Influence of copper(II) catalyst on the oxidation of l-histidine by platinum(IV) in alkaline medium: A kinetic and mechanistic study

Article abstract of DOI:10.1007/s11243-014-9834-9

The kinetics of oxidation of l-histidine (His) by platinum(IV) in the absence and presence of copper(II) catalyst was studied using spectrophotometry in alkaline medium at a constant ionic strength of 0.1 mol dm^-3 and at 25°C. In both cases, the reactions exhibit a 1:1 stoichiometry ([His]:[Pt^IV]). The rate of the uncatalyzed reaction is dependent on the first power of each of the concentrations of oxidant, substrate and alkali. The catalyzed path shows a first-order dependence on both [Pt^IV] and [Cu^II], but the order with respect to both [His] and [OH^-] is less than unity. The rate constants increase with increasing ionic strength and dielectric constant of the medium. The catalyzed reaction has been shown to proceed via formation of a copper(II)^-histidine intermediate complex, which reacts with the oxidant by an inner-sphere mechanism leading to decomposition of the complex in the rate-determining step. Platinum(IV) is reduced to platinum(II) by the substrate in a one-step two-electron transfer process. This is followed by other fast steps, giving rise to the oxidation products which were identified as 2-imidazole acetaldehyde, ammonia and carbon dioxide. A tentative reaction mechanism is suggested, and the associated rate laws are deduced. The activation parameters with respect to the slow step of the mechanism are reported and discussed.

Full text of DOI:10.1007/s11243-014-9834-9

Transition Met Chem
DOI 10.1007/s11243-014-9834-9
Influence of copper(II) catalyst on the oxidation of L-histidine
by platinum(IV) in alkaline medium: a kinetic and mechanistic
study
Ahmed Fawzy
Received: 2 April 2014 / Accepted: 2 May 2014
Ó Springer International Publishing Switzerland 2014
Abstract The kinetics of oxidation of L-histidine (His) by
platinum(IV) in the absence and presence of copper(II)
catalyst was studied using spectrophotometry in alkaline
medium at a constant ionic strength of 0.1 mol dm-3 and
at 25 °C. In both cases, the reactions exhibit a 1:1 stoi-
chiometry ([His]:[PtIV]). The rate of the uncatalyzed
reaction is dependent on the first power of each of the
concentrations of oxidant, substrate and alkali. The cata-
lyzed path shows a first-order dependence on both [PtIV]
and [CuII], but the order with respect to both [His] and
[OH-] is less than unity. The rate constants increase with
increasing ionic strength and dielectric constant of the
medium. The catalyzed reaction has been shown to proceed
via formation of a copper(II)_histidine intermediate com-
plex, which reacts with the oxidant by an inner-sphere
mechanism leading to decomposition of the complex in the
rate-determining step. Platinum(IV) is reduced to plati-
num(II) by the substrate in a one-step two-electron transfer
process. This is followed by other fast steps, giving rise to
the oxidation products which were identified as 2-imidaz-
ole acetaldehyde, ammonia and carbon dioxide. A tentative
reaction mechanism is suggested, and the associated rate
laws are deduced. The activation parameters with respect
to the slow step of the mechanism are reported and
discussed.
Keywords Kinetics ꢀ Mechanism ꢀ Platinum(IV) ꢀ
L-histidine ꢀ Copper(II) catalysis
Introduction
There is growing interest in the chemistry of six-coordinate
platinum(IV) complexes because of their anticancer
activity [1–3]. It is generally believed that platinum(IV)
compounds are inert in ligand substitution reactions rela-
tive to their platinum(II) analogues [4]. They require
reduction to PtII species to act as potential anticancer drugs.
The anticancer activity of such complexes is likely to be
due to effective PtIV transport into the cell, followed by
reduction to the more reactive PtII compounds.
Kinetic investigations of the oxidation of organic [5–8]
and inorganic substrates [9–12] by platinum(IV) have been
reported. The reduction of platinum(IV) complexes can be
carried out with both outer-sphere [13] and inner-sphere
[14–17] single-electron reductants. Although the reduction
of some platinum(IV) complexes by various substrates
proceeds via a free radical mechanism, i.e., one-electron
transfer process [13, 18–20], the alternative path where
platinum(IV) undergoes a two-electron reduction has also
been proposed [16].
The study of the oxidation of amino acids is of interest
because of their biological significance and selectivity
toward oxidants to yield different products [21, 22]. L-
histidine (His) is an essential amino acid that finds exten-
sive applications as a reducing agent in chemical and
biochemical systems. The oxidation of histidine with sev-
eral different reagents has been reported [23–28]. Cop-
per(II) complexes of histidine [29] play an essential role in
biochemistry and pharmacology, because copper is an
essential trace element and histidine plays a pivotal role in
A. Fawzy (&)
Chemistry Department, Faculty of Science, Assiut University,
Assiut 71516, Egypt
e-mail: afsaad13@yahoo.com
A. Fawzy
Chemistry Department, Faculty of Applied Sciences, Umm Al-
Qura University, Makkah Al-Mukarramah 13401, Saudi Arabia
Kingdom
123

Transition Met Chem
copper transport before its entry into cellular transport
systems and incorporation into enzymes and proteins. A
small fraction of CuII bound to histidine maintains an
exchangeable pool of copper in equilibrium with albumin
in human blood. Copper(II) acts as a catalyst in the oxi-
dation of several amino acids [30]. The mechanism of
catalysis can be quite complicated due to formation of
different intermediate complexes. Although the mechanism
of catalysis depends on the nature of the substrate, oxidant
and the experimental conditions, it has been shown [31]
that metal ions act as catalysts by a few different paths such
as the formation of complexes with reactant, or direct
oxidation of the substrate, or through the formation of free
radicals.
molar
extinction
coefficient
(e = 1.26 0.026 9
104 dm3 mol-1 cm-1) was found to be in a good agree-
ment with that reported previously [32]. Solutions of PtIV
and the mixture containing His and alkali (and CuII catalyst
where required) were separately thermostatted for ca. 1 h.
The solutions were then mixed and transferred to the cell of
path length 1 cm, and 3–4 experimental readings were
taken for each run. Good straight lines were obtained for at
least three half-lives. The pseudo-first-order rate constants
of the uncatalyzed (kU) and catalyzed (kC) reactions were
calculated from plots of ln (absorbance) versus time. The
gradients of the plots were calculated by the least-squares
method. The rate constants were reproducible to within
4 %. The total rate constant (kT) is equal to the sum of the
rate constants kU and kC.
To the best of my knowledge, no studies have been
reported on the oxidation of histidine by PtIV in alkaline
medium in either the absence or presence of a catalyst. The
present report deals with the title reactions in order to
understand the active species of oxidant and catalyst, to
examine the activity of the catalyst and to propose an
appropriate mechanism.
Results
Spectral changes
Spectra recorded during the course of the reactions in
alkaline medium in the absence and presence of CuII cat-
alyst are shown in Fig. 1. The figure shows a gradual dis-
appearance of the PtIV band at 262 nm, with a slow rate in
the case of the uncatalyzed reaction, Fig. 1a, but a signif-
icantly faster one in the presence of CuII catalyst, Fig. 1b.
The simultaneous appearance of a new broad band in the
region above 330 nm was observed with an isosbestic point
located at 310 nm.
Experimental
Reagent grade chemicals and doubly distilled water were
used throughout the work. A stock solution of L-histidine
(Merck) was prepared afresh as required in bidistilled
water. Chloroplatinic acid solution (Johnson Matthey) was
used without further purification. The required solution of
oxidant was freshly prepared before each experiment by
proper dilution of the stock solution, which was standard-
ized spectrophotometrically [32]. The solution was stored
away from light and re-standardized periodically. All other
reagents were of analytical grade, and their solutions were
prepared by dissolving requisite amounts of the samples in
doubly distilled water.
Stoichiometry and product analysis
Reaction mixtures containing a known excess of [PtIV]
over [His] at fixed [OH-] were kept overnight, until the
reactions had gone to completion, and then filtered to
remove the solid black residue. The unreacted plati-
num(IV) was determined spectrophotometrically by mea-
suring the absorbance at 262 nm, showing that one mole of
histidine consumed one mole of platinum(IV). The reac-
tions may thus be represented stoichiometrically as in
Scheme 1.
Kinetic measurements
The kinetic studies were carried out under pseudo-first-
order conditions with the concentration of His in a large
excess (0.01 mol dm-3) relative to that of PtIV. The ionic
strength, I, of the reaction mixture was adjusted to
0.1 mol dm-3 using NaClO4. The reaction temperature
(25.0 °C) was controlled to within 0.1 °C. The rate of
disappearance of PtIV was followed spectrophotometrically
by monitoring the decrease in the absorption maximum at
262 nm. The spectrophotometer, Shimadzu UV-1800 PC
automatic scanning double-beam, had a cell compartment
kept at constant temperature by circulating water from a
thermostat. The applicability of Beer’s law was verified at
262 nm under the reaction conditions, and the determined
The stoichiometric equation shown in Scheme 1 is
consistent with the results of product analysis. The alde-
hyde product was estimated quantitatively as its 2,4-DNP
derivative [33]. The ammonia by-product was identified by
Nessler’s reagent [34], and CO2 was qualitatively detected
by bubbling nitrogen gas through the acidified reaction
mixture, followed by a tube containing lime water. Similar
oxidation products with different experimental conditions
have been reported earlier [23–28]. The formation of PtII
was confirmed by the black precipitate obtained after
standing of the reaction mixture for several hours. This
123

Transition Met Chem
Fig. 1 Spectral changes in the
oxidation of histidine by
platinum(IV) in alkaline
medium. [His] = 0.01,
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Platinum(IV)
Platinum(IV)
(a)
(b)
4 min.
[PtIV] = 8 9 10-5
,
[OH-] = 0.05 and
I = 0.1 mol dm-3 at 25 °C,
a in the absence of CuII, b in the
presence of 1 9 10-5
mol dm-3 CuII
II
Cu
1 min.
His
His
200
225
250
275
300
325
350
200
225
250
275
300
325
350
Wavelength, nm
Wavelength, nm
Scheme 1 The stoichiometric
equation
Table 1 Effects of [PtIV], [His], [OH-] and ionic strength, I, on the
pseudo-first-order rate constant value in the uncatalyzed oxidation of
histidine by platinum(IV) in alkaline medium at 25 °C
There is literature evidence [36] to indicate that plati-
num(II) oxide exists as Pt(OH)2.
a
105 [PtCl6
]
102 [His]
(mol dm-3
102 [OH-]
(mol dm-3
I (mol dm-3
)
105 kU
(s-1
2-
(mol dm-3
)
)
)
)
Reactions order and reagents effects
4.0
6.0
8.0
10.0
12.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
a
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
1.2
1.4
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
1.0
3.0
5.0
7.0
10.0
5.0
5.0
5.0
5.0
5.0
0.1
0.1
3.21
3.39
3.32
3.35
3.33
1.88
2.46
3.32
3.93
4.54
0.73
1.96
3.32
4.39
6.30
3.32
3.88
4.51
5.45
6.32
The orders with respect to the reactants for both the un-
catalyzed and catalyzed reactions were determined from
the slopes of the log kU and log kC versus log(concen-
tration) plots using the relationship (log k = log
k0 ? n log C) by varying the concentrations (C) of sub-
strate, alkali and catalyst, in turn, while keeping other
conditions constant.
0.1
0.1
0.1
0.1
0.1
0.1
The pseudo-first-order rate constants (kU and kC) were
determined at different [PtIV] in the region:
(4–12) 9 10-5 mol dm-3 but at constant [His], [OH-],
ionic strength, temperature and [CuII] for the catalyzed
reaction. The values of kU and kC were independent of
initial [PtIV], Tables 1 and 2. Hence, the order of the
reactions with respect to the oxidant is confirmed to be one.
In order to determine the effect of [His], the rate con-
stants were determined at different initial [His] keeping all
other reactants concentrations constant, including that of
the copper(II) catalyst for the catalyzed reaction. The val-
ues of the rate constants for both the uncatalyzed and
catalyzed reactions increased with increasing concentration
of histidine, as listed in Tables 1 and 2. In the uncatalyzed
path, the plot of kU versus [His] was found to be fairly
linear passing through the origin, whereas the plot of kC
versus [His] gave a straight line with a positive intercept on
the kC axis (Fig. 2). These results confirm that the order
with respect to histidine thus changes from first to less than
unity from the uncatalyzed path to the catalyzed one.
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.10
0.12
0.15
0.17
0.20
Experimental error 3 %
black precipitate is formed as a result of transformation of
[PtCl4]2- in alkaline medium into hydrated PtO, as indi-
cated by thermogravimetric and IR spectral analysis [35].
2ꢂ
½PtCl4ꢁ þ 2OHꢂ ! PtðOHÞ2þ 4 Clꢂ
123

Transition Met Chem
Table 2 Effects of [PtIV], [His], [OH-], [CuII] and ionic strength, I, on the pseudo-first-order rate constant value in the copper(II)-catalyzed
oxidation of histidine by platinum(IV) in alkaline medium at 25 °C
105 [PtCl6
]
102 [His]
(mol dm-3
102 [OH-]
(mol dm-3
105 [CuII]
(mol dm-3
)
I (mol dm-3
)
105 kC (s-1
)
2-
(mol dm-3
)
)
)
4.0
6.0
8.0
10.0
12.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
1.2
1.4
1.0
1.0
1.0
1.0
1.0
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
1.0
3.0
5.0
7.0
10.0
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
1.0
1.0
1.0
1.0
1.0
0.5
1.0
1.5
2.0
2.5
1.0
1.0
1.0
1.0
1.0
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.10
0.12
0.15
0.17
0.20
47.3
49.9
48.7
47.5
50.2
29.3
39.4
48.7
55.0
63.5
17.5
34.9
48.7
61.1
76.3
21.5
48.7
68.9
92.3
119.1
48.7
59.3
71.4
79.2
88.2
Experimental error 4 %
10
8
100
80
70
60
7
6
Uncatalyzed reaction
Catalyzed reaction
Uncatalyzed reaction
Catalyzed reaction
5
4
3
2
1
0
50
40
60
6
30
20
10
4
40
20
2
0
1.6
0
12
0
0
2
4
6
8
10
0.0
0.4
0.8
1.2
102 [OH-], mol dm-3
102 [His], mol dm-3
Fig. 3 Plots of kU and kC versus [OH-] in the uncatalyzed and
copper(II)-catalyzed oxidation of histidine by platinum(IV) in alkaline
medium. [His] = 0.01, [PtIV] = 8 9 10-5 and I = 0.1 mol dm-3 at
25 °C. [CuII] = 1 9 10-5 mol dm-3
Fig. 2 Plots of kU and kC versus [His] in the uncatalyzed and
copper(II)-catalyzed oxidation of histidine by platinum(IV) in alkaline
medium. [PtIV] = 8 9 10-5, [OH-] = 0.05 and I = 0.1 mol dm-3 at
25 °C. [CuII] = 1 9 10-5 mol dm-3
123

Transition Met Chem
Next, the reaction rates were measured at constant [His],
[PtIV], [CuII] (for the catalyzed reaction), ionic strength and
temperature but with various [OH-] (0.01–0.1 mol dm-3).
The rates of both the reactions were found to increase with
[OH-], Tables 1 and 2. Similar to the case of histidine, the
order with respect to [OH-] changes from first to less than
unity from the uncatalyzed path to the catalyzed one, as
also illustrated in Fig. 3.
The copper(II) catalyst concentration was varied from
5 9 10-6 to 25 9 10-6 mol dm-3, at constant concentra-
tions of platinum(IV), histidine and alkali and at constant
ionic strength and temperature. The rate of the reaction was
directly proportional to [CuII] as listed in Table 2 and
shown in Fig. 4, showing that the order in [CuII] is unity.
This was also confirmed from the linearity of the plot of log
kC versus log[CuII] with a slope &1.
140
120
100
80
60
The effect of ionic strength was investigated. At con-
stant concentrations of PtIV, His, CuII and OH-, and at
constant temperature, the addition of NaClO4 to the reac-
tion medium was found to accelerate the reactions. The
results, presented in Tables 1 and 2, show that the pseudo-
first-order rate constants kU and kC increase with increasing
ionic strength of the medium, and the Debye–Hu¨ckel plots
were found to be linear with positive slopes (Fig. 5).
The dielectric constant of the medium, D, was varied by
varying the ratio of t-butyl alcohol to water in the solvent.
The D values were calculated from equation (1):
40
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
105 [CuII], mol dm-3
Fig. 4 A plot of kC versus [CuII] in the copper(II)-catalyzed oxidation
of histidine by platinum(IV) in alkaline medium. [His] = 0.01,
[PtIV] = 8 9 10-5, [OH-] = 0.05 and I = 0.1 mol dm-3 at 25 °C
D ¼ DwVw þ DBVB
ð1Þ
-2.8
-3.2
where Dw and DB are dielectric constants of pure water
and t-butyl alcohol, respectively, and Vw and VB are the
volume fractions of components water and t-butyl alcohol,
respectively, in the total mixture. Decreasing the dielectric
constant of the reaction medium decreased the rate of
reactions. Plots of log kU and log kC versus 1/D gave
straight lines with negative slopes.
-3.6
Uncatalyzed reaction
Catalyzed reaction
-4.0
The effect of chloride ion on the reaction was investi-
gated by the addition of different concentrations of NaCl
solution ranging from 1 9 10-3 to 1 9 10-2 mol dm-3
,
-4.4
keeping other variables constants. The values of the rate
constants were found to be independent of NaCl
concentration.
-4.8
0.30
0.33
0.36
1/2, mol1/2 dm-3/2
0.39
0.42
0.45
I
The influence of temperature on the value of kU was
studied at 20, 25, 30 and 35 °C. It was found that raising
the temperature increases the kU value. The activation
parameters for the second-order rate constant of the slow
step of the mechanism, k1, were obtained from the
Fig. 5 Debye–Hu¨ckel plots in the uncatalyzed and copper(II)-cata-
lyzed oxidation of histidine by platinum(IV) in alkaline medium.
[His] = 0.01, [PtIV] = 8 9 10-5 and [OH-] = 0.05 mol dm-3 at
25 °C. [CuII] = 1 9 10-5 mol dm-3
Table 3 Activation parameters of the second-order rate constant (k1) in the uncatalyzed oxidation of histidine by platinum(IV) in alkaline
medium. [His] = 0.01, [PtIV] = 8 9 10-5, [OH-] = 0.05 and I = 0.1 mol dm-3 b
Temp. K
293
298
303
308
104 k1, dm3 mol-1 s-1
DS=, J mol-1 K-1
2.31
3.42
4.29
5.27
DH=, kJ mol-1
DG=, kJ mol-1
E=a , kJ mol-1
A, mol-1 s-1
-177.50
39.99
82.38
40.58
4.14 9 103
b
Experimental errors 5 %
123

Transition Met Chem
Arrhenius and Eyring equations by the least-squares
method and are presented in Table 3.
Consequently, under the present experimental conditions,
[PtCl5OH]2- is probably the sole oxidizing species.
It is well known in aqueous solutions [42], amino acid
exists as zwitterions, whereas in aqueous alkaline media,
they exist in anionic form according to the following
equilibria:
Finally, the possibility of involvement of free radicals
was examined as follows; the reaction mixture, to which a
known quantity of acrylonitrile monomer had been added,
was kept in an inert atmosphere for 2 h. On diluting the
reaction mixtures with methanol, no white precipitate was
formed, indicating the absence of free radicals in these
reactions.
þ
Aqꢀ
RCHðNH2ÞCOOH ꢀ RCHðN H3ÞCOOꢂ
ð6Þ
OHꢂ
ꢀ RCH(NH2ÞCOOꢂ
The observed enhancement of the reaction rates with
increasing pH of the medium suggests deprotonation of
histidine prior to the rate-determining step, such that the
deprotonated form is the principal reactive species.
Discussion
The mechanism of reduction of platinum(IV) by different
reductants has been the subject of much discussion [13]
centered mainly on the possibility of platinum(III) forma-
tion as an intermediate. The reductions of some plati-
num(IV) complexes by different reductants have been
shown to proceed via a free radical mechanism in two
successive one-electron transfer steps [37]. However,
alternative mechanistic paths whereby platinum(IV)
undergo a simultaneous two-electron reduction have also
been reported [10, 16]. In the present study, the absence of
free radical intervention in the reactions suggests the pos-
sibility of a two-electron transfer mechanism.
Mechanism of uncatalyzed oxidation
Since platinum(IV) complexes are generally substitution
inert [4], initial complex formation prior to electron
transfer does not normally take place in reductive-elimi-
nation reactions [43], [44]and electron transfer by a sub-
stitution controlled inner-sphere mechanism is unlikely.
Halide-mediated reductive-elimination reactions of plati-
num(IV) complexes involving various inorganic [45, 46]
and biological [47] reductants have been suggested to take
place via nucleophilic attack of the reductant on a halide
ligand on a good leaving center such as PtIV. Such reac-
tions are formally equivalent to a transfer of Cl1 from the
oxidizing PtIV center to the reducing nucleophile followed
by loss of the trans-ligand [45, 46].
It has been reported [38] that hexachloroplatinate(IV) is
hydrolyzed in aqueous solutions according to the following
equation:
2ꢂ
½PtCl6ꢁ þ H2O ! ½PtCl5H2Oꢁꢂ þ Clꢂ
The hydrolysis can be followed by rapid deprotonation of
ð2Þ
Under the present experimental conditions, initial com-
plex formation between [PtCl5OH]2- and histidine can be
excluded because of the substitution inertness of PtIV
complexes. This is confirmed by the absence of any spec-
troscopic evidence for the formation of such a complex in
the UV–Vis spectra, as shown in Fig. 1a. Furthermore, the
independence of [His] with respect to the well-known
Michaelis–Menten kinetics, 1/kU versus 1/[His], is con-
sidered as kinetic evidence against intermediate formation.
The reaction between platinum(IV) and histidine in
alkaline medium in the absence of catalyst exhibits a
stoichiometry of 1:1, with first-order dependences on
[PtIV], [His] and [OH-]. Based on the experimental results
and product analysis, a mechanism can be proposed as
discussed below. Thus, the amino acid substrate is first
deprotonated by the alkali,
the aqua-complex upon increasing pH to form
[PtCl5OH]22
;
Ka
2ꢂ
½PtCl5H2Oꢁꢂ ꢀ ½PtCl5OHꢁ þ Hþ
ð3Þ
where Ka is the acid dissociation constant (Ka = 3.5) [39]
Furthermore, there is literature evidence [40] to indicate
that in alkaline medium (pH [ 8), [PtCl6]2- is converted to
[PtCl5OH]2- in a fast step as follows:
2ꢂ
2ꢂ
½PtCl6ꢁ þ OHꢂ ! ½PtCl5OHꢁ þ Clꢂ
ð4Þ
In the present work, addition of Cl- to the reaction mixture
failed to inhibit the rate, indicating that step (4) is irre-
versible. A metathesis reaction between [PtCl5OH]2- and
OH- to give [PtCl4(OH)2]2-, followed by the oxidation of
histidine by [PtCl4(OH)2]2- can be ruled out, since the
dihydroxy platinum(IV) complex is unstable in aqueous
solution and readily disproportionates [41] according to the
following equation:
K1
His þ OHꢂ ꢀ Hisꢂ þ H2O
ð7Þ
followed by its attack on the active oxidant species,
[PtCl5OH]2-, in the rate-determining step to give rise to
Â
Ã
Â
Ã
2ꢂ
2 PtCl4ðOHÞ 2ꢂ! PtCl2ðOHÞ
þ ½PtCl6ꢁ
ð5Þ
2ꢂ
2
4
123

Transition Met Chem
Scheme 2 Mechanism of
uncatalyzed and copper(II)-
catalyzed oxidation of histidine
by platinum(IV) in alkaline
medium
the initial oxidation products in a one-step two-electron
transfer process
where the carboxylate group is deprotonated, it should be
able to attack [PtCl5OH]2-. Such a reaction may be visu-
alized as a nucleophilic substitution on the halogen, with
the platinum moiety as the leaving group. As already
pointed out [50, 51], there is no sharp distinction between a
two-electron redox change involving atom transfer and
such a nucleophilic substitution.
k1
Hisꢂ þ ½PtCl5OHꢁ2ꢂ ꢂ! initial products
ð8Þ
slow
This was indicated by the absence of free radicals during
the reaction. The observed kinetic behavior on varying the
ionic strength and dielectric constant confirms that the
reaction is between two similarly charged ions [48, 49],
consistent with [PtCl5OH]22 and His-. This is followed by
other fast steps to yield the final oxidation products as
illustrated in Scheme 2.
The redox reaction follows the second-order rate law
given by
ꢂd½PtCl5OH2ꢂꢁ þd ½PtIIꢁ
2ꢂ
¼
¼ k1½Hisꢁ½PtCl5OHꢁ
ð9Þ
dt
dt
Histidine has both carboxylate and amine groups which
may act as nucleophiles toward a chloride ligand in the
platinum(IV) complex, depending on the pH. At high pH,
Under pseudo-first-order conditions, the rate law can be
expressed by Eq. (10)
123

Transition Met Chem
10
8
Mechanism of copper(II)-catalyzed reaction
It has been reported [29, 30] that copper(II) acts as an
efficient catalyst in some redox reactions involving amino
acids, particularly in alkaline media. In aqueous alkaline
media [52], CuII is known to give the tetrahydroxycup-
rate(II) ion, [Cu(OH)4]2-. The reaction between [PtCl5-
OH]2- and histidine in the presence of microquantities of
copper(II) catalyst is similar to that of the uncatalyzed
reaction with respect to stoichiometry and effects of ionic
strength and dielectric constant, addition of CuII changed
the order in both [His] and [OH-] from unity to less than
unity. In addition, the reaction is first-order with respect to
copper(II) catalyst.
20 oC
25 oC
30 oC
35 oC
6
4
2
0
0
2
4
6
8
10
12
14
16
102 [His], mol dm-3
The observed order in [His] of less than unity presum-
ably results from complex formation between the cop-
per(II) catalyst and substrate prior to reaction with the
oxidant. Spectroscopic evidence to support complex for-
mation between CuII and histidine was obtained from the
UV–Vis spectra, which showed a new band with an isos-
bestic point as shown in Fig. 1b. The formation of the
complex was also indicated kinetically by a nonzero
intercept in the plot of [CuII]/kC versus 1/[His] (Fig. 8).
Similar complexes between histidine and copper(II) cata-
lyst have been reported in earlier studies [30].
Fig. 6 Plots of kU versus [His] at different temperatures in the
uncatalyzed oxidation of histidine by platinum(IV) in alkaline
medium. [PtIV] = 8 9 10-5, [OH-] = 0.05 and I = 0.1 mol dm-3
ꢂd½PtCl5OH2ꢂꢁ
2ꢂ
¼ kU½PtCl5OHꢁ
ð10Þ
dt
Comparing Eqs. (9) and (10), the following relationship is
obtained
kU ¼ k1½Hisꢁ
ð11Þ
I suggest that the deprotonated histidine combines with
[Cu(OH)4]2- to form an intermediate complex, [His–
Cu(OH)4]3-, which then reacts in a slow step with one mole
of [PtCl5OH]2- to give the initial products. This is followed
by other fast steps to yield the final oxidation products. The
results are accommodated in the following sequence
According to Eq. (11), plots of kU against [His] at
constant [H?] should be linear passing through the origin,
as was observed experimentally at four different tempera-
tures (Fig. 6). The pH-dependent second-order rate con-
stant, k1, obtained from the slopes of these plots, along with
its activation parameters which were calculated by the
least-squares method from Arrhenius and Eyring plots
(Fig. 7a, b), is listed in Table 3. The observed large neg-
ative value of DS= suggests that the reactants form a
transition state which is more ordered than the reactants,
while the positive values of both DH= and DG= indicate
endothermic formation of the transition state and its non-
spontaneity, respectively.
Â
Ã
Â
Ã
K2
2ꢂ
Hisꢂ þ CuðOHÞ4
ꢀ HisꢂCuðOHÞ4 3ꢂðCÞ
ð12Þ
ð13Þ
k2
2ꢂ
C þ ½PtCl5OHꢁ ! products þ Cu2þ
slow
According to the proposed mechanism, the rate of dis-
appearance of [PtCl5OH]2- or formation of the interme-
diate complex (C) can be expressed by the following rate
law equation:
-36.8
Fig. 7 a Arrhenius and
-7.4
-7.6
-7.8
-8.0
-8.2
-8.4
-8.6
b Eyring plots for the rate
constant of the slow step (k1) in
the uncatalyzed oxidation of
histidine by platinum(IV) in
alkaline medium. [His] = 0.01,
(a)
(b)
-37.0
-37.2
-37.4
-37.6
-37.8
-38.0
[PtIV] = 8 9 10-5
OH-] = 0.05 and
I = 0.1 mol dm-3
,
3.25
3.30
3.35
3.40
3.45
3.25
3.30
3.35
3.40
3.45
103 (1/T), K-1
103 (1/T), K-1
123

Transition Met Chem
1/[OH-], dm3 mol-1
100
Appendix
0
50
150
200
80
According to the suggested mechanism for copper(II)-cat-
alyzed oxidation reaction,
ꢂd½PtIVꢁ þd½Cꢁ
Â
Ã
60
40
20
RateðCÞ
¼
¼
¼ k2½Cꢁ PtIV
ð14Þ
dt
dt
From reaction (7),
½Hisꢂꢁ
K1 ¼
ð18Þ
½Hisꢁ½OHꢂꢁ
Therefore,
½Hisꢂꢁ ¼ K1½Hisꢁ½OHꢂꢁ
From reaction (12) and Eq. (19),
ð19Þ
ð20Þ
0
0
5
10
15
20
1/[His], dm3 mol-1
½Cꢁ
½Cꢁ
K2 ¼
¼
½Hisꢂꢁ½CuIIꢁ K1½Hisꢁ½OHꢂꢁ½CuIIꢁ
Fig. 8 Verification of the rate law (15) in the form of Eq. (17) in the
copper(II)-catalyzed oxidation of histidine by platinum(IV) in alka-
line medium. [His] = 0.01 (when varying [OH-]), [OH-] = 0.05
(when varying [His]), [PtIV] = 8 9 10-5 and I = 0.1 mol dm-3 at
25 °C
Therefore,
Â
Ã
½Cꢁ ¼ K1K2½Hisꢁ½OHꢂꢁ CuII
ð21Þ
ð22Þ
ð23Þ
Substituting Eq. (21) into Eq. (14) yields
Â
ÃÂ
Ã
RateðCÞ ¼ k2K1K2½Hisꢁ½OHꢂꢁ CuII PtIV
ꢂd½PtIVꢁ þd½Cꢁ
Â
Ã
RateðCÞ
¼
¼
¼ k2½Cꢁ PtIV
ð14Þ
ð15Þ
dt
dt
The total concentration of histidine is given by
k2K1K2½Hisꢁ½OHꢂꢁ½CuIIꢁ½PtIVꢁ
½HisꢁT¼ ½HisꢁF þ ½Hisꢂꢁ þ ½Cꢁ
where [His]T and [His]F stand for total and free concen-
trations of histidine.
Rate ¼
ð1 þ K1½OHꢂꢁÞð1 þ K1K2½His][OHꢂꢁÞ
Under pseudo-first-order conditions,
Substituting Eqs. (19) and (21) into Eq. (23) gives
Â
½HisꢁT¼ ½HisꢁF 1 þ K1½OHꢂꢁ þ K1K2½OHꢂꢁ CuII
ꢂd½PtIVꢁ
dt
Â
Ã
¼ kC PtIV
ð16Þ
Ã
½HisꢁT¼ ½HisꢁF þ K1½HisꢁF½OHꢂꢁ þ K1K2½HisꢁF½OHꢂꢁ CuII
Comparing Eqs. (15) and (16) and rearranging, we obtain
ꢀ
ð24Þ
ð25Þ
ꢁ
À
Â
ÃÁ
½CuIIꢁ
1 þ K1½OHꢂꢁ
1
¼
þ K0
ð17Þ
kC
k2K1K2½OHꢂꢁ ½His]
Therefore,
where K0 = (1 ? K1[OH-])/k2.
½HisꢁT
½HisꢁF ¼
According to Eq. (17), plots of [CuII]/kC versus 1/[His],
at constant [OH-], and of [CuII]/kC versus 1/[OH-], at
constant [His], should be linear with positive intercepts on
[CuII]/kC axes. The experimental results satisfied this
requirement, as shown in Fig. 8.
ð26Þ
1 þ K1½OHꢂꢁ þ K1K2½OHꢂꢁ½CuIIꢁ
Similarly,
Â
Ã
Â
Ã
CuII ¼ CuII þ ½Cꢁ
ð27Þ
T
F
The rate constant for the slow step (k2) could not be
evaluated because of non-availability of the deprotonation
constants of histidine (K1) and/or the formation constants
of the copper(II)-histidine complex (K2) under the present
experimental conditions and at different temperatures.
Some attempts were made to evaluate these constants from
the experimental data, but the results were unsatisfactory.
The overall mechanisms indicated by this work is shown in
Scheme 2.
Substituting Eq. (21) into Eq. (27) and rearrangement gives
½CuIIꢁT
Â
Ã
CuII
¼
ð28Þ
1 þ K1K2½His][OHꢂꢁ
F
In view of the concentrations of [OH-] and [PtIV] used
½OHꢂꢁT¼ ½OHꢂꢁF
and,
ð29Þ
123

Transition Met Chem
Â
Ã
Â
Ã
PtIV ¼ PtIV
ð30Þ
14. Bakac A, Hand TD, Sykes AG (1975) Inorg Chem 14:2540
15. Beattie JK, Starink J (1975) Inorg Chem 14:996
16. Moodley KG, Nicol MJ (1977) J Chem Soc Dalton Trans 239
17. Al-Jibori S, Crocker C, Shaw BL (1981) J Chem Soc Dalton
Trans 319
T
F
Substituting Eqs. (26), (28), (29) and (30) into Eq. (21)
(and omitting ‘‘T’’ and ‘‘F’’ subscripts), we get
18. Beattie JK, Basolo F (1971) Inorg Chem 10:486
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55:425
23. Shukla R, Upadhyay SK (2008) Ind J Chem 47A:551
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35:73
25. Sony V, Sindal RS, Mehrotra RN (2005) Polyhedron 24:1167
26. Jose TP, Nandibewoor ST, Tuwar SM (2005) E J Chem 2:75
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RateðCÞ
k2K1K2½Hisꢁ½OHꢂꢁ½CuIIꢁ½PtIVꢁ
¼
ð1þK1½OHꢂꢁþK1K2½OHꢂꢁ½CuIIꢁÞð1þK1K2½His][OHꢂꢁÞ
ð31Þ
In view of low concentration of [CuII] and [OH-] used, the
term K1K2[OH-][CuII] in the denominator of Eq. (31) can
be neglected. Therefore, Eq. (31) becomes
k2K1K2½Hisꢁ½OHꢂꢁ½CuIIꢁ½PtIVꢁ
RateðCÞ
¼
ð32Þ
ð1 þ K1½OHꢂꢁÞð1 þ K1K2½His][OHꢂꢁÞ
Under pseudo-first-order conditions, the rate law can be
expressed as
ꢂd½PtIVꢁ
dt
Â
Ã
RateðCÞ
¼
¼ kC PtIV
ð33Þ
32. Georgieva M, Andonovski B (2003) Anal Bioanal Chem 375:836
33. Vogel AI (1973) In Text book of practical organic chemistry, 3rd
edn, ELBS Longman, London, p 332, 679
Comparing Eqs. (32) and (33), the following relation-
ship is obtained
34. Ghosh MC, Reed JW, Bose RN, Gould ES (1994) Inorg Chem
33:73
35. Pal B, Sen Gupta KK (2012) Trans Met Chem 37:671
k2K1K2½Hisꢁ½OHꢂꢁ½CuIIꢁ
kC
¼
ð34Þ
ð1 þ K1½OHꢂꢁÞð1 þ K1K2½His][OHꢂꢁÞ
and with rearrangement, it becomes
ˆ
36. Bailar JC Jr, Emele-Aus HJ, Nyholm R, Trotman-Dickenson AF
(1973) Comprehensive inorganic chemistry. Pergamon Press,
Oxford, p 1330
ꢀ
ꢁ
½CuIIꢁ
1 þ K1½OHꢂꢁ
1
1 þ K1½OHꢂꢁ
37. Peloso A, Basato M (1972) J Chem Soc Dalton Trans 2040
38. Anderson JR (1975) Structure of metallic catalysts. Academic
Press, New York
39. Shelimov B, Lambert JF, Che M, Didillon B (1999) J Am Chem
Soc 121:545
¼
þ
ð35Þ
kC
k2K1K2½OHꢂꢁ ½His]
k2
40. Sen Gupta KK, Sen PK (1977) J Inorg Nucl Chem 39:1651
41. Grinberg AA (1962) The chemistry of complex compounds.
Pergamon Press, Oxford, p 279
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