Oxidative degradation of l-histidine by manganese dioxide (MnO2) nano-colloid in HClO4 medium with/without using TX-100 catalyst: A kinetic approach

Article abstract of DOI:10.1039/c6ra14920k

The kinetics of the oxidative degradation of l-histidine (His) in perchloric acid medium by freshly prepared manganese dioxide (MnO₂) nano-colloid have been investigated with and without using TX-100 catalyst at a constant temperature of 35 °C. The progress of the oxidation reaction was checked by monitoring the decrease in absorbance of MnO₂ spectrophotometrically at 360 nm. The reaction was observed to be first order with respect to [MnO₂] and fractional with respect to [His] and [HClO₄] in both uncatalysed and TX-100-catalysed paths. The catalytic effect of TX-100 on the oxidation reaction has been explained by the Tuncay model. Protonated l-histidine is bound to TX-100 and forms a [TX-100-His] intermediate complex which further reacts with MnO₂ and decomposes into the oxidation products: 2-imidazole acetaldehyde, Mn(ii), NH₄⁺ and CO₂. On the basis of the kinetics results, a possible reaction mechanism is proposed.

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Oxidative degradation of L-histidine by manganese
dioxide (MnO2) nano-colloid in HClO4 medium
with/without using TX-100 catalyst: a kinetic
approach†
Cite this: RSC Adv., 2016, 6, 101162
ab
and Deogratius Jaganyib
*
Mohammad Altaf
The kinetics of the oxidative degradation of L-histidine (His) in perchloric acid medium by freshly prepared
manganese dioxide (MnO2) nano-colloid have been investigated with and without using TX-100 catalyst at
a constant temperature of 35 ꢀC. The progress of the oxidation reaction was checked by monitoring the
decrease in absorbance of MnO2 spectrophotometrically at 360 nm. The reaction was observed to be
first order with respect to [MnO2] and fractional with respect to [His] and [HClO4] in both uncatalysed
and TX-100-catalysed paths. The catalytic effect of TX-100 on the oxidation reaction has been explained
by the Tuncay model. Protonated L-histidine is bound to TX-100 and forms a [TX-100–His] intermediate
complex which further reacts with MnO2 and decomposes into the oxidation products: 2-imidazole
Received 8th June 2016
Accepted 12th October 2016
DOI: 10.1039/c6ra14920k
+
acetaldehyde, Mn(II), NH4 and CO2. On the basis of the kinetics results, a possible reaction mechanism
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is proposed.
groups bind to the nonpolar hydrophobic core. This inherent
micro-heterogeneity of the micellar solubilisation environment
1. Introduction
could play an important role in the catalysis of a reaction. The
effects of micelles on redox reactions have been investigated
because they change the reaction rate which involves the
assimilation of reactants in the micellar pseudo-phase.11–16
Manganese dioxide (MnO2) is well-known in both engi-
neered and natural aquatic systems. It is formed in engineered
systems by the oxidation of Mn(II) or the reduction of Mn(VII),
while in natural aquatic systems MnO2 is produced as a by-
product in the pretreatment of water to transform organic and
inorganic contaminants by using Mn(VII) as an oxidant.17,18
There are also various routes to forming MnO2 in natural water,
like weathering of minerals and biological catalysis of dissolved
manganese. The MnO2 surface reactions i.e. adsorption/
desorption and redox reactions might impact its fate and
transport in aquatic environments. Manganese dioxide has
a deciency in oxidizing power due to the insolubility of MnO2
under normal conditions. Fortunately, over the past few
decades, a perfectly transparent and water soluble solution of
MnO2 has been prepared by reducing neutral or slightly acidic
Mn(VII) with sodium thiosulfate.19,20 The water soluble MnO2
colloid has the advantage over water-insoluble MnO2 in terms of
compatibility with conventional UV-vis spectrophotometers.
The water soluble MnO2 colloid has been used to study the
oxidative degradation of a number of compounds.21–43
Approximately 80% of the human body is made up of amino
acids which are vital to every part of human function. The
kinetics of oxidative degradation of the amino acids is important
due to their biological signicance. L-Histidine (His) is one of the
essential amino acids that is mandatory for the production of
RBC/WBC and also acts as a neurotransmitter/neuromodulator
in the mammalian central nervous system, including in the
retina.1–3 Rheumatoid arthritis, anaemia, ulcers, and allergic
diseases have been cured by using L-histidine. Histidine is
directly involved in catalysis because of the high reactivity of its
imidazole group. In previous studies, researchers have found
that 2-imidazole acetaldehyde, ammonia and carbon dioxide are
the nal products of the oxidation of L-histidine by numerous
reagents in different media.4–10
Over the past few decades, chemical reactivity in self-organ-
ised assemblies (e.g. micelles, microemulsion droplets, and
vesicles) has become a subject of great attention to chemists
because of their catalytic activities in organic and inorganic
reactions, and to biochemists due to their resemblance to bio-
logical membrane/globular proteins. In micelles, the reactant's
polar groups interact with the outer charged shell while nonpolar
aCentral Laboratory, College of Science, King Saud University, PO Box 2455, Riyadh
11451, Saudi Arabia. E-mail: altafamu@gmail.com
bSchool of Chemical and Physical Sciences, University of KwaZulu-Natal, Private Bag
In the literature, no reports have been found on the oxidative
degradation of L-histidine by a MnO2 nano-colloid in HClO4
medium without or with using TX-100 catalyst. This study may
X01, Scottsville 3209, Pietermaritzburg, South Africa
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra14920k
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enable us to understand the complicated biological reactions in transmission electron microscope was used to determine the
living systems as well as the behaviour of L-histidine in enzy- morphology of the MnO2 colloid particles at a 200 kV accelera-
matic environments and simultaneously, will also assist us in tion voltage. A few drops of the resultant solution were placed
understanding the catalytic activity of TX-100 along with the onto the copper grid supported by an amorphous carbon lm
oxidative capacity of MnO2. On the basis of the above points, we and kept overnight at room temperature to evaporate the water.
studied the title reaction.
The TEM image shows that the MnO2 particles are spherical with
a 6.0 ꢂ 1 nm diameter (Fig. 1B), which was calculated by using
ImageJ soware and shown as a histogram (Fig. 1B inset). Fig. 1C
shows the elemental composition of the resultant solution. The
EDX pattern showed that the sample was composed of 60.9%
manganese and 31.2% oxygen, suggesting that the MnO2 nano-
colloid had been successfully synthesized.
2. Experimental
2.1. Materials
All chemicals i.e., L-histidine (Fluka, USA, 99%), potassium
permanganate (E. Merck, USA, 98.5%), sodium thiosulfate
(E. Merck, USA, 99%), perchloric acid (ACE, Glenvista, 70%) and
TX-100 (Sigma, USA, 99%) were analytical grade reagents and
used as received without further purication. All reagent solu-
tions were prepared in ultra-pure water. Permanganate solu-
tions were stored in a dark glass bottle.
2.3. Kinetics measurements
All kinetics runs were followed under pseudo-rst-order
conditions where [His] > [MnO2] at a temperature of 35 ꢂ 0.1 ꢀC,
unless specied. The kinetics measurements were obtained
using a UV-1800 Shimadzu UV-visible spectrophotometer. The
reaction was started by addition of MnO2 into a mixture of L-
histidine and HClO4 solutions for the uncatalysed reaction and
into a mixture of L-histidine, HClO4 and TX-100 solutions for the
TX-100 catalysed reaction. The progress of the reactions was
followed spectrophotometrically at 360 nm by monitoring the
decrease in absorbance of MnO2. The reaction mixture was
prepared in a N2-atmosphere to check the effect of dissolved
oxygen on the reaction rate and no signicant difference was
found between the results obtained under N2 and air. Prior to
the kinetics runs, Beer's law was veried and the extinction
coefficient was found to be 3360 nm ¼ 10 500 ꢂ 50 molꢁ1 dm3
cmꢁ1. It was established that other species present in the
reaction mixture have negligible interference at this
wavelength.
2.2. Preparation and characterization of manganese dioxide
nano-colloids
In a 250 ml beaker, 158.03 mg of KMnO4 was dissolved in 100
ml of ultra-pure water and stirred for 3 hours at room temper-
ature. 118.58 mg of sodium thiosulfate was dissolved in 7.5 ml
of ultra-pure water in a separate ask and was then added to the
permanganate solution. The colour of the solution changed
rapidly from purple to dark brown (Fig. 1A, the onset of the
formation of MnO2), which was perfectly transparent and stable
for several months. The reaction taking place under the above
conditions in neutral aqueous solution may be described by the
following stoichiometric equation:
8MnO4 + 3S2O3 + 2H+ / 8MnO2 + 6SO4 + H2O (1)
ꢁ
2ꢁ
2ꢁ
UV-vis spectroscopy, transmission electron microscopy
(TEM), and energy-dispersive X-ray spectrometry (EDX) are used
2.4. Polymerization tests
to characterize the obtained solution. The UV-vis spectra The possibility of formation of free radicals was examined by
(Fig. 1A) show a permanganate absorption peak at lmax ¼ 525 nm adding 10% (v/v) acrylonitrile to the partially oxidized reaction
while a broad distinctive peak of high intensity at lmax ¼ 360 nm mixtures without/with the TX-100 catalyst. Aer 24 h both
was observed for the obtained solution of MnO2 28–35 JEOL-2010 reaction mixtures were diluted with methanol. No precipitation
Fig. 1 Characterization images of the as prepared MnO2 nano-colloid: (A) UV-visible spectra of KMnO4 and MnO2, (B) a TEM image of MnO2,
particle size distribution graph, inset, and (C) an EDX spectrum.
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was observed in either reaction mixture, conrming that no free in Fig. 2. A plot of rate constant versus [His] is nonlinear passing
radical was formed in these reactions.
through the origin. The linear plot (Fig. 2 inset) between log kobs
vs. log[His] yields a slope equal to 0.6948 (r ¼ 0.9897), indicating
a fractional order with respect to L-histidine concentration.
3. Results and discussion
3.1. Stoichiometry and product analysis
3.3. Effects of MnO2 concentration on the reaction rate
Different sets of reaction mixtures containing a 10 fold molar
excess of MnO2 to L-histidine, without/with the TX-100 catalyst
with a constant amount of HClO4, were kept overnight in
a closed vessel under an inert atmosphere at 35 ꢀC. The
remaining (unreacted) MnO2 concentration was estimated
spectrophotometrically at 360 nm. The results indicate a 1 : 1
stoichiometry as shown:
To see the effect of MnO2 concentration on the reaction rate, the
reaction was carried out with MnO2 concentrations ranging from
2.0 ꢃ 10ꢁ5 to 12.0 ꢃ 10ꢁ5 mol dmꢁ3. The temperature (35 C)
ꢀ
and concentration of L-histidine (¼4.0 ꢃ 10ꢁ3 mol dmꢁ3) and
HClO4 (¼6.0 ꢃ 10ꢁ5 mol dmꢁ3) were kept constant. The obtained
pseudo-rst-order rate constants (kobs) are shown in Table 1. The
result shown in Fig. 3 shows that the rate constant decreases with
increasing MnO2 concentration. The same behavior of MnO2 as
observed in the present study has also been observed for the
oxidative degradation of a number of compounds,21–30 which is
contrary to the existing hypothesis of the concentration inde-
pendence of the rst-order rate constant (the pseudo-rst-order
rate constants should be independent of the initial concentration
of the reactant indefect). The decrease in rate constant with the
increase in concentration may be due to the occulation of the
MnO2 nano-colloid particles.
C6H9O2N3 + MnO2 + H3O+
/
C5H6ON2 + Mn2+ + NH4 + CO2 + 2OHꢁ (2)
+
where C6H9O2N3 and C5H6ON2 is L-histidine and 2-imidazole
acetaldehyde, respectively. The stoichiometric eqn (2) is
consistent with the results of the product analysis. The product,
2-imidazole acetaldehyde was estimated quantitatively by the
presence of its 2,4-DNP derivative.44 Mn2+ was qualitatively
identied by reaction with indole.45,46 Other products were
identied as discussed earlier.47,48 Similar reaction products
were observed during the oxidation of amino acids by the
permanganate ion49 or by other oxidants.50
3.4. Effects of HClO4 concentration on the reaction rate
In the present study, HClO4 was preferred because it possesses
only strong acid features and no oxidizing properties at room
temperature and because ClO4ꢁ is weakly nucleophilic (non-
complexing in nature). It is well known that colloidal MnO2 can
be precipitated by adding electrolytes21,34 hence it is important
to reduce as much as possible the amount of additives in order
to avoid any possible uctuations in the properties of the
micelles. Therefore, no buffer solution was used to maintain the
pH of the micellar solutions. On the other hand, the stability of
the colloidal particles is strongly dependent upon the pH of the
reaction medium.51,52 For this reason the concentration of
HClO4 was kept constant (6.0 ꢃ 10ꢁ5 mol dmꢁ3) throughout,
except for determination of the effect of HClO4 concentration.
The effect of [HClO4] was determined by carrying out a series of
kinetics runs at different [HClO4] with xed [MnO2] (¼8.0 ꢃ
10ꢁ5ꢀ mol dmꢁ3), [His] (¼4.0 ꢃ 10ꢁ3 mol dmꢁ3) and temperature
(35 C). The rate constant (kobs) values are given in Table 1. A
plot of kobs and perchloric acid concentration shows a non-
linear slope with a positive intercept, which indicates that the
oxidative degradation of L-histidine by MnO2 nano-colloid is
independent and dependent on HClO4 concentration (Fig. 4). A
double logarithm plot of the rate constant and [HClO4] was
linear (Fig. 4 inset) with a slope of 0.235 (r ¼ 0.997), indicating
the fractional-order dependence on [HClO4].
3.2. Effects of [histidine] on the reaction rate
The dependence of the rate constant on [His] was determined by
carrying out kinetics experiments at different concentrations of
L-histidine while keeping constant the [MnO2] ¼ 8.0 ꢃ 10ꢁ5 mol
dmꢀꢁ3, [HClO4] ¼ 6.0 ꢃ 10ꢁ5 mol dmꢁ3, and the temperature ¼
35 C. The rate constant increased with L-histidine concentra-
tion which is summarized in Table 1 and also shown graphically
Table 1 Effects of [histidine], [MnO2] and [HClO4] on the oxidative
degradation of L-histidine by a MnO2 nano-colloid at 35 ꢀ
C
103 [histidine]
105 [MnO2]
105 [HClO4]
104 kobs
(mol dmꢁ3
)
(mol dmꢁ3
)
(mol dmꢁ3
)
(sꢁ1
)
1.0
2.0
3.0
4.0
5.0
6.0
4.0
8.0
6.0
1.5
2.6
3.5
4.2
4.8
5.1
9.1
7.3
5.5
4.2
3.7
3.5
3.2
3.8
4.2
4.4
4.7
4.9
2.0
4.0
6.0
6.0
8.0
10.0
12.0
8.0
3.5. Effect of TX-100 concentration on the reaction rate
To see the catalytic role of TX-100 on the reaction rate, different
experiments were carried out with various TX-100 concentra-
tions at constant [His] (¼4.0 ꢃ 10ꢁ3 mol dmꢁ3), [MnO2] (¼8.0 ꢃ
10ꢁ5 mol dmꢁ3), [HClO4] (¼6.0 ꢃ 10ꢁ5 mol dmꢁ3) and
temperature (¼35 ꢀC). The reaction was found to be accelerated
by TX-100 (Table 2). The rate constants were increased with an
4.0
2.0
4.0
6.0
8.0
10.0
12.0
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Fig. 2 Effect of [His] on the rate constant for the oxidative degradation of L-histidine by MnO2 nano-colloid. Inset, a double logarithm plot of the
rate constant and [His]. Reaction conditions: [MnO2] ¼ 8.0 ꢃ 10ꢁ5 mol dmꢁ3, [HClO4] ¼ 6.0 ꢃ 10ꢁ5 mol dmꢁ3, temperature ¼ 35 ꢀC.
Fig.
3 Effect of MnO2 nano-colloid concentration on the rate
Fig. 4 Effect of [HClO4] on the rate constant for the oxidative
degradation of L-histidine by a MnO2 nano-colloid. Inset, a double
logarithm plot of the rate constant and [HClO4]. Reaction conditions:
constant for the oxidative degradation of L-histidine by MnO2 nano-
colloid. Reaction conditions: [His] ¼ 4.0 ꢃ 10ꢁ3 mol dmꢁ3, [HClO4] ¼
6.0 ꢃ 10ꢁ5 mol dmꢁ3, temperature ¼ 35 ꢀC.
[MnO2] ¼ 8.0 ꢃ 10ꢁ5 mol dmꢁ3, [His] ¼ 4.0 ꢃ 10ꢁ3 mol dmꢁ3
,
temperature ¼ 35 ꢀC.
increase in TX-100 concentration and reached a plateau at
higher concentration (Fig. 5).
a decrease in the free energy difference between the initial and
the transition state and/or at the TX-100 micellar interface, and
therefore the frequency of the molecular collisions of the reac-
tants increases. The presence of TX-100 molecules in the
interfacial region with a number of donor groups may be an
additional attractive interaction for the L-histidine molecules
The nature of the reactants and the reaction conditions alter
the value of the cmc of surfactants. In our study, the cmc did not
signicantly affect the results of the variation in values of the
kinetics. The catalytic effect of TX-100 may be due to changes in
the environment of the reactants. It could also be due to
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Table 2 Effects of TX-100 concentration on the oxidative degradation
of L-histidine by a MnO2 nano-colloid
the manganese dioxide nano-colloid. It was found that a plot of
log kJ vs. log[TX-100] was linear with a slope of x ¼ 0.1833 and
an intercept of y ¼ 2.7223 (r ¼ 0.99729) Fig. 5 (inset a).
104 [TX-100] (mol dmꢁ3
)
104 kc (sꢁ1
)
Furthermore, the Michaelis–Menten kinetic equation can
also be used to explain the catalytic effect of TX-100 on the
reaction rate according to which a reciprocal plot of a rate
constant and a surfactant concentration should follow eqn (4).
0.0
3.0
5.0
3.2
4.1
4.5
5.2
5.6
5.9
6.1
6.3
6.4
6.5
6.5
6.5
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
1
b
¼ a þ
(4)
ðkJ ꢁ kobs
Þ
½TX-100ꢄ
Fig. 5 (inset b) shows the fullment of the above relation. The
values of a ¼ 2.365 ꢃ 103 s and b ¼ 2.7022 mol dmꢁ3 s were
calculated from the intercept and slope of Fig. 5 (inset b).
4. Mechanism
4.1. Mechanism of the oxidation reaction without using the
TX-100 catalyst
which helps in bringing the L-histidine and MnO2 together in
a small area and may orient them in a manner suitable for
a redox reaction followed by rearrangement of the TX-100
molecules.
To explain the catalytic effect of TX-100 on the redox reaction
by the nano-colloid, Tuncay et al. proposed a mathematical
model53 according to which a double logarithmic plot of a rate
constant and the concentration of a surfactant should be linear:
The reaction between the MnO2 nano-colloid and L-histidine in
perchloric acid medium shows a stoichiometry of 1 : 1, a rst-
order dependency with respect to [MnO2] and a fractional-rst-
order dependency with respect to [His] and [HClO4]. These
results suggest that L-histidine is rst protonated, then forms an
intermediate complex (C1) in combination with MnO2
(Scheme 1). The order of less than unity in L-histidine can be
(3) attributed to complex (C1) formation before the rate deter-
mining step. Indeed, it is noted that a reciprocal plot of the rate
constant vs. L-histidine concentration shows an intercept in
agreement with complex formation (Fig. 6). Detection of the
intermediate complex by spectrophotometry fails due to the low
log kJ ¼ x log[TX-100] ꢁ y
where x and y are empirical constants.
Therefore, above model was applied to explain the catalytic
effect of TX-100 on the oxidative degradation of L-histidine by
Fig. 5 Effect of TX-100 concentration on the rate constant for the oxidative degradation of L-histidine by a MnO2 nano-colloid. Inset, (a) a plot of
log kc vs. log[TX-100] and (b) a plot of 1/(kc ꢁ kobs) vs. 1/[TX-100]. Reaction conditions: [MnO2] ¼ 8.0 ꢃ 10ꢁ5 mol dmꢁ3, [HClO4] ¼ 6.0 ꢃ 10ꢁ5 mol
dmꢁ3, [His] ¼ 4.0 ꢃ 10ꢁ3 mol dmꢁ3, temperature ¼ 35 ꢀC.
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Scheme 1 Mechanism of the oxidative degradation of L-histidine by a MnO2 nano-colloid in perchloric acid medium without/with the TX-100
catalyst.
K1
*
)
concentration of reactants used and/or the fast subsequent
þ
His
His þ Hþ
(5)
(6)
decomposition of the intermediate in comparison to its
formation. The following equations assume the decomposition
of the intermediate complex in the rate determining step;
K2
þ
þ
*
His þ MnO2 )
½His ꢁ MnO2ꢄ
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Fig. 6 shows the fullment of eqn (11). Due to the non-avail-
ability of the protonation constant (K1) of L-histidine and the
formation constant (K2) of the [His–MnO2] complex, under the
present conditions with temperature variation, evaluation of
the rate constant (k1) along with its activation parameters has
not been possible. Some kinetics runs were performed in order
to evaluate the rate constants from observed data, but the
results were not satisfactory.
4.2. Mechanism of the oxidation reaction with the TX-100
catalyst
To conrm whether the mechanism of the oxidative degrada-
tion of L-histidine by a MnO2 nano-colloid without/with TX-100
is similar, kinetics runs were performed as functions of [His],
[MnO2], and [HClO4] with [TX-100] (¼20.0 ꢃ 10ꢁ4 mol dmꢁ3) at
35 ꢀC. The experimental results are shown in Table 3. The
stoichiometry and order of the reaction with respect to [His],
[MnO2], and [HClO4] remain unchanged, therefore, it can be
concluded that the mechanism of the reaction remains
unchanged in the presence of TX-100. The fractional order with
respect to [His] is probably due to complex formation between
TX-100 and L-histidine before reaction with MnO2. The
formation of the complex was also proved kinetically by the
plot of [TX-100]/kJ vs. 1/[His] displaying a nonzero intercept
(Fig. 7).
Fig. 6 Verification of the rate law in the form of eqn (10) in aqueous
medium for the oxidative degradation of L-histidine by a MnO2 nano-
colloid, [MnO2] ¼ 8.0 ꢃ 10ꢁ5 mol dmꢁ3 at 35 ꢀC.
k1
ƒƒƒƒ!
½His ꢁ MnO2ꢄþ
products
(7)
slow
The following rate law was derived from the suggested
mechanism:
On the basis of the above discussion, we suggest that the
intermediate complex [His–TX-100] (C2) forms by combination
of the protonated L-histidine and TX-100 which then reacts with
one mole of MnO2 in the rate determining step and decomposes
into the oxidation products with regeneration of the catalyst,
TX-100 (Scheme 1). These results are shown in the following
sequence of equations;
Â
Ã
d MnO2
Rate ¼ ꢁ
dt
Â
Ã
k1K1K2½MnO2ꢄ½Hisꢄ Hþ
À
Â
Ã
Â
ÃÁÀ
Â
ÃÁ
¼
1 þ K1 Hþ þ K1K2½MnO2ꢄ Hþ 1 þ K1K2½Hisꢄ Hþ
(8)
The term K1K2[H+][MnO2] in the denominator can be
neglected because of the low concentration of MnO2 used.
Therefore, eqn (8) becomes
Table 3 Effects of [histidine], [MnO2] and [HClO4] on the oxidative
degradation of L-histidine by a MnO2 nano-colloid with TX-100 (20.0
ꢃ 10ꢁ4 mol dmꢁ3) at 35 ꢀ
C
Â
Ã
Â
ÃÂ
Ã
d MnO2
k1K1K2½MnO2ꢄ His Hþ
À
Â
ÃÁÀ
Â
ÃÁ
Rate ¼ ꢁ
¼
(9)
1 þ K1 Hþ 1 þ K1K2½Hisꢄ Hþ
103 [histidine]
105 [MnO2]
105 [HClO4]
104 kc
(sꢁ1
dt
(mol dmꢁ3
)
(mol dmꢁ3
)
(mol dmꢁ3
)
)
1.0
2.0
3.0
4.0
5.0
6.0
4.0
8.0
6.0
2.9
4.4
5.3
5.9
6.4
6.7
13.1
10.1
7.8
5.9
5.1
4.9
4.7
5.4
5.9
6.2
6.7
7.0
This rate law is consistent with all the observed reaction
orders with respect to different species.
Under pseudo-rst order conditions
d½MnO2ꢄ
Rate ¼ ꢁ
¼ kobs½MnO2ꢄ
(10)
dt
2.0
4.0
6.0
6.0
Comparing eqn (9) and (10) and upon rearrangement we
obtain
8.0
10.0
12.0
8.0
ꢀ
ꢁꢀ
ꢁ
1
ð1 þ K1½HþꢄÞ
1
¼
þ C
(11)
ðk1K1K2½HþꢄÞ
4.0
2.0
4.0
6.0
8.0
10.0
12.0
kobs
½Hisꢄ
where C ¼ (1 + K1[H+])/k1.
According to eqn (11), the reciprocal plots of {kobs vs. [His]} at
constant L-histidine concentration and {kobs vs. [H+]} at constant
HClO4 concentration should be linear with positive intercepts.
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d½MnO2ꢄ
dt
Rate ¼ ꢁ
¼ kJ½MnO2ꢄ
(18)
ꢀ
ꢁꢀ
ꢁ
½TX-100ꢄ
1 þ K1½Hþꢄ
1
¼
þ C
(19)
k2K1K3½Hþꢄ
kJ
½Hisꢄ
where C ¼ (1 + K1[H+])/k2.
According to eqn (19), the plots of {[TX-100]/kJ vs. 1/[His]}
and {[TX-100]/kJ vs. 1/[H+]} at constant L-histidine and
perchloric acid concentrations, respectively, should be linear
with positive intercepts on the y-axes, which satises the
experimental results (Fig. 7).
5. Conclusions
This paper presents a detailed kinetics study of the oxidative
degradation of L-histidine by a MnO2 nano-colloid without and
with use of TX-100 as a catalyst. Analysis of the kinetics data
suggested that the rate-determining step takes place on the
manganese dioxide nano-colloid. The reaction has been found
to be fractional-order with respect to L-histidine as well as
HClO4, and rst-order with respect to MnO2. On the basis of
these results, a rate law equation was established. On the other
hand it has also been conrmed that the oxidative degradation
of L-histidine is catalyzed by TX-100 which has been discussed
and explained in terms of the mathematical model of Tuncay
et al. A suitable mechanism, consistent with the experimental
observations, has been proposed.
Fig. 7 Plots confirming a rate law in the form of eqn (18) for the
oxidative degradation of L-histidine by a MnO2 nano-colloid in
perchloric acid medium with the TX-100 catalyst, [MnO2] ¼ 8.0 ꢃ 10ꢁ5
mol dmꢁ3 at 35 ꢀC.
K1
þ
His
His þ Hþ
)
(12)
*
K3
þ
þ
*
His þ TX-100 )
½His ꢁ TX-100ꢄþ
½His ꢁ TX-100ꢄ
(13)
(14)
k2
ƒƒƒƒ!
products
slow
Acknowledgements
According to the above mechanism, the rate of disappear-
ance of [MnO2], or formation of the intermediate complex, can
be expressed by the following rate law equation:
This work was supported by King Saud University, Deanship of
Scientic Research, College of Science Research Center.
d½MnO2ꢄ
dt
d½His ꢁ TX-100ꢄ
¼ þ
dt
Rate ¼ ꢁ
References
¼ k2½His ꢁ TX-100ꢄ½MnO2ꢄ
(15)
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Â
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