
-140.84
-146.74
-160.62
-5
32.82
38.27
40.18
37.27
78.41
80.24
83.91
85.14
-5
35.71
41.24
43.09
40.21
Cr(III)
Pd(II)
Zr(IV)
a
-2
?
-3
-3
[
Asn] = 5.0 9 10 , [HCP] = 7.6 9 10 , [H ] = 1.0 and I = 2.0 mol dm . [M(Y)] = 5.0 9 10 mol dm for Ag(I) and Cr(III); and
-
4
.0 9 10 mol dm for Pd(II) and Zr(IV). Experimental error ± 4 %
-3
5
catalyst and substrate prior to reaction with the oxidant.
Complex formation is also indicated kinetically by the
nonzero intercepts of the plots of [M(Y)]/kC versus
spectrophotometrically detect such intermediate complexes
might be explained by either the low concentrations of the
reactants or fast subsequent decomposition of the inter-
mediates relative to their rates of formation. However,
1
/[Asn], as shown in Fig. 4a–d. The failure to
+
3
-1
+
3
-1
1
/[H ], dm mol
1/[H ], dm mol
0
1
2
3
4
5
6
(b)
0
1
2
3
4
5
6
(
a)
4
3
2
1
0
0
0
0
0
40
30
20
10
0
80
60
40
20
0
80
60
40
20
0
0
0
20
1
40
60
80
100
5
120
0
20
1
40
60
80
100
120
3
-1
3
-1
1
/[Asn], dm mol
1/[Asn], dm mol
+
3
-1
+
3
-1
1
/[H ], dm mol
1/[H ], dm mol
(
c)
2
3
4
6
(d) 500
0
2
3
4
5
6
400
300
200
100
0
400
500
400
300
200
100
0
4
3
2
1
00
00
00
00
0
300
200
100
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
3
-1
3
-1
1
/[Asn], dm mol
1/[Asn], dm mol
-
3
Fig. 4 a–d Verification of the rate law in the form of Eq. (14) in the
transition metal ion-catalyzed oxidation of L-asparagine by hex-
achloroplatinate(IV) in sulfuric acid medium. [HCP] = 7.6 9 10
and I = 2.0 mol dm at 25 °C for a Ag(I), b Cr(III), c Pd(II) and
d Zr(IV)
-
5
1
23

Transition Met Chem (2015) 40:587–594
593
Scheme 1 Mechanism of
transition metal ion-catalyzed
H
H
K1
R C COOH + H+
(5)
R C COOH
oxidation of L-asparagine by
hexachloroplatinate(IV) in
sulfuric acid medium.
NH2
NH
3
+
-
2
R = H
2
NC(O)CH
,
M(Y) = Ag(I), Pd(II), Cr(III) or
H
H O
Zr(IV)
K2
R C COOH + M(Y)
R C C O M(Y)
(C)
(6)
NH
NH
H
3
3
+
+
H O
k1
+
+
-
C + [PtCl6]2-
R C C O + H + [PtCl ] + 2 Cl + M(Y)
2-
(7)
(8)
(9)
4
slow
NH
3
+
H O
+
fast
+
R C C O
NH
R C H + CO + H
2
NH
3
2
+
+
fast
+
R C H + H O
R C H + NH4
2
NH
O
2
+
þ
complexes between amino acids and Ag(I) [16, 31, 35],
Pd(II) [43, 44], Cr(III) [45–47] and Zr(IV) [48] have been
reported in the literature.
k K K ½AsnŠ½H Š½MðYފ½HCPŠ
1
1
2
Rate ¼
ð12Þ
ð13Þ
þ
þ
ð1 þ K ½H ŠÞð1 þ K K ½AsnŠ½H ŠÞ
Then, under pseudo-first-order conditions,
1
1
2
A reaction mechanism consistent with the experimental
results is given in Scheme 1. It is suggested that the pro-
Àd½HCPŠ
?
tonated asparagine (Asn ) combines with M(Y) to form an
Y?1
¼ kC½HCPŠ
dt
intermediate complex, [Asn–M]
, which then reacts in a
On comparing Eqs. (12) and (13) and rearrangement,
Eq. (14) is obtained,
slow step with one mole of HCP to give rise to the oxi-
dation products with regeneration of the catalyst, as illus-
trated in the Scheme.
ꢀ
According to Eq. (14), plots of [M(Y)]/k against
ꢁ
þ
þ
½
MðYފ
1 þ K1½H Š
1
1
¼
þ
ð14Þ
The proposed mechanism leads to the following rate
law,
kC
k1K1K2½H Š ½AsnŠ k1
C
?
1/[Asn] at constant [H ] and [M(Y)]/k against 1/[H ] at
?
Àd½HCPŠ
C
¼
k1½CŠ½HCPŠ
ð10Þ
dt
constant [Asn] should be linear with positive intercepts on
the [M(Y)]/k axes. The experimental results satisfied this
C
The relationship between the rate of complex formation
and the substrate, hydrogen ion, catalyst and oxidant con-
centrations has been deduced (Appendix shown in sup-
plementary material) to give the following rate law
expression,
requirement, as shown in Fig. 4a–d.
Unfortunately, the value of the rate constant for the slow
reaction step (k ) and its associated activation parameters
1
could not be determined because the protonation constant
þ
(K ) for Asn and/or the formation constants K are
1 2
k1K1K2½AsnŠ½H Š½MðYފ½HCPŠ
Rate =
þ
þ
þ
unknown for M(Y)–Asn complexes under the present
experimental conditions. Attempts to determine these
constants from the experimental data were unsuccessful.
The activation parameters listed in Table 2 lead to the
ð1 þ K1½H Š þ K1K2½H Š½MðYފÞð1 þ K1K2½AsnŠ½H ŠÞ
ð11Þ
In view of the low concentrations of [M(Y)] used, the
?
=
term K1 K [H ][M(Y)] in the denominator can be
2
following conclusion. The large negative values of DS
indicate the compactness of the Asn–M(Y) complexes
neglected. Therefore, Eq. (11) becomes,
123

5
94
Transition Met Chem (2015) 40:587–594
formed. These activated complexes are more ordered than
the reactants due to the loss of some degrees of freedom.
18. Sen Gupta KK, Das S, Sen Gupta S (1987) Transit Met Chem
1
2:417
9. Sen Gupta KK, Das S, Sen Gupta S (1988) Transit Met Chem
3:155
1
=
=
The values of DH and DS are both favorable for an
1
=
electron transfer process. The positive value of DH
20. Choi S, Filotto C, Bisanzo M, Delaney S, Lagasee D, Whitworth
JL, Jusko A, Li C, Wood NA, Willingham J, Schwenker A,
Spaulding K (1988) Inorg Chem 37:2500
indicates an endothermic transition state. This mechanism
is also consistent with the proposition made by Stewart
et al. [49, 50]. They reported that the entropy of activation
tends to be more positive for outer-sphere mechanisms,
whereas it is more negative for inner-sphere reactions.
2
2
1. Senagar SKS, Yadav BS (1988) J Indian Chem Soc 65:88
2. Sanjeevagowda TP, Mahantesh AA, Abdulazizkhan LH (2008) J
Solut Chem 37:1795
23. Khalid MAA (2007) J Sci Eng 33:199
2
2
4. Zahedi M, Bahrami H (2004) Kinet Catal 45:351
5. Fawzy A, Ashour SS, Musleh MA (2014) React Kinet Mech
Catal 111:443
Conclusion
26. Fawzy A, Ashour SS, Musleh MA, Hassan RM, Asghar BH
2015) J Saudi Chem Soc (in press)
7. Shukla A, Gupta S, Upadhyay SK (1991) Int J Chem Kinet
3:279
8. Pecci L, Montefoschi G, Musci G, Cavallini D (1997) Amino
Acids 13:355
(
2
2
The reaction between Asn and HCP in sulfuric acid pro-
ceeds with a measurable rate in the absence of transition
metal ion catalysts but is significantly faster in their pres-
ence. The catalytic efficiencies of the metals increase in the
order: Ag(I) [ Cr(III) [ Pd(II) [ Zr(IV). Electron transfer
is facilitated by adducts between metal and the carboxylate
oxygen on Asn. The reaction mechanism proposed here is
consistent with product, kinetic and mechanistic studies.
2
29. Shetti NP, Hosamani RR, Nandibewoor ST (2009) Open Catal J
:130
2
3
3
0. Georgieva M, Andonovski B (2003) Anal Bioanal Chem 375:836
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3
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5. Lemma K, Sargeson AM, Elding LI (2000) J Chem Soc Dalton
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