Dual mechanism of oxidation of dl-methionine by diperiodatoargentate(III) in aqueous alkaline medium (stopped flow technique)

Article abstract of DOI:10.1016/j.molstruc.2007.09.013

The kinetics of oxidation of dl-methionine by diperiodatoargentate(III) (DPA) has been studied spectrophotometrically in a wide range, 0.01-1.0 mol dm^-3, of alkali at constant ionic strength of 0.50 mol dm^-3. In a lower range, 0.01-0

Full text of DOI:10.1016/j.molstruc.2007.09.013

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Journal of Molecular Structure 882 (2008) 88–95
www.elsevier.com/locate/molstruc
Dual mechanism of oxidation of DL-methionine by
diperiodatoargentate(III) in aqueous alkaline medium
(stopped flow technique)
*
K.A. Thabaj, S.A. Chimatadar, S.T. Nandibewoor
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India
Received 21 April 2007; received in revised form 2 September 2007; accepted 18 September 2007
Available online 25 September 2007
Abstract
The kinetics of oxidation of DL-methionine by diperiodatoargentate(III) (DPA) has been studied spectrophotometrically in a wide
range, 0.01–1.0 mol dmꢀ3, of alkali at constant ionic strength of 0.50 mol dmꢀ3. In a lower range, 0.01–0.20 mol dmꢀ3 and in the higher
range 0.3–1.0 mol dmꢀ3 of OHꢀ ion concentrations the reaction exhibits first order with respect to DPA concentration. But in the lower
and higher range of OHꢀ ion concentration the order with respect to DL-methionine concentration changes from fractional order to zero
order. In the lower range of OHꢀ ion concentration the rate is independent of periodate concentration and in the higher range of OHꢀ
ion concentration, periodate retarded the rate of the reaction. At constant DPA, DL-methionine, periodate and at constant ionic strength,
the order with respect to OHꢀ ion concentration changes from negative fractional (ꢀ0.36) to positive fractional (0.44). The increase in
dielectric constant of the medium, increases the rate of the reaction in lower and higher range of the OHꢀ ion concentration. The added
products did not have any significant effect on the rate of reaction. The main products were identified by spot test and IR spectroscopy.
A dual free radical mechanisms have been proposed for both lower and higher range of OHꢀ ion concentrations. The active species of
Ag(III) was found to be diperiodatoargentate(III) in lower range of OHꢀ ion concentration and monoperiodatoargentate(III) in higher
range of OHꢀ ion concentration. The rate constants of the slow step of the mechanisms were determined. The activation parameters were
evaluated and discussed.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Diperiodatoargentate(III); DL-Methionine; Kinetics and mechanism
1. Introduction
energy source of living organisms, as well. Methionine sup-
plementation may prove especially useful in cases of acet-
aminophen poisoning.
Methionine is the least abundant of all amino acids.
Despite this verity, methionine is essential, and is an impor-
tant chelating agent that assists in both metabolic and
growth processes. Methionine (along with choline and ino-
sitol) is included in a group of compounds referred to as
lipotropics and these aid the liver in processing fats by
increasing the production of lecithin. Methionine serves
as a catalyst for choline and inositol functioning. It actively
participates in the formation of D-glucose, the primary
Methione’s most important function in human physiol-
ogy may be its antioxidant activities. This aminoacid has
been shown to protect the body’s cells from harmful free
radical damage. The protection against free radicals and
toxic compounds may coincide to the sulfur which metho-
nine possesses in its chemical makeup. This may also pro-
vide a feasible explanation as to why methionine is ascribed
as an anti-hepatotoxic agent. The oxidation of methionine
plays an important role in vivo [1], during biological condi-
tions of oxidative stress, as well as for protein stability
in vitro. Depending on the nature of the oxidizing species,
methionine may undergo a two-electron oxidation to
*
Corresponding author. Tel.: +91 0836 2215286.
E-mail address: stnandibewoor@yahoo.com (S.T. Nandibewoor).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.09.013

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K.A. Thabaj et al. / Journal of Molecular Structure 882 (2008) 88–95
89
methionine sulfoxide or one-electron oxidation to methio-
nine radical cations. In vivo, methionine sulfoxide is sub-
ject to reduction by the methionine sulfoxide reductase
system, suggesting that some methionine sulfoxide residues
may only be transiently involved in the deactivation of pro-
teins through reactive oxygen species. Other methionine
sulfoxide residues (Msr) may accumulate, depending on
the accessibility to Msr. Amino acids have been oxidized
by a variety of oxidising agents [2]. The oxidation of amino
acids is of interest as the oxidation products differ for dif-
ferent oxidants [3]. The oxidation of methionine to the sulf-
oxide form is of distinct interest and important in
biopharmaceutical industry as it has been shown to occur
on a wide variety of proteins and often reduces or elimi-
nates biological activity, causes aggregation, increases
immunogenicity, encourages proteolysis, etc [4].
2. Experimental
2.1. Materials
All chemicals used were of reagent grade and double dis-
tilled water was used throughout the work. DL-Methionine
(S.D. Fine Chem.) was used as received. The purity of the
sample was checked by TLC and from its melting point (lit.
279 ꢁC). The required concentration of DL-methionine was
used from its aqueous stock solution. KNO3 and KOH
were used to maintain ionic strength and alkalinity of the
reaction respectively. Aqueous solution of AgNO3 was
used to study the product effect, Ag(I).
2.2. Preparation of DPA
Diperiodatoargentate(III) (DPA) is a powerful oxidizing
agent in alkaline medium with the reduction potential,
1.74 V [5]. It is widely used as a volumetric reagent for
the determination of various organic and inorganic species
[6]. Jayaprakash Rao et al. [7] have used DPA as an oxidiz-
ing agent to study kinetics of oxidation of various organic
substrates. They normally found that the reaction order
with respect to both oxidant and substrate was unity and
OHꢀ ion concentration was found to enhance the rate of
reaction. They suggested the possible active species of
DPA in alkali and on the other hand they proposed reac-
tion mechanisms by generalizing the DPA as [Ag (HL)
L](x+1)ꢀ. However, Anil Kumar et al. [8] put an effort to
find an evidence for the reactive form of DPA at highly
alkaline pH. The present investigation was carried out in
the wide range of OHꢀ ion concentration and found
entirely different kinetic results in both lower and higher
OHꢀ ion concentration and obtained the evidence for the
reactive species for DPA in various OHꢀ ion concentra-
tion. The DPA is a metal. complex with Ag in +3 oxidation
state like Cu3+ in diperiodatocuprate(III) (DPC) and Fe3+
in hemoglobin and Ni4+ in diperiodatonickelate(IV)
(DPN). However, former are single equivalent oxidants,
having a structural similarity with DPA; and latter is a
two equivalent oxidant and has structural dissimilarity
with DPA [9]. Apart from this, during the study of the
reaction as the OHꢀ ion concentration increases, the rate
decreases and reaches a minimum and again increases. At
the same time, orders with respect to reactive species also
changes. This article will provide detailed mechanistic
schemes for the reaction of diperiodatoargentate(III) with
methionine. The major oxidation product was found to
be methionine sulfoxide, which is the same product found
in vivo oxidation of methionine [1,10]. The product profile
can, in some cases, provide valuable information on the
species involved. By using this product (in vitro studies)
one can study the effect of accumulation of such damaged
proteins on human pathologies. Hence, it was important
and interesting for the detailed investigation of DPA oxida-
tion of DL-methionine in a wide range of aqueous alkaline
medium to explore the possible mechanisms.
DPA was prepared by oxidizing Ag(I) as described else-
where [11]: the mixture of 28 g of KOH and 23 g of KIO3 in
100 cm3 of water along with 8.5 g AgNO3 was heated just
to boiling and 20 g of K2S2O8 was added in several lots
with stirring then allowed to cool. It was filtrated through
a medium porosity fritted glass filter and 40 g of NaOH
was added slowly to the filtrate, whereupon a voluminous
orange precipitate agglomerates. The precipitate was fil-
tered as above and washed three to four times with cold
water. The pure crystals were dissolved in 50 cm3 water
and warmed to 80 ꢁC with constant stirring thereby some
solid was dissolved to give a red solution. The resulting
solution was filtered when it was hot and on cooling at
room temperature, the orange crystals separated out and
were recrystallised from water.
The complex was characterized from its UV–vis spec-
trum, exhibited three peaks at 216, 225 and 362 nm. These
spectral features were identical to those reported earlier for
DPA [11]. The magnetic moment study revealed that the
complex is diamagnetic. The compound prepared were
analyzed [12] for silver and periodate by acidifying a solu-
tion of the material with HCl, recovering and weighing the
AgCl for Ag and titrating the iodine liberated when excess
KI was added to the filtrate for IO4ꢀ. The aqueous solution
of DPA was used for the required DPA concentration in
the reaction mixture.
2.3. Kinetic procedure
All kinetic measurements were performed under pseudo-
first order conditions with DL-methionine in excess over
DPA at constant ionic strength of 0.50 mol dmꢀ3. The
reaction was initiated by mixing previously thermostatted
solutions of DL-methionine and DPA, which also contained
the necessary quantities of KOH and KNO3 to maintain
the required alkalinity and ionic strength, respectively.
The temperature was maintained at 20.0 0.1 ꢁC. Since
the initial reaction was too fast to be monitored by usual
methods, the course of reaction was followed by monitor-
ing the decrease in absorbance of DPA in a 1 cm quartz cell
of a Varian CARY 50 Bio UV–vis Spectrophotometer

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90
K.A. Thabaj et al. / Journal of Molecular Structure 882 (2008) 88–95
connected to a rapid kinetic accessory (HI-TECH SFA-12)
at its absorption maximum of 360 nm as a function of time.
The first order rate constants, kobs were evaluated by the
plots of log[absorbance] versus time and were linear in
almost all cases upto 80% completion of the reaction and
were reproducible to within 5%.
chloride solution [14,15]. The precipitate N-benzoyl methi-
onine sulfoxide was confirmed by its mp 183 ꢁC (lit.
184 ꢁC).
The product was separated by adding acetone-ethanol
mixture (1:1) to the reaction mixture, which resulted in
the precipitate [15,16] and is identified by its mp 233 ꢁC
(lit. 235). The IR spectra showed that the stretching for
ANH2 and ACOOH remained same. However the new
band for >S@O is appeared at 1032 cmꢀ1 for the product.
The reaction products do not undergo further oxidation
under the present kinetic conditions.
During the variation of OHꢀ ion concentration, as the
OHꢀ ion concentration increases from 0.01 to 0.20 mol dmꢀ3
the rate of reaction decreases and further increase in the
OHꢀ ion concentration from 0.3 to 1.0 mol dmꢀ3 the rate
of reaction increases. Hence the detailed kinetics studies
were made in the lower (0.01–0.20 mol dmꢀ3) and higher
(0.3–1.0 mol dmꢀ3) range of OHꢀ ion concentration.
3.2. Reaction order
In the kinetics
a
constant concentration viz.
1.0 · 10ꢀ4 mol dmꢀ3 of KIO4 was used throughout the
study unless otherwise stated. Thus, the possibility of oxi-
dation of DL-methionine by periodate was checked and
found that there was no significant interference due to
KIO4 under experimental condition. The effect of dissolved
oxygen on the rate of the reaction was checked by prepar-
ing the reaction mixture and following the reaction in an
atmosphere of N2. No significant difference between the
results obtained under N2 and in presence of air was
observed. In view of the ubiquitous contamination of basic
solutions by carbonate, the effect of carbonate on the reac-
tion was also studied. Added carbonate had no effect on
the reaction rate. However, fresh solutions were used when
conducting the experiment.
The reaction orders have been determined from the
slopes of log kobs versus log (concentration) plots by vary-
ing the concentration of DL-methionine, DPA and alkali in
turn while keeping others constant.
3.3. Effect of diperiodatoargentate(III)
The DPA concentration was varied in the range of
2.0 · 10ꢀ5 to 1.0 · 10ꢀ4 mol dmꢀ3 under constant con-
centrations of DL-methionine and periodate with con-
stant ionic strength in the lower (0.01–0.20 mol dmꢀ3
)
and in the higher (0.30–1.0 mol dmꢀ3) range of OHꢀ
ion concentration. The non-variation in the pseudo-first
order rate constants at various concentrations of DPA
indicates the unit order dependence on DPA concentra-
tion (Table 1). This was also confirmed from the linear-
ity of plots of log(absorbance) versus time up to 80%
completion of the reaction (Fig. 1) (r P 0.9984,
S 6 0.0081).
Regression analysis of experimental data to obtain the
regression coefficient, r and standard deviation, S of points
from the regression line was performed using Microsoft
Excel-2003 programme.
3. Results and discussion
3.4. Effect of DL-methionine
3.1. Stoichiometry and product analysis
The substrate, DL-methionine was varied in the wide
range with all other reactant concentrations and conditions
kept constant. It was observed that the rate of reaction was
increased with increase in methionine concentration from
5.0 · 10ꢀ5–5.0 · 10ꢀ4 mol dmꢀ3 and became optimum at
6.0 · 10ꢀ4; further increase in methionine concentration
the rate of reaction remains constant. Order with respect
to methionine concentration in both lower and higher
OHꢀ ion concentration changed from fractional (0.40) to
zero order (Table 1).
Different sets of reaction mixtures containing different
concentrations of DL-methionine and DPA at constant
ionic strength and alkali were kept for ca. 6 h at
20.0 0.1 ꢁC in an inert atmosphere in a closed vessel.
When DPA concentration was higher than DL-methionine
concentration, the unreacted DPA was found spectropho-
tometrically by measuring the absorbance at 360 nm. The
results indicated that one mole of DPA consumed by one
mole of DL-methionine as in Eq. (1).
O
S
O
O
S
+
-
+ Ag(I)
+
H
ð1Þ
+ Ag(III)
+ OH
HO
HO
CH3
CH3
NH2
NH2
3.5. Effect of alkali
The main reaction product was identified as DL-methionine
sulphoxide, by spot test [13] and it was also confirmed as
below. The reaction mixture was allowed to stand for a
few hours. Then sodium bicarbonate was added and stirred
vigorously, followed by a dropwise addition of benzoyl
Kinetics of oxidation of DL-methionine by alkaline DPA
was studied in a wide range of OHꢀ ion concentration at
constant DPA concentration, DL-methionine concentra-

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K.A. Thabaj et al. / Journal of Molecular Structure 882 (2008) 88–95
91
Table 1
concentration in descending series was found to be ꢀ0.35
and in ascending series +0.45 (Table 2). Thus, the kinetics
of the reaction was studied at both higher and lower range
of OHꢀ ion concentration.
Effects of [DPA], [methionine] and ½IO4ꢀꢁ on the diperiodatoargentate(III)
oxidation of methionine in both lower and higher alkaline medium at
I = 0.50 mol dmꢀ3
[DPA] · 105 [methionine] · 104 ½IO4ꢀꢁ ꢂ 104 [OHꢀ] mol dmꢀ3
(mol dmꢀ3
)
(mol dmꢀ3
)
(mol dmꢀ3
)
0.03
kobs · 102 kobs · 102
(sꢀ1 (sꢀ1
0.3
3.6. Effect of periodate
)
)
Periodate was varied in the range from 1.0 · 10ꢀ4 to
1.0 · 10ꢀ3 at constant [DPA], [DL-methionine], ionic
strength and at both higher (0.30) and lower (0.03) concen-
tration of alkali. Interestingly it was observed that the rate
constants decreased by increasing ½IO4ꢀꢁ in higher alkali
concentration and remains almost constant in lower
concentration of alkali (Table 1). The order with respect
to ½IO4ꢀꢁ was found to be ꢀ0.49 at higher alkali
concentration.
2.0
5.0
6.0
7.0
8.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1.39
1.39
1.38
1.37
1.39
1.39
1.41
0.58
0.58
0.58
0.57
0.58
0.57
0.60
9.0
10.0
0.5
0.5
0.5
0.5
0.5
0.5
0.9
2.0
3.0
5.0
0.1
0.1
0.1
0.1
0.1
0.58
0.81
0.98
1.15
1.41
0.23
0.33
0.43
0.52
0.61
3.7. Effect of added products
0.5
0.5
0.5
0.5
0.5
5.0
7.0
9.0
11.0
13.0
0.1
0.1
0.1
0.1
0.1
1.42
1.68
1.66
1.69
1.66
0.61
0.96
0.96
0.95
0.96
In both lower and higher range of OHꢀ ion concentra-
tion, the one of the product Ag(I) concentration was varied
from 1.0 · 10ꢀ5 to 2.0 · 10ꢀ4 mol dmꢀ3 with all other reac-
tant concentrations and conditions kept constant. The
added Ag(I) did not has any significant effect on the rate
of reaction.
0.5
0.5
0.5
0.5
0.5
5.0
5.0
5.0
5.0
5.0
1.0
2.0
4.0
6.0
8.0
1.42
1.44
1.39
1.40
1.39
0.59
0.46
0.31
0.25
0.22
3.8. Effect of ionic strength (I) and dielectric constant of the
medium (D)
The addition of KNO3 was used to increase the ionic
strength of the reaction medium. There was no effect on
the ꢀrate of reaction at constant DPA, DL-methionine,
IO4 concentration in both higher and lower OHꢀ ion
concentrations.
The dielectric constant of the medium, ‘D’ was varied by
varying the t-butyl alcohol and water percentage. The D
values were calculated from the equation,
1.2
0.9
0.6
0.3
0
(7)
(6)
(5)
(4)
(3)
D ¼ DwV w þ DBV B
(2)
(1)
Table 2
Effect of [alkali] on diperiodatoargentate(III) oxidation of methionine in
alkaline medium
0
1
2
3
[OHꢀ] (mol dmꢀ3
)
kobs · 102 (sꢀ1
)
Time (min)
0.01
0.02
0.03
0.05
0.10
0.20
0.30
0.50
0.60
0.70
1.00
1.92
1.64
1.42
1.17
0.84
0.54
0.60
0.84
0.88
0.92
0.94
Fig. 1. First order plots of the oxidation of methionine by diperiodat-
oargentate(III) in aqueous alkaline medium. [DPA] · 105 mol dmꢀ3 = (1)
2.0, (2) 5.0, (3) 6.0, (4) 7.0, (5) 8.0, (6) 9.0, (7) 10.0.
tion, KIO4 concentration and at constant ionic strength
(Table 1). It was observed that the rate of reaction was
decreased with increase in OHꢀ ion concentration from
1.0 · 10ꢀ2 to 1.0 · 10ꢀ1 mol dmꢀ3 and reaches minimum
at 2.0 · 10ꢀ1; further increase in OHꢀ ion concentration
from 2.0 · 10ꢀ1 to 1.0 mol dmꢀ3 the rate of reaction
was increased. The order with respect to OHꢀ ion
[DPA] = 5.0 · 10ꢀꢀ45 mol dmꢀ3
;
½methionineꢁ ¼ 5:0 ꢂ 10ꢀ4 mol dmꢀ3
;
½IO4ꢀꢁ ¼ 1:0 ꢂ 10 mol dmꢀ3; I = 0.50 mol dmꢀ3
.