Periodate influencing diperiodatocuprate(III) oxidation of sulfur containing amino acid in aqueous alkaline medium

Article abstract of DOI:10.1007/s10953-008-9307-x

The diperiodatocuprate(III) (DPC) oxidation of DL-methionine, a sulfur containing amino acid, was studied spectrophotometrically in alkaline solution. The reaction rate was first order in the concentration of DPC and fractional order in the concentration

Full text of DOI:10.1007/s10953-008-9307-x

J Solution Chem (2008) 37: 1217–1225
DOI 10.1007/s10953-008-9307-x
Periodate Influencing Diperiodatocuprate(III) Oxidation
of Sulfur Containing Amino Acid in Aqueous Alkaline
Medium
K. Sharanabasamma · Manjali S. Salunke ·
Suresh M. Tuwar
Received: 12 December 2007 / Accepted: 4 April 2008 / Published online: 18 July 2008
©
Springer Science+Business Media, LLC 2008
Abstract The diperiodatocuprate(III) (DPC) oxidation of DL-methionine, a sulfur contain-
ing amino acid, was studied spectrophotometrically in alkaline solution. The reaction rate
was first order in the concentration of DPC and fractional order in the concentration of
−
DL-methionine. Increasing the OH concentration decreased the rate of reaction, whereas
−
adding IO4 enhanced the rate. The reaction was preceded by a small initiation period of
about 0.8 minutes. This initiation time decreased when the concentration of IO4 or DPC
−
increased. Adding the reaction products did not alter the rate of reaction. A mechanism
including the intervention of a DL-methionine free radical is proposed and the correspond-
ing rate law is derived. The reaction rate constants are evaluated as well as the activation
parameters.
Keywords Kinetics · Oxidation · Mechanism · Diperiodatocuprate(III) · DL-methionine
1
Introduction
It is known that copper exists in three different oxidation states, +1, +2 and +3, along with
its elemental form. Among these, only the +2 state is stable. However, in the recent past
[
1], some relatively stable copper(III) complexes have been prepared, viz., the periodate,
3
+
2+
guinidine and tellurate complexes. The Cu /Cu reduction potential is −1.8 V in alkaline
solution [2]. Hence, these compounds have been widely used as oxidants in the analysis of
inorganic as well as organic compounds in alkaline media. The other reason for this use
is that no other versatile oxidizing agents (in their stable form) are available in alkaline
media for quantitative analyses. Moreover, it undergoes reduction by a single equivalent.
The kinetics of oxidation by trivalent copper, in particular copper(III) periodate, has received
considerable attention in recent years [3]. Its active forms in alkaline media are known to
be the diperiodatocuprate(III) (DPC) and monoperiodatocuprate(III) (MPC) complexes [4].
K. Sharanabasamma · M.S. Salunke · S.M. Tuwar (ꢀ)
Department of Chemistry, Karnatak Science College, Dharwad 580001, India
e-mail: sureshtuwar@yahoo.co.in

1218
J Solution Chem (2008) 37: 1217–1225
Nevertheless, their molecular formulae have not been described [3, 5]. Thus, the present
study was undertaken to arrive at a plausible molecular formula for its active form.
The sulfur containing amino acid, DL-methionine, is an essential amino acid. It has been
widely used in the treatment of liver diseases, allergy and asthma, rheumatoid arthritis, uri-
nary tract infections, in the immune system for wound healing [6], etc. Daily supplement
of methionine may prevent the onset of diseases related to smoking [7]. A recent study
reports that DL-methionine is an essential molecule in the treatment for Alzheimer’s dis-
ease because the patient normally suffers from sulfur deficiency. DL-methionine is utilized
in the body on par with L-methionine, because D-methionine is deaminated to the corre-
sponding α-keto acid, which produces L-methionine by transamination. However, in the
normal course of oxidation of α-amino acids, deamination and decarboxylation take place.
DL-methionine is one molecule in which no such possibilities exist. Instead, it undergoes
oxidation at its ‘S’ site. Hence, the present investigation was undertaken to understand the
oxidation of DL-methionine and to provide evidence for the molecular formula of the active
form of the copper periodate complex.
2
Experimental
2.1 Materials and Methods
All chemicals used were of reagent grade and double distilled water was used through-
out. Stock solutions of DL-methionine were prepared by dissolving the required quantity
of sample “as received” (S.D. fine), i.e. without further purification. Aqueous solutions of
−
KOH and KNO3 were used to maintain the required [OH ] and ionic strength, respectively.
A constant concentration of periodate was also maintained by using a KIO4 solution in the
reaction mixture, unless otherwise stated.
2.1.1 Preparation of DPC
The diperiodatocuprate(III), DPC, was prepared [8, 9] by oxidizing Cu(II) in an alkaline
medium as follows. Copper sulfate (3.54 g), potassium metaperiodate (6.8 g), potassium
3
persulfate (2.2 g) and KOH (9 g) were added to 250 cm of water. The mixture was heated
to boiling on a hotplate, with constant stirring. The boiling mixture turned intensely red and
the boiling was continued for another 20 minutes to allow the reaction to reach completion.
The mixture was then cooled, filtered through a sintered glass crucible (G-4) and diluted
3
to 250 cm with distilled water. The resulting solution was found to be fairly stable for
several months at room temperature. The DPC was characterized by its UV–Vis spectrum
and compared with literature results [8, 9]. The concentration of the solution was ascertained
by a reported method [8, 9], and also confirmed spectrophotometrically at 415 nm.
2.2 Kinetic Measurements
The reaction kinetics were studied under a pseudo-first-order condition where [DL-M] >
DPC] by at least 20 fold. The reaction was initiated by mixing DPC with DL-M that also
[
contained the required quantities of KOH, KNO3 and KIO4 needed to maintain a constant
alkali concentration, ionic strength and ligand concentration, respectively. The progress of
◦
the reaction was monitored spectrophotometrically at 30 C by measuring the decrease in
absorbance of DPC at its absorption maximum λmax = 415 nm with a molar absorbency

J Solution Chem (2008) 37: 1217–1225
1219
Fig. 1 Effect of the DPC(III)
concentration on the periodate
induced oxidation of
DL-methionine by
diperiodatocuprate(III) in
aqueous alkaline medium at
◦
−4
3
7
3
0 C: [DPC] = 1.0 × 10 (1),
−
5
−5
.7 × 10 (2), 6.1 × 10 (3),
−
5
−5
.8 × 10 (4), 1.8 × 10 (5)
−
3
mol·dm ; [DL-methionine]
−
3
−3
=
1 × 10 mol·dm
,
−
−4
−3
[
IO ] = 2.0 × 10 mol·dm
,
4
−
−2
−3
[
OH ] = 2.0 × 10 mol·dm
,
−
3
and I = 0.1 mol·dm
3
−1
−1
‘
ε’ = 6500± 50 dm ·mol ·cm . Earlier, it was confirmed that there were no other species
present that absorb significantly at this wavelength, and Beer’s law was verified for DPC at
15 nm.
The oxidation of DL-methionine by DPC was initially very slow (up to 0.8 minutes).
4
Hence, pseudo-first-order rate constants were determined only after an appropriate lapse of
time (Fig. 1). The ‘kobs’ values were calculated from the slopes of log (conc.) versus time
1
0
plots, which were found to be linear up to 80% completion of reaction and to be reproducible
to within ± 5%. The reaction orders for the various species were determined from the slopes
of plots of log kobs versus log (conc.), except for DPC for which no variation of ‘kobs’ was
1
0
10
observed for all its concentrations, as expected, due to the reaction condition.
2.3 Stoichiometry and Product Analysis
−
Different sets of concentrations of DL-methionine and DPC were kept at constant [IO ],
4
−
◦
[
OH ] and ionic strength in an inert atmosphere at 30 C for more than 6 hours. When [DPC]
was in excess of [DL-M], the unreacted DPC was estimated by measuring its absorbance at
15 nm. The results indicated that one mole of DL-methionine was consumed by the two
moles of DPC as shown in reaction 1.
4
(1)
The oxidation product of [DL-methionine] was found to be DL-methionine sulfoxide,
which was characterized by the spot test [10] and also confirmed as described below. Af-
ter completion of the reaction, sodium carbonate was added with vigorous stirring along
with drop-wise addition of benzyl chloride solution to give a precipitate of N-benzoyl me-
◦
thionine, whose identity was confirmed by its melting point (183 C) [11]. The product, DL-
methionine sulfoxide, was also confirmed by its IR spectrum that showed a strong absorption
−
1
at 1045 cm , in addition to the normal characteristic bands of the ionized carboxylic group
−
1
and of an amine salt occurring between 3130 and 2500 cm
.

1220
J Solution Chem (2008) 37: 1217–1225
3
Results and Discussion
3.1 Reaction Order
−
−
The reaction orders with respect to [DL-methionine], [IO ] and [OH ] were determined
4
from log kobs versus log (conc.) plots. These orders were obtained by varying the concen-
trations of DL-methionine, IO4 and OH , in turn, while keeping the other concentrations
constant.
1
0
10
−
−
3.2 Effect of [DPC]
−
5
The effect of [DPC] on the reaction rate was studied by varying the [DPC] from 2.0 × 10
−
4
−3
to 2.0 × 10 mol·dm at constant concentrations of DL-methionine, alkali, periodate and
ionic strength as given in Table 1. It was observed that the slope of the plot of log (conc.)
1
0
versus time remained constant for the all of the different [DPC] values. It was also confirmed
by the linearity of the first-order plot after a small initial period. Hence, the order in [DPC]
was concluded to be unity.
3.3 Effect of [DL-Methionine]
−
4
−3
−3
The DL-methionine concentration was varied from 6.0×10 to 5.0×10 mol·dm while
the concentrations all other reactants were held constant. The increase in [DL-methionine]
increased the rate of reaction by an order of ∼0.6 (Table 1).
−
−
3
.4 Effect of [OH ] and [IO ]
4
−
−3
The effect of [OH ] on the reaction rate was studied in the concentration range 5.0 × 10
−
2
−3
to 5.0 × 10 mol·dm while the concentrations all other reactants were held constant
−
(
Table 1). It was noticed that there was a decrease in ‘kobs’ with an increase in [OH ], with
a reaction order of about −0.35.
−
−
−5
Similarly, the effect of [IO ] was also studied from [IO ] = 2.0 × 10 to 4.0 ×
4
4
−
4
−3
1
0
mol·dm , while the concentrations all other reactants were held constant. Adding
−
[
IO ] accelerated the rate by an order of ≈0.3 (Table 1).
4
3.5 Effect of Added Products
The added products, CuSO4 and DL-methionine sulfoxide, did not alter the rate of reaction
in the studied concentration range of 1.0 × 10− to 2.0 × 10 mol·dm
5
−4
−3
.
3.6 Effect of Ionic Strength and Dielectric Constant of the Medium
The ionic strength of the reaction medium was varied by using different amounts of KNO3
−
3
from I = 0.02 to 0.4 mol·dm , while the concentrations all other reactants were held con-
stant. The rate constants decreased with an increase in the ionic strength of the reaction
√
medium. The plot of log kobs versus I was linear. The slope was found to be −0.9
1
0
(Fig. 2).
The effect of polarity of the solvent medium on the rate of reaction was studied by de-
creasing the solvent polarity using t-butyl alcohol + water mixtures as the reaction medium.
The dielectric constants of the medium with various ratios (v/v) of t-butyl alcohol and wa-
ter were calculated by a known method [12]. The kobs values were found to vary little with
a decrease in the dielectric constant of the medium (t-butyl alcohol + water).

J Solution Chem (2008) 37: 1217–1225
1221
−
−
Table 1 Effect of the variation of [DPC], [DL-methionine], [OH ] and [IO ] concentrations on the oxida-
4
◦
−3
tion of DL-methionine by [DPC] at 30 C and I = 0.1 mol·dm
−
4
−3
−
−2
−
−4
−2 −1
)
[
DPC] ×10
[DL-M] ×10
−3
[OH ] × 10
[IO ] × 10
k
× 10 (s
4
obs
−
3
−3
−3
a
b
(mol·dm
)
(mol·dm
)
(mol·dm
)
(mol·dm
)
Exptl.
Calc.
2.0
4.0
6.0
8.0
0
0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.34
1.37
1.34
1.36
1.36
1.36
1.40
1.40
1 40
1.40
1.40
1.40
1
2
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
2.0
4.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.96
1.20
1.36
2.10
2.82
3.20
0.96
1.20
1.40
2.16
2.90
3.18
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
0.8
1.0
2.0
4.0
5.0
2.0
2.0
2.0
2.0
2.0
2.0
2.00
1.73
1.63
1.36
1.05
0.95
1.84
1.72
1.66
1.40
1.08
0.97
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
0.2
0.4
0.6
1.0
2.0
4.0
0.60
0.82
1.02
1.12
1.36
1.60
0.54
0.82
0.99
1.19
1.40
1.55
a
Experimental
Calculated
b
3.7 Polymerization Study
The intervention of free radicals generated during the oxidation of DL-methionine with a
single equivalent of oxidant, DPC, was expected as it is a non-complimentary reaction. This
possibility was tested by adding a free-radical scavenger, acrylonitrile, while the reaction
was in progress. On diluting the reaction mixture with methyl alcohol after the reaction
was complete, a copious precipitate resulted, indicating that the oxidation occurred via the
intervention of a free radical. Earlier, it was ascertained that no precipitate formed with
either the oxidant or reductant alone.

1222
J Solution Chem (2008) 37: 1217–1225
Fig. 2 Effect of ionic strength
on the oxidation of
DL-methionine by DPC at 30 C:
◦
−
4
−3
[
[
DPC] = 1.0 × 10 mol·dm
,
−
3
DL-methionine] = 1.0 × 10
−
−
3
−
−4
mol·dm , [IO ] = 2.0 × 10
4
3
mol·dm , and
−
−2
−3
[
OH ] = 2.0 × 10 mol·dm
Table 2 Effect of temperature
on the oxidation of
(A) Effect of temperature
DL-methionine by alkaline DPC:
−2 −1
)
Temp. (K)
k
× 10 (s
obs
−
4
−3
[
DPC] = 1.0 × 10 mol·dm
,
−
3
[
DL-methionine] = 1.0 × 10
−
3
−
−
−4
303
308
313
1.34
2.48
3.92
6.59
mol·dm , [IO ] = 2.0 × 10
4
−
3
−2
mol·dm , [OH ] = 2.0 × 10
−
3
mol·dm , and I = 0.1
−
3
mol·dm
318
(B) Activation parameters
−
1
)
Ea (kJ·mol
85 ± 2
#
−1
)
ꢀ
ꢀ
ꢀ
H
(kJ·mol
82 ± 2
#
−1
−1
S
(J·K ·mol
)
−9.0 ± 0.2
85 ± 2
#
−1
G (kJ·mol
)
log
A
0
12.7 ± 0.2
1
3.8 Effect of Temperature
The effect of temperature on the reaction rate was studied at various temperatures (Table 2)
at constant concentrations of oxidant and reductant, with the other reaction conditions being
kept constant. The kobs values increased with increasing temperature. The activation para-
meters were calculated and are tabulated in Table 2.
A powerful oxidant, the copper(III) periodate complex in alkaline medium is relatively
stable compared to other copper(III) complexes. Malaprade et al. [1] and Rozovski et al.
[
13, 14], and Jensovsky and Rabe [15] have prepared a copper(III) periodate complex in
2
pure form. It was found to be crystalline, diamagnetic and have a dsp configuration with a
square-planar structure.
Water soluble DPC in alkaline medium shows an UV-Vis spectrum [15] with two strong
bands at 413 and 265 nm, one fairly strong band at 334 nm, and a weak one at 560 nm. When
solid K3H3[Cu(IO6)2] · 4H2O is dissolved in water it undergoes ionization to give a com-
x
y
z
plex ion [16]. In alkaline solution it may exist as [Cu(HL)2] , [CuL(HL)] and [Cu(HL)] ,
where, HL contains an indefinite number of protons and L denotes the unprotonated perio-
date molecule in the complex.

J Solution Chem (2008) 37: 1217–1225
1223
−
2−
6
3−
However, it is well known that periodate exists [17] as H5IO6, H4IO , H3IO , H2IO6
6
and dimeric forms, depending on the pH of the medium and total concentration of perio-
−
−3
date. In the present study, the [OH ] was varied from 0.005 to 0.05 mol·dm . Thus, the
2
6
−
3−
6
periodate species should be in the form of H3IO and H2IO . The other species are ex-
pected to occur in acidic medium. The existence of dimeric forms can be ruled out as they
−
−
occur only at higher [IO ]. Consequently, the active forms of IO can be considered as
4
4
being H3IO3 and H2IO . Thus, as an alternative to the notation HL and L, respectively,
−
2−
6
6
x
y
z
and on incorporating the same into the [Cu(HL) ], [CuL(HL) ] and [Cu(HL) ] formulae,
2
−
2−
+
leads the structures [Cu(H IO6) ], [Cu(H IO6)(H IO6) ] and [Cu(H3IO6)(H2O) ]. These
3
2
2
3
2
structures retain the dsp configuration with square-planar structures. The bidentate ligands
H3IO2 and H2IO6 satisfy the coordination number of Cu . Similar structures of peri-
−
3−
3+
6
odate complexes have been deduced for the Ni(IV) and Ag(III) periodate complexes [18,
1
9].
In the present study, the observations that the reaction is first order in [DPC], fractional
−
order in [DL-methionine], retarded by increasing [OH ], and enhanced by increasing [peri-
odate] can be accommodated in the following mechanism:
K1
+
3−
2−
Cu(H IO6)(H O) + H2IO →Cu(H IO6)(H IO6) + 2H2O
3
2
2
6
3
2
←
K2
2
−
−
2
−
Cu(H IO6)(H IO6) + H2O→Cu(H IO6) + OH
3
2
3
←
K3
−
2
Cu(H IO6) + DL − M→complex(C)
3
←
•
k
complex(C) → DL−M+Cu + 2H3IO 62 −
2
+
slow
•
DL − M+Cu(H IO6) →Cu + DL − MSO + 2H3IO 62 −
−
2+
3
2
fast
Scheme 1
In most of the studies of kinetics of periodate related metal complexes, the added peri-
odate either had a retarding effect or no effect on the rate of reaction. In the present study,
the accelerating effect of periodate on the reaction rate might result from the formation of
DPC from MPC(monoperiodatocuprate(III)). This could occur during the small initial time
lapse after the start of the reaction. Evidence for this is the relatively small initial reaction
rate. This indicates that an optimum concentration of [Cu(H3IO6)2] is necessary to trans-
form DL-methionine to DL-methionine sulfoxide, which is formed by the combination of
monoperiodatocuprate(III) and periodate (Steps 1 and 2).
Scheme 1 also incorporates the intervention of a free radical during the transformation
−
of DL-methionine to DL-methionine sulfoxide. The retarding effect of [OH ] on the rate
of reaction was due to the formation of cuprate(III) periodate by the hydrolysis of the less
protonated diperiodatocuprate(III) (Step 2). Scheme 1 leads to the following rate law:
d[DPC]
−
dt
2
6
−
kK1K2K3[Cu(III)]T[H3IO ][DL-M]T
=
rate =
−
−
2−
2−
2−
6
[
OH ] + K1[OH ][H3IO ] + K1K2[H3IO ] + K1K2K3[H3IO ][DL-M]
6
6
(2)

1224
J Solution Chem (2008) 37: 1217–1225
Fig. 3 Verification of the rate
law for oxidation of
DL-methionine by alkaline DPC
◦
at 30 C (conditions as in
Table 1)
2
6
−
kK1K2K3[H3IO ][DL-M]T
kobs =
(3)
−
−
2−
2−
[
OH ] + K1[OH ][H3IO ] + K1K2[H3IO ] + K1K2K3[H3IO 26 −][DL-M]
6
6
−
Rate law (3) in the form of (4) is verified by plotting 1/kobs versus [OH ], 1/[DL-M]
2
−
and 1/[H3IO6 ], all of which should be linear and is found to be so as shown in Fig. 3.
−
−
1
[OH ]
[OH ]
1
1
=
+
+
+ k
.
(4)
−
6
2
kobs
kK K K [H IO ][DLM] kK2K3[DLM] kK3[DL-M]
1
2
3
3
−
2
−1
s
The slopes and intercepts of such plots lead to the constants k = 4.63 × 10
, K1 =
3
3
−1
−2
−3
2
3
−1
3
.28 × 10 dm ·mol , K2 = 6.71 × 10 mol·dm , and K3 = 7.70 × 10 dm ·mol
.
These values were used in (3) to regenerate the kobs values and the results were compared
with experimental results. They are found to be reasonably consistent with each other (Ta-
ble 1). This strengthens the evidence for the proposed mechanism.
The mechanism written in Scheme 1 does not account for the decrease in rate with in-
creasing ionic strength, as it is generally expected that an amino acid in alkaline solution
exists in its anionic form. The anion of DL-methionine and other anionic forms of DPC are
involved in a pre-equilibrium step for which a positive effect of ionic strength is anticipated.
However, such a contradicting observation may be explained by assuming that some of the
amino acids in alkaline pH exists in a zwitterionic form [20–22] that can be described as
(5)
The positive part of this dipolar molecule may be oriented toward the negatively charged
DPC, which would retard the rate of reaction when the ionic strength of the reaction media
is increased.
The effect of polarity on the reaction medium is in the expected direction if the oppositely
charged species that are involved in the mechanism form a single activated complex sphere,
by losing a degree of freedom in the low dielectric constant medium. This is strongly sup-
#
ported [23] by a negative value of ꢀS and large value of log A (Table 2). A relatively large
1
0

J Solution Chem (2008) 37: 1217–1225
1225
value of kobs also strengthens the argument that the reaction occurs through the interaction
of oppositely charged ions to form a single sphere at the slow step of the reaction.
Acknowledgements Suresh M. Tuwar thanks the UGC, Regional Office Bangalore, for providing financial
assistance and Mrs. Sharanabasamma K. is grateful to the principal, Proudhadevaraya Institute of Technology,
Hospet for supporting the research work.
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