Kinetic and mechanistic study of oxidation of L-methionine by Waugh-type enneamolybdomanganate(IV) in perchloric acid

Article abstract of DOI:10.1007/s00726-009-0285-0

The reaction between methionine and enneamolybdomanganate(IV) in perchloric acid was carried out under pseudo-first-order conditions keeping large excess of methionine. The orders in oxidant and substrate were found to be unity and 0.91, respectively. The reaction proceeds with rapid formation of complex between the reactants followed by its decomposition in a rate determining step. The accelerating effect of hydrogen ions on the reaction is due to the formation of active hexaprotonated oxidant species. The product of the reaction was found to be methionine sulfoxide. The reaction involves direct two-electron transfer step without any free radical intervention. The effect of ionic strength, solvent polarity and the activation parameters were also in support of the mechanism proposed.

Full text of DOI:10.1007/s00726-009-0285-0

Amino Acids (2010) 38:789–795
DOI 10.1007/s00726-009-0285-0
ORIGINAL ARTICLE
Kinetic and mechanistic study of oxidation of L-methionine
by Waugh-type enneamolybdomanganate(IV) in perchloric acid
V. M. Gurame Æ A. R. Supale Æ Gavisiddappa S. Gokavi
Received: 12 October 2008 / Accepted: 24 March 2009 / Published online: 15 April 2009
Ó Springer-Verlag 2009
Abstract The reaction between methionine and enne-
amolybdomanganate(IV) in perchloric acid was carried out
under pseudo-first-order conditions keeping large excess of
methionine. The orders in oxidant and substrate were found
to be unity and 0.91, respectively. The reaction proceeds
with rapid formation of complex between the reactants
followed by its decomposition in a rate determining step.
The accelerating effect of hydrogen ions on the reaction is
due to the formation of active hexaprotonated oxidant
species. The product of the reaction was found to be
methionine sulfoxide. The reaction involves direct two-
electron transfer step without any free radical intervention.
The effect of ionic strength, solvent polarity and the acti-
vation parameters were also in support of the mechanism
proposed.
viz., O, N and S, it has been reported by McAuley and
coworkers that the sulfur is more susceptible to attack by
oxidants like CrVI (Olatunji and McAuley 1977; McAuley
and McCann 1975). Methionine, a thioether containing
amino acid behaves differently than other amino acids
towards many oxidants (Sharma et al. 1996; Mahadevappa
et al. 1985), due to the presence of electron rich sulfur
which is easily oxidizable. Oxidations of different amino
acids by manganese are of special significance because of
their biological relevance (Boucher 1972). Molybdenum
forms clusters with the sulfur compounds; these clusters are
also important in enzyme catalysis (George et al., 2000).
Chemistry of heteropoly acids received much attention
in the recent past due to their applications in biochemistry
and material chemistry (Katsoulis 1998; Sadakane and
Steckhan 1998). These polyoxometalate salts are used
(Katsoulis 1998; Sadakane and Steckhan 1998; Weinstock
1998) as oxidants and catalysts (both redox and acid) for
various organic transformations. Tunable physical proper-
ties like redox potential and acidity of transition metal
substituted heteropolyoxometalates make them attractive
from catalytic and electrocatalytic point of view
(Kozhevnikov 1998). Most of the electron transfer reactions
of polyoxometalate (POM) anions and their mechanistic
studies are concentrated (Ayoko 1990; Hasure and Gokavi
2003; Saha et al. 1988; Mehrotra and Mehrotra 2003;
Satpathy et al. 2006; Bhosale and Gokavi 2003, 2004a, b,
2006; Baciocchi et al. 1996a, b; Shah et al. 1999; Goyal
et al. 1998; Dholia et al. 1997), on the Keggin type salts
containing hetero atom in its higher oxidation state. Vari-
eties of organic and inorganic substrates have also been
oxidized (Sadakane and Steckhan 1998), by these POMs
and the mechanisms generally follow outer-sphere path-
way. POMs contain a hetero atom which is surrounded by
number of oxoanions; thus, making the central hetero atom
Keywords Kinetics ꢀ Mechanism ꢀ Oxidation ꢀ
Methionine ꢀ Waugh-type enneamolybdomanganate(IV)
Introduction
Molybdenum is found in several tissues of human body and
is required for the activity of some enzymes which are
involved in the catabolism of purines and sulfur containing
amino acids. Methionine is a sulfur containing amino acid
which is involved in the wide range of biochemical reac-
tions. Although methionine has three co-ordinating centers,
V. M. Gurame ꢀ A. R. Supale ꢀ G. S. Gokavi (&)
Kinetics and Catalysis Laboratory, Department of Chemistry,
Shivaji University, Kolhapur 416004, India
e-mail: gsgokavi@hotmail.com
123

790
V. M. Gurame et al.
Kinetic measurements
unavailable for the inner sphere interaction with the
substrate. In comparison with Keggin and Dawson type of
salts, less attention is given to other type of POM anions. In
continuation of our interest (Hasure and Gokavi 2003;
Bhosale and Gokavi 2003, 2004a, 2004b, 2006; Gurame
and Gokavi 2008), in the electron transfer reactions
involving POMs, the present study of oxidation of methi-
onine by enneamolybdomanganate(IV), a Waugh-type
heteropolyacid, was carried out in perchloric acid medium.
Enneamolybdomanganate(IV) contains one MnO6 unit
surrounded by nine MoO6 octahedra. The redox potential
of the MnIV/MnII couple also decreases (Dunne et al. 1992)
from 1.51 to 1.035 V at pH 3.94 in presence of molybdate
ions leading to the formation of POM; thus, making the
MnIV ion stable in aqueous solution.
The reaction between [MnIVMo9O32]6- and methionine
was studied under pseudo-first-order conditions at a
constant temperature of 26.0 0.1°C. The reaction was
initiated by mixing the previously thermostated solu-
tions of methionine and [MnIVMo9O32]6-, which also
contained the required amount of perchloric acid,
and doubly distilled water. The reaction was fol-
lowed by determining the concentration of remaining
[MnIVMo9O32]6- iodometrically. The pseudo-first-order
rate constants were determined from the plots of log
[MnIVMo9O32]6-against time and the rate constants
were reproducible within 6%.
Atomic absorption analysis
Therefore, the oxidation of methionine by enne-
amolybdomanganate(IV) ion was undertaken to understand
the mechanism of the reaction as both the ions are prone to
form complex with molybdate ion. Methionine has been
oxidized by various oxidants (Gensch and Higuchi 1967;
Young and Hsich 1978; Mansour 2002; Natile et al. 1975;
Johnson and Read 1996), and the mechanisms generally
involve formation of a complex between the two reactants.
The polyoxometalates like enneamolybdomanganate(IV)
are considered as outer-sphere electron transfer reagents;
therefore, it would be of interest to understand the mech-
anism of title reaction between two redox centers
interacting with molybdate ion.
The complex (NH4)6[MnIVMo9O32]ꢀ3H2O, was analyzed by
AAS. The solution for AAS analysis was prepared by dis-
solving 100 mg of recrystalized sample in double distilled
water and 5 ml of this stock solution was diluted to 100 ml.
The diluted solution was used for AAS analysis of Mn and
Mo using Perkin–Elmer AAnalyst-300 instrument. The
complex (NH4)6[MnIVMo9O32]ꢀ3H2O shows (theoretical):
Mn, 3.501% (3.4493%) and Mo, 54.217% (54.2125%).
Spectrophotometric measurements and FTIR-spectral
study
The visible spectra of (NH4)6[MnIVMo9O32] complex, and
the reaction mixture were recorded by using Elico SL-177
spectrophotometer between the wavelength region 400–
650 nm. The spectrum of the reaction mixture at various
time intervals is shown in Fig. 1.
Materials and methods
All the solutions were prepared in doubly distilled water.
The solution of L-methionine was prepared by dissolving
methionine (SD Fine) in water and standardized by
reported method (Lavine 1993). The solution of sodium
thiosulphate was freshly prepared every day and stan-
dardized against standard potassium dichromate solution
iodometrically (Vogel 1961). Enneamolybdomangnate
complex (NH4)6[MnIVMo9O32] was prepared by reported
method (Baker and Weakly 1966) as follows: 50 g of
ammonium molybdate was dissolved in 200 ml of water
and excess of hydrogen peroxide as oxidant was added to it
and the resultant solution was heated to 95°C. To this hot
solution, 5 g of MnSO4ꢀH2O in 50 ml of water was added
slowly with constant stirring. The resultant orange-red
colored solution was boiled for 10 min and quickly filtered
and cooled. The orange-red colored crystals were recrys-
talized thrice from hot (70°C) water. The solution of
(NH4)6[MnIVMo9O32] was standardized by treating known
amount of complex solution with excess of AsIII, and back
titrating AsIII. The oxidation state of hetero atom was also
confirmed to be four, by iodometric method.
1.4
Time(min)
2
4
1.2
6
1
0.8
0.6
0.4
0.2
0
8
10
12
14
16
400
450
500
550
600
650
wavelength (nm)
Fig. 1 The spectra of the reaction mixtures at different time intervals
6-
at 26°C 103[MnIVMo9O32
]
= 1.8 mol dm-3, 102[Methionine] =
3.0 mol dm-3 10[HClO4] = 3.5 mol dm-3
123

Kinetic and mechanistic study of oxidation of L-methionine
791
The FTIR-Spectra of (NH4)6[MnIVMo9O32] complex
was taken in KBr by using Perkin–Elmer Spectrumone
spectrophotometer. The complex (NH4)6[MnIVMo9O32]
contains MnIV as a hetero atom surrounded by octahedral
groups of MoO62-. In the IR-spectrum of MnIV complex,
the peaks obtained at 3521, 933, 918 and 900 cm-1 cor-
responds to lattice water molecules (Nakamoto 1977), Mo–O
stretching (Martin et al. 2004), MnIV–O stretching and
tetrahedral MoO42- groups (Nakamoto 1977), respectively.
mixture containing excess of oxidant was kept undisturbed up
to 24 h and the complete disappearance of amino acid was
confirmed by thin layer chromatography. The product
methionine sulfoxide was isolated from the reaction mixture
by reported method (Natile et al. 1975). Acetone–ethanol (1:1
volume ratio) was added to the reaction mixture previously
brought to pH 4 by adding sodium bicarbonate and precipi-
tated methionine sulfoxide was identified by its melting point
(Natile et al. 1975).
Test for free radicals
Reaction order
The reaction was also studied in presence of added acry-
lonitrile to understand the intervention of free radicals
(Kolthoff et al. 1953; Mahesh et al. 2004), in the reaction.
There was no effect of added acrylonitrile on the reaction
and also no precipitate due to the polymerization of the
added acrylonitrile was observed thus confirming the
absence of any free radical formation in the reaction.
The reaction was carried out under pseudo-first-order
conditions, keeping the concentration of perchloric acid
constant at 0.35 mol dm-3 and varying either methionine
from 0.01 to 0.077 mol dm-3 at constant [MnIVMo9O32]6-
(1.8 9 10-3 mol dm-3) or [MnIVMo9O32]6- from 2.2 9
10-4 to 1.8 9 10-3 mol dm-3 at constant methionine
(3.0 9 10-2 mol dm-3). The pseudo-first-order rate con-
stants were found to be increased with increase in the
concentration of the reductant (Table 1) and the order was
found to be 0.91 as determined by log (kobs) against log
[methionine] plot. The order in oxidant concentration was
found to be unity as the pseudo-first-order plots were found
to be linear and the pseudo-first-order rate constants were
fairly constant as the concentration of oxidant is varied
(Table 1). Added molybdate ion over the concentration
range of 0.1 9 10-2 to 1.0 9 10-2 mol dm-3 did not
affect the rate of reaction.
Results and discussion
Stoichiometry and product analysis
The stoichiometry was studied by keeping concentration of
[MnIVMo9O32]6- constant at 5 9 10-3 mol dm-3 and vary-
ing concentration of methionine from 0.5 9 10-3 to
3.0 9 10-3 mol dm-3. Different reaction mixtures contain-
ingvaryingconcentrationofmethioninewereprepared, which
also contained required amount of perchloric acid. The con-
centration of unreacted [MnIVMo9O32]6- was determined
after 24 h iodometrically as well as spectrophotometrically at
468 nm. The stoichiometry was found to be one mole of
[MnIVMo9O32]6- per mole of methionine. The reaction
Effect of hydrogen ion concentration
The effect of hydrogen ion concentration on the reaction
was studied to know probable prior protonation equilibria
Table 1 Effect of
concentration of methionine,
perchloric acid and
6-
]
103[MnIVMo9O32
10[HClO4]
(mol dm-3
102[Methionine]
(mol dm-3
)
103 kobs s-1
(mol dm-3
)
)
enneamolybdomanganate(IV)
ion, on the reaction between
methionine and
enneamolybdomanganate(IV) at
26°C
0.22
0.45
0.9
1.3
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
0.38
0.7
1.9
3.5
3.0
3.0
3.0
3.0
3.0
1.0
2.0
3.0
5.0
7.7
3.0
3.0
3.0
3.0
1.3
1.4
1.3
1.3
1.4
0.6
1.0
1.4
2.2
3.7
0.2
0.4
0.7
1.4
123

792
V. M. Gurame et al.
of either the oxidant or the substrate and to identify the
active reactant species. The concentration of H? ion was
varied from 0.0385 to 0.35 mol dm-3 keeping all other
concentrations constant. It was found that the reaction rate
increases with the concentration of H? ions (Table 1) and
the graph of kobs against [H?] was found to be linear
without intercept (Fig. 2) and the order in [H?] was found
to be 0.9.
reaction, there is no possibility of a prior equilibrium
involving it. The oxidant, [MnIVMo9O32]6-, is a hetero-
polyoxometalate of MnIV and molybdate ion; the effect of
added molybdate ion signifies the stability of the [MnIV-
Mo9O32]6- ion under experimental conditions of the
reaction.
Enneamolybdomanganate(IV) is one of the stable het-
eropolymolybdate containing MnIV as a hetero atom and is
noncentrosymmetric. It also shows a characteristic charge
transfer band at 468 nm which has been assigned to
4A2g?4T2g transition of MnIV. This band is sensitive (Saito
et al. 1997), to the changes in the electronic state of the
MnIV ion and any perturbations due to interaction of MnO6
groups with neighboring ions will result in the shift of
band. The arrangements of oxygen atoms (Saito et al.
1997), around [MnMo9O32]6- are of two different types A
and B as shown in the Fig. 3. The negative charge of the
oxygen atoms at site A is greater than that of the oxygen at
site B. From the spectrophotometric investigation (Saito
et al. 1997), on interaction of various cations with the
anion, it was concluded that metal cations will bind to site
A due to their larger negative charge in comparison with
that of site B. Such type of cation interaction at site A
perturbs the electronic state of the central hetero atom,
manganese, resulting in change of absorption spectra of the
anion. In aqueous solution the UV–VIS spectrum of the
Effect of solvent polarity, ionic strength
and temperature
The effects of ionic strength, solvent polarity and temperature
were studied keeping concentration of [MnIVMo9O32]6-
,
methionine and perchloric acid constant at 1.8 9 10-3, 0.03
and 0.35 mol dm-3, respectively, at 26°C. Sodium perchlo-
rate and acetonitrile were used to vary the ionic strength and
solvent polarity, respectively. The rate of the reaction was
found to be increased as ionic strength increases from 0.39 to
0.75 mol dm-3. The rate of reaction decreases as percentage
of acetonitrile increases from 0 to 57% v/v. This decrease in
the rate with decrease in the dielectric constant is in confor-
mity with Amis concept for ion–dipole interactions. The effect
of temperature was studied at 15, 20, 26, 30 and 35°C and the
activation parameters were determined DH# = 46.78
6 kJmol-1, DG# = 55.50 6 kJ mol-1, DS# = -30.18
5 JK-1 mol-1
.
The order in methionine concentration was found to be
close to unity (0.91) and the pseudo-first-order plots were
linear as well as the pseudo-first-order rate constants were
constant as the concentration of oxidant was increased
indicating an order of unity in its concentration also.
Therefore, the mechanism of the reaction involves prior
interaction between the oxidant and methionine. Since, the
added molybdate ion did not influence the rate of the
16
14
12
10
8
6
4
2
6-
(from
6- anion.
Fig. 3 Arrangements of oxygen atoms in [MnMo9O32
reference Saito et al., 1997). a Structure of the [MnMo9O32
]
]
0
0
0.1
0.2
0.3
0.4
(shaded octahedron in a is MnO6). b Two arrays of oxygen atoms as
the potential metal binding sites. (large solid circles in b are oxygen
atoms, small solid circles are Mo atoms and an open circle is a Mn
atom.)
[H+] mol.dm-3
Fig. 2 Plot of kobs against [H?](conditions as in Table 1)
123

Kinetic and mechanistic study of oxidation of L-methionine
793
anion, [MnMo9O32]6-, shows a peak at 468 nm and in
presence of acid two isosbestic points are obtained at 435
and 520 nm due to its protonation. At high metal ion
concentrations, the peak at 468 nm reappeared (Saito et al.
1997) as a result of metal ion substitution on the anion at
oxygen atoms of A type. In order to understand the inter-
The pKa of the anion, [MnMo9O32]6-, is reported (Saito
et al. 1997), to be around 3.0 and at pH = 2 approximately
90% of the anion is totally protonated and it is completely
deprotonated at pH = 4. Therefore, at a pH bellow 2, used
in the present study, the accelerating effect of hydrogen ion
can be accounted due to the formation of hexaprotonated,
H6[MnMo9O32], anion in a prior equilibrium between
hydrogen ion and the pentaprotonated, H5[MnMo9O32]-,
anion. Therefore, under the reaction conditions employed
in the present study the hexaprotonated anion,
H6[MnMo9O32], and the protonated methionine are the
reactive species.
action between the methionine and [MnMo9O32]6-
,
UV–VIS spectra of the reaction mixture was examined.
The anion shows a characteristic peak at 468 nm in aque-
ous solution and in 0.35 mol dm-3 perchloric acid the
spectrum of protonated anion contain two isosbestic points
at 435 and 520 nm due to its protonation. To an acidic
solution of the anion when methionine is added the reap-
pearance of peak at 468 nm was not observed which is
expected if the interaction of added methionine would have
been through oxygen atoms of site A. Therefore, the
Under the pseudo-first-order condition, methionine is
oxidized to methionine sulfoxide only. Oxidation product
of methionine depends upon the oxidants, with some oxi-
dants it is oxidized to sulfoxide stage (Gensch and Higuchi
1967; Young and Hsich 1978; Mansour 2002; Natile et al.
1975), and with certain others it is oxidized to sulfone stage
(Mahadevappa et al. 1983; Wani et al. 1994). In all these
reactions, electrophilic attack of oxidant take place at sul-
fur site due to the higher electronegativity of S rather than
O and N. The stoichiometry of the reaction was found to be
1:1 in presence of excess of enneamolybdomanganate(IV)
when analyzed after 24 h. The mechanism for the forma-
tion of methionine sulfoxide reaction product with the
observed stoichiometry 1:1 can be represented as in
Scheme 1 in terms of the active species of the reactants.
The mechanism of the reaction represented in Scheme 1
involves a complex formation between the substrate and
the oxidant through electrophilic attack of the oxidant on
the nucleophilic sulfur atom of methionine. The product of
the reaction under present conditions is methionine sulf-
oxide only and further oxidation does not takes to the
sulfone stage even in presence of stoichiometrically excess
oxidant as noticed in the product analysis. The nucleo-
philicity of the sulfur atom in the methionine sulfoxide
decreases as it binds to an oxygen atom, thus retarding the
attack of the electrophilic oxidant which prevents the fur-
ther oxidation to the corresponding sulfone. The formation
of polyoxometalate around a hetero atom like Mn(IV) also
reduces its oxidizing capacity of form 1.51–1.03 V(Dunne
et al. 1992) thus making it a mild oxidant which also
methionine is interacting with the anion, [MnMo9O32]6-
,
through the oxygen atoms of site B without perturbing the
electronic state of MnO6 octahedra. The spectra of the
reaction mixtures at different time intervals are given in
Fig. 1 which indicates the decrease in absorbance of the
anion as the reaction proceeds.
The dependence of rate of the reaction on hydrogen ion
concentration is shown in Table 1 and the order in its
concentration was found to be 0.9 as found from the plot of
log kobs against log [H?] plot. The plot of kobs against [H?]
is also found to be linear (Fig. 2). The order of close to
unity in [H?] indicates a prior protonation equilibrium. The
pKa values of the methionine are 2.22, 9.27 (Weast et al.
1986), corresponding to the protonation of amino and
carboxylic acid groups. In strongly acidic medium, the
carboxylic acid group remains un-dissociated and the
amino group is also protonated bellow pH 4 as shown in
Eq. 1. Therefore, under the present reaction conditions
methionine exists completely in the (CH3)SR?H2.
CH3
NH3
CH3
NH3
CH3
NH2
K2,H+
H+
K1,
+
+
S
(1)
S
S
pH< 4.0
OH
O-
pH>8.0
O-
O
O
O
(CH3)SR+H2
(CH3)SRH
(CH3)SR-
Scheme 1
K3
H6[MnIVMo9O32]
H5[MnIVMo9O32]- + H+
Kc
k1
(CH3)SR+H2
H6[MnIVMo9O32]
Complex
+
(CH3)SOR+H2 + H4[MnIIMo9O32] + 4H+
Complex + H2O
H4[MnIIMo9O32]
4H2O
fast
-
MnII + 9HMoO4 + 3H+
+
123

794
V. M. Gurame et al.
References
renders further oxidation of the sulfoxide to the sulfone
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ÃꢁÂ
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k1KcK3 H5MnIVMo9O32 ðCH3Þ2SRþH2 ½Hþꢂ
Rate ¼
ð1 þ K3½HþꢂÞ
ð2Þ
ð3Þ
Â
Ã
k1KcK3 ðCH3Þ2SRþH2 ½Hþꢂ
kobs
¼
ð1 þ K3½HþꢂÞ
The mechanism according to Scheme 1 proceeds with
the rapid formation of a complex between the methionine
and [H6MnIVMo9O32] and subsequent rate determining
decomposition of complex to give methionine sulfoxide
and [MnIIMo9O24]10-. The interaction between methionine
and [H6MnMo9O32] during complex formation occurs
through the oxygen atoms of the site B. The reduced
polyoxometalate, [MnIIMo9O24]10-, undergoes fast decom-
position to generate aqueous Mn2? ion and molybdate ions.
In an earlier ESR study the interaction between aqueous
Mn2? and molybdate ions was examined and the alteration
of linewidth of the Mn2? signal in presence of molybdate
(Dunne et al. 1993) was found to be comparable with that
in presence of acetate ions. From this result, it has been
concluded that the interaction between molybdate ions and
Mn2? ions in aqueous solutions is weak. Therefore, the
dissociation of the [MnIIMo9O24]10- under the present
reaction conditions is considered to be fast. Acrylonitrile,
free radical scavenger, did not affect the reaction rate
indicates direct two-electron transfer from methionine to
[MnIVMo9O32]6-. The effect of ionic strength and the
solvent polarity on the rate qualitatively explains the
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