A kinetic and mechanistic study on the oxidation of L-cystine by alkaline diperiodatocuprate(III): A free radical intervention

Article abstract of DOI:10.1134/S0023158409040090

The kinetics of oxidation of L-cystine (L-CYS) by diperiodatocuprate (III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.20 mol/1 was studied spectrophotometrically at 298 K. The reaction between DPC and L-cystine in alkaline medium e

Full text of DOI:10.1134/S0023158409040090

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2009, Vol. 50, No. 4, pp. 530–539. © Pleiades Publishing, Ltd., 2009.
A Kinetic and Mechanistic Study on the Oxidation
of LꢀCystine by Alkaline Diperiodatocuprate(III):
A Free Radical Intervention1
R. R. Hosamani, N. P. Shetti, and S. T. Nandibewoor
Post Graduate Department of Studies in Chemistry, Karnatak University, Dharwad, India
eꢀmail: stnandibewoor@yahoo.com
Received March 21, 2008
Abstact—The kinetics of oxidation of Lꢀcystine (LꢀCYS) by diperiodatocuprate (III) (DPC) in aqueous
alkaline medium at a constant ionic strength of 0.20 mol/1 was studied spectrophotometrically at 298 K. The
reaction between DPC and Lꢀcystine in alkaline medium exhibits 1 : 4 stoichiometry (Lꢀcystine: DPC =1 : 4).
The reaction is of first order in [DPC] and has less than unit order in [LꢀCYS] and [alkali], negative fractional
order in [periodate] and intervention of free radicals was observed in the reaction. The oxidation reaction in
alkaline medium has been shown to proceed via a monoperiodatocuprate(III)ꢀLꢀystine complex, which
decomposes slowly in a rate determining step followed by other fast steps to give the products. The main prodꢀ
ucts were identified by spot test, IR and GCꢀMS. The reaction constants involved in the different steps of the
mechanism are calculated. The activation parameters with respect to slow step of the mechanism are comꢀ
puted and discussed and thermodynamic quantities were also determined.
DOI: 10.1134/S0023158409040090
1
Amino acids act not only as the building blocks in tellurate complexes of copper in its trivalent state have
protein synthesis but they also play a significant role in
metabolism and have been oxidized by different oxiꢀ
dizing agents. The study of the oxidation of amino
acids is of interest because of their biological signifiꢀ
cance and selectivity towards the oxidant to yield the
different products [1–3].
been extensively used in the analysis of several organic
compounds [7]. The kinetics of selfꢀdecomposition of
these complexes was studied in some detail [8]. Copꢀ
per(III) is shown to be an intermediate in the copꢀ
per(II) catalyzed oxidation of amino acids by peroxyꢀ
disulphate [9]. The oxidation reaction usually involves
the copper (II)ꢀcopper(I) couple and such aspects are
detailed in different reviews [10, 11]. The use of dipeꢀ
riodatocuprate(III) (DPC) as an oxidant in alkaline
medium is new and restricted to a few cases due to the
fact of its limited solubility and stability in aqueous
medium. DPC is a versatile oneꢀelectron oxidant for
various organic compounds in alkaline medium and its
use as an analytical reagent is now well recognized
[12]. Copper complexes have occupied a major place
in oxidation chemistry due to their abundance and relꢀ
evance in biological chemistry [13, 14]. Copper(III) is
involved in many biological electron transfer reactions
[15]. They have also been used [16] in the differential
titration of organic mixtures, in the estimation of
chromium, calcium and magnesium from their ores,
antimony, arsenic and tin from their alloys. Since mulꢀ
tiple equilibria between different copper(III) species
are involved, it would be interesting to know which of
the species is the active oxidant.
Lꢀcystine is a covalently linked dimeric nonꢀessenꢀ
tial amino acid formed by the oxidation of cystine.
Two molecules of Lꢀcysteine are joined together by a
disulfide bridge (S–S) to form Lꢀcystine. Lꢀcystine is
sulfur containing chemical substance, which naturally
occurs as a deposit in the urine, and can form a calcuꢀ
lus (hard mineral formation) when deposited in the
kidney. Lꢀcystine is required for proper vitamin B6
utilization and is also helpful in the healing of burns
and wounds breaking down mucus deposits in illnesses
such as bronchitis as well as cystic fibrosis. Lꢀcystine
also assists in the supply of insulin to the pancreas.
In recent years, the study of highest oxidation state
of transition metals has intrigued many researchers.
Transition metals in a higher oxidation state can be
stabilized by chelation with suitable polydentate
ligands. Metal chelates such as diperiodatocuꢀ
prate(III) [4], diperiodatoargentate(III) [5] and dipeꢀ
riodatonickelate(IV) [6] are good oxidants in a
medium with an appropriate pH value. Periodate and
1
The article is published in the original.
530

A KINETIC AND MECHANISTIC STUDY ON THE OXIDATION OF LꢀCYSTINE
531
Absorbance, arb. units
0.4
1
0.3
0.2
0.1
0
2
3
4
5
300
400
500 600
Wavelength, nm
–5
Fig. 1. Spectroscopic changes occurring in the oxidation of Lꢀcystine by diperiodatocuprate(III) at 25°C ([DPC] = 5.0
× 10 ,
ꢀ4
–
[LꢀCYS] = 5.0
×
10 , [OH ] = 0.08, [I] = 0.20 mol/l. Scanning time interval 0.5 min (curves 1–5).
Relative intensity, %
150
100
80
60
130
50
40
20
0
102
77
64
38
25
50
75
100
125
150 m/z
Fig. 2. GCꢀmass spectrum of acetaldehyde (2ꢀoxoꢀethyldisulfanyl) with its molecular ion peak at 150 amu.
Literature survey reveals that there are no reports verified by its UVꢀvis spectrum, which showed an
on the oxidation of Lꢀcystine (LꢀCYS) by diperiodaꢀ
tocuprate(III). The present study deals with the title
reaction to investigate the redox chemistry of DPC in
alkaline media to arrive at a suitable mechanism on the
basis of kinetic and spectral results and to compute the
thermodynamic quantities of various steps of Scheme.
absorption band with maximum absorption at 415 nm.
The aqueous solution of copper(III) was standardized
by iodometric titration and gravimetrically by thiocyꢀ
anate method [18]. The copper(II) solutions were preꢀ
pared by dissolving the known amount of copper sulꢀ
phate (BDH) in distilled water. Periodate solution was
prepared by weighing out the required amount of samꢀ
ple in hot water and used after keeping it for 24 h to
maintain the equilibrium. Its concentration was ascerꢀ
tained iodometrically [19] at neutral pH by phosphate
buffer. Since periodate is present in excess in DPC, the
possibility of oxidation of Lꢀcystine by periodate in
alkaline medium at 25°C was tested. The progress of
the reaction was followed iodometrically. It was found
that there was no significant reaction under the experꢀ
imental conditions employed compared to the DPC
oxidation of Lꢀcystine. KOH and KNO3 (BDH, AR)
were employed to maintain the required alkalinity and
EXPERIMENTAL
All chemicals used were of reagent grade and douꢀ
ble distilled water was used throughout the work.
A solution of Lꢀcystine (BDH Industries Ltd.) was
prepared by dissolving an appropriate amount of
recrystallised sample in double distilled water. The
purity of Lꢀcystine sample was checked its m.p. 239°C
(literary m.p. 240°C). The required concentration of
Lꢀcystine was used from its stock solution. The copꢀ
per(III) periodate complex was prepared by standard
procedure [17]. Existence of copper(III) complex was ionic strength respectively in reaction solutions.
KINETICS AND CATALYSIS Vol. 50
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532
HOSAMANI et al.
A [arb. units]
3 + log
3.0
2.5
2.0
1.5
1.0
0.5
5
4
3
2
1
0
2
4
6
8
Time, min
Fig. 3. First order plots for the oxidation of Lꢀcystine by diperiodatocuprate(III) in aqueous alkaline medium at 25°C. [Diperioꢀ
5
datocuprate(III)] × 10 , mol/l: (1) 1.0, (2) 3.0, (3) 5.0, (4) 8.0, (5) 10.0.
The kinetic measurements were performed on a ence in the results was obtained under a N2 atmoꢀ
Varian CARY 50 Bio UVꢀVisible Spectrophotometer. sphere and in the presence of air. In view of the ubiqꢀ
The kinetics was followed under pseudoꢀfirst order uitous contamination of carbonate in the basic
condition where [LꢀCYS] > [DPC] at 25 0.1°C
,
medium, the effect of carbonate was also studied.
unless specified. The reaction was initiated by mixing Added carbonate had no effect on the reaction rates.
the DPC to Lꢀcystine solution which also contained The spectral changes during the reaction are shown in
required concentration of KNO3, KOH, and KIO4
;
(Fig. 1). It is evident from the figure that the concenꢀ
and the progress of reaction was followed spectrophoꢀ tration of DPC decreases at 415 nm.
tometrically at 415 nm by monitoring the decrease in
absorbance due to DPC with the molar absorbancy
Regression analysis of experimental data to obtain
the regression coefficient and standard deviation
of points from the regression line was performed using
Microsoft Excelꢀ2003 programme.
r
S,
index
= 6230 100 1 mol–1 cm–1. It was verified that
ε
there is a negligible interference from other species
present in the reaction mixture at this wavelength.
The pseudoꢀfirst order rate constants (kobs) were
determined from the log (Absorbance) versus time
plots. The plots were linear up to 85% completion of
reaction under the range of [OH–] used. During the
RESULTS AND DISCUSSION
Stoichiometry and Product Analysis
Different sets of reaction mixtures containing varyꢀ
ing ratios of DPC to Lꢀcystine in presence constant
amounts of OH– and KNO3 were kept for 6 h in closed
vessel under inert atmosphere. The remaining DPC
concentration was estimated by spectrophotometriꢀ
cally at 415 nm. The results indicate 1 : 4 (LꢀCYS :
kinetics a constant concentration viz. 1.0
10–4 mol/1
×
of KIO4 was used throughout the study unless otherꢀ
wise stated.
Kinetics runs were also carried out in N2 atmoꢀ
sphere in order to understand the effect of dissolved
oxygen on the rate of reaction. No significant differꢀ DPC) stoichiometry as given in Eq. (I).
–
HOOC–CH–CH –S–S–CH –CH–COOH + 4[Cu(OH) (H IO )]
2 2 2 3 6
(I)
NH
NH
2
2
2H O
2
2–
+
OHC–CH –S–S–CH –CHO + 2NH + 4Cu(OH) + 4H IO + 2CO + 4H
2 2 3 6 2
3
2
The main reaction product was identified as band at 2854 cm–1 due to aldehydic
−
CH stretching.
(2 oxoꢀethyl disulfanyl)ꢀacetaldehyde by its IR specꢀ Nessler’s reagent identified ammonia. Preparing its
trum (KBr): carbonyl stretching at 1742 cm–1 and a 2,4ꢀDNP derivative identified the product aldehyde.
KINETICS AND CATALYSIS Vol. 50
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A KINETIC AND MECHANISTIC STUDY ON THE OXIDATION OF LꢀCYSTINE
533
Table 1. Effect of [DPC], [LꢀCYS], [OH–], and [IO4–] on the oxidation of Lꢀcystine by diperiodatocuprate(III) in alkaline
medium at 25°C (I = 0.20 mol/l)
[IO4–]
×
104, mol/l
1.0
[DPC]
×
105, mol/l [LꢀCYS]
×
104, mol/l
[OH–], mol/l
kobs
×
103, s–1 kcal
×
103, s–1
1.0
5.0
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.02
0.05
0.08
0.10
0.20
0.08
0.08
0.08
0.08
0.08
6.1
6.0
6.1
5.9
6.0
4.3
6.1
6.2
6.2
6.2
6.2
6.2
4.2
6.3
3.0
5.0
8.0
10.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3.0
5.0
10.0
20.0
30.0
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
0.5
0.8
1.0
3.0
5.0
10.0
10.0
14.9
17.5
4.7
5.5
6.1
6.5
7.0
7.0
6.4
6.1
5.1
4.2
14.3
16.7
4.6
5.8
6.3
6.4
6.7
6.7
6.4
6.3
4.9
4.1
Additionally GCꢀMS analysis was carried out on a constant kobs values indicate that order with respect to
17A Shimadzu gas chromatograph with a QPꢀ5050A
Shimadzu mass spectrometer.
[DPC] was one (Table 1). This was also confirmed by
linearity of the plots of log versus time ( 0.983,
A
r
≥
S
≤
0.012) up to 80% completion of the reaction (Fig. 3).
The effect of Lꢀcystine on the rate of reaction was
studied at constant concentrations of alkali, DPC and
periodate at a constant ionic strength of 0.20 mol/l.
The substrate Lꢀcystine was varied in the range of
The mass spectra confirmed the presence of
(2ꢀoxoꢀethyldisulfanyl)ꢀacetaldehyde (Fig. 2), which
showed both molecular ion peak and base peak at
m/z = 150. All other peaks observed in GCꢀMS can be
interpreted in accordance with the observed structure
of the (2ꢀoxoꢀethyldisulfanyl)ꢀacetaldehyde.
3.0
×
10–4 to 3.0 10–3 mol/l. The kobs values increased
×
with increase in concentration of Lꢀcystine. The order
with respect to [Lꢀcystine] was found to be less than
Reaction Orders
unity (Table 1) (
r
≥
0.998,
S
≤
0.008). The effect of
The reaction orders were determined from the
slope of logkobs versus log (Concentration) plots by
varying the concentrations of Lꢀcystine, alkali and
periodate in turn while keeping all other concentraꢀ
tions and conditions constant.
alkali on the reaction has been studied in the range of
0.02 to 0.20 mol/l at constant concentrations of Lꢀcysꢀ
tine, DPC and a constant ionic strength of 0.20 mol/l.
The rate constants increased with increasing [alkali]
and the order was found to be less than unity
The oxidant DPC concentration was varied in the
(
r
≥
0.981,
S 0.006) (Table 1). The effect of increasꢀ
≤
range of 1.0
×
10ꢀ5 to 1.0
×
10–4 mol/l and the fairly ing concentration of periodate was studied by varying
KINETICS AND CATALYSIS Vol. 50
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2009

534
HOSAMANI et al.
the periodate concentration from 5.0
×
10–5 to 5.0
×
employed in alkaline medium, the main species are
10–4 mol/l keeping all other reactant concentrations
constant. It was found that the added periodate had a
retarding effect on the rate of reaction. The order with
respect to periodate concentration being negative less
than unity (Table 1).
2−
expected to be
and
3−. At higher concenꢀ
H3IO6
H2IO6
trations, periodate also tends to dimerise [23]. Howꢀ
ever, formation of this species is negligible under conꢀ
ditions employed for kinetic study. Hence, at the
pH employed in this study, the soluble copper(III)
periodate complex exists as diperiodatocuprate(III),
[Cu(H3IO6)2(OH)2]3–, a conclusion also supported by
earlier work [24].
It was found that ionic strength and dielectric conꢀ
stant of the medium had no significant effect on the
rate of reaction. The externally added products,
(2ꢀoxoꢀethyldisulfanyl)ꢀacetaldehyde and copper(II)
The reaction between the diperiodatocuprate(III)
complex and Lꢀcystine in alkaline medium had a stoꢀ
ichiometry 1 : 4 (LꢀCYS : DPC) with a first order
dependence on [DPC], apparent less then unite order
in [LꢀCYS] and [OH] and a negative fractional order
in [periodate]. No effect of added products was
observed.
The result of increase in rate of reaction with
increase in alkalinity (Table 1) can be explained
in terms of prevailing equilibrium of formation of
[Cu(OH)2(H3IO6)(H2IO6)]4– from [Cu(OH)2(H3IO6)2]3–
deprotonated DPC as given in the following Eq. (V).
(
CuSO4), did not have any significant effect on the
rate of the reaction.
The intervention of free radicals in the reaction was
examined as follows. The reaction mixture, to which a
known quantity of acrylonitrile monomer was initially
added, was kept for 2 h in an inert atmosphere.
On diluting the reaction mixture with methanol, a
white precipitate of polymer was formed, indicating
the intervention of free radicals in the reaction. The
blank experiments of either DPC or Lꢀcystine alone
with acrylonitrile did not induce any polymerization
under the same condition as those induced for reacꢀ
tion mixture. Initially added acrylonitrile decreased
the rate of reaction indicating free radical intervenꢀ
tion, which is the case in earlier work [20].
The kinetics was studied at four different temperaꢀ
tures under varying concentrations of Lꢀcystine, alkali
and periodate, keeping other conditions constant. The
rate constants were found to increase with increase in
temperature. The rate constant (
Scheme were obtained from the slopes and intercepts
[Cu(OH)2(H3IO6) ]3– + OH–
2
(V)
K
[Cu(OH)2(H3IO6)(H2IO6)]4– + H2O.
1
[H3IO62−]
Also decrease in rate with increase in
k) of the slow step of
(Table I) suggest that equilibrium of copper(III) periꢀ
odate complex to form monoperiodatocuptrate(III)
(MPC) species as given in Eq. (VI) is established.
2−
of 1/kobs versus 1/[LꢀCYS], 1/[OH–], and
plots at four different temperatures and were used to
calculate the activation parameters. The energy of
activation corresponding to these constants was evaluꢀ
ated from the Arrhenius plot of log
(
[H3IO6 ]
4–
[Cu(OH)2(H3IO6)(H2IO6)]
k
versus 1/
T
(VI)
K
–
3–
2
r
≥
0.9802, 0.009) and other activation parameꢀ
S
≤
[Cu(OH)2(H3IO6)] + [H2IO6]
.
ters obtained are tabulated in Table 2.
The waterꢀsoluble copper(III) periodate complex
is reported [21] to be [Cu(HIO6)2(OH)2]7–. However,
in an aqueous alkaline medium and at a high pH range
as employed in the study, periodate is unlikely to exist
Such type of equilibria (V) and (VI) have been well
noticed in literature [25]. It may be expected that a
lower periodate complex such as MPC is more imporꢀ
tant in the reaction than the DPC. The inverse fracꢀ
2−
as
4− (as present in the complex) as is evident from
HIO6
tional order in
might also be due to this reaꢀ
[H3IO6 ]
its involvement in the multiple equilibria [22] (II)–
(IV) depending on the pH of the solution.
son. Therefore, MPC might be the main reactive form
of the oxidant.
The less than unit order in [LꢀCYS] presumably
results from formation of a complex (C) between the
MPC species and Lꢀcystine prior to the formation of
the products. This complex C decomposes in a slow
step to form a free radical derived from Lꢀcystine. This
free radical species further reacts with another moleꢀ
cule of MPC in a fast step to form 2ꢀaminoꢀ3ꢀpropiꢀ
onic acid intermediate, This intermediate then reacts
with two more molecules of MPC in a further fast steps
H4IO6−
H3IO62−
H2IO36−
H5IO6
H4IO6−
H3IO62−
+ H+,
+ H+,
+ H+.
(II)
(III)
(IV)
Periodic acid exists in acid medium as H5IO6 and as
H4IO6−
around pH 7. Thus, under the conditions to form the products as given in Scheme.
KINETICS AND CATALYSIS Vol. 50
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A KINETIC AND MECHANISTIC STUDY ON THE OXIDATION OF LꢀCYSTINE
535
K
[Cu(OH)2(H3IO6) ]3– + OH–
[Cu(OH)2(H3IO6)(H2IO6)]4– + H2O
1
2
K
[Cu(OH)2(H3IO6)] + H2IO63–
4–
–
2
[Cu(OH)2(H3IO6)(H2IO6)]
K
3
–
[Cu(OH) (H IO )] + HOOC–CH–CH –S–S–CH –CH–COOH
2
Complex(C)
3
6
2
2
NH
NH
2
2
•
HOOC–CH–CH –S–S–CH –CH + Cu(OH) + H IO + H + CO
2 2 2 3 6 2
Slov
2–
+
Complex(C)
k
NH
NH
2
2
•
HOOC–CH–CH –S–S–CH –CH + [Cu(OH) (H IO )]
–
2
2
2
3
6
NH
NH
2
2
fast
2–
+
HOOC–CH–CH –S–S–CH –CHO + NH + H + Cu(OH) + H IO
6
2
2
3
2
3
H O
2
NH
2
•
CH–CH –S–S–CH –CHO
2 2
– fast
HOOC–CH–CH –S–S–CH –CHO + [Cu(OH) (H IO )]
2 2 2 3 6
NH
NH
2
2
2–
+
+ Cu(OH) + H IO + CO + H
2 3 6 2
•
CH–CH –S–S–CH –CHO + [Cu(OH) (H IO )]
fast
–
OHC–CH –S–S–CH –CHO
2 2
2
2
2
3
6
H O
2
NH
2
2–
+
+ NH + Cu(OH) + H IO + H
3 2 3 6
Scheme.
Since Scheme is in accordance with the generally wellꢀ Spectroscopic evidence for the complex formation
accepted principle of nonꢀcomplementary oxidations takꢀ
ing place in sequence of oneꢀelectron steps, the reaction
between the substrate and oxidant would afford a radical
intermediate. A free radical scavenging experiment
revealed such a possibility (see IR spectra). This type of
radical intermediate has also been observed in earlier work
between oxidant and substrate was obtained from
UVꢀVis spectra of Lꢀcystine (5.0
10–4 mol/l), DPC
5.0
0–5 mol/l), [OH–] = 10.08 (mol/l) and mixꢀ
×
(
×
ture of both. A bathochromic shift of about 4 nm
from 365 to 369 nm in the spectra of Lꢀcystine was
observed. The Michael isꢀMenten plot also proved
the complex formation between DPC and Lꢀcystine,
which explains the less than unit order dependence
on [LꢀCYS]. Such type of complex between a subꢀ
strate and an oxidant has been observed in other
studies [26].
[26]. The probable structure of the complex is given by:
–
O
OH
O
OH
Cu
I
O
HO
CH
OH
OH
OH
O
C
HOOC
CH
CH2
S
S
CH2
NH2
NH2
Scheme leads to the rate law (1):
kK1K2K3[LꢀCYS][OH−][Cu(OH)2(H3IO6)2]3−
[H2IO36−]
−d[DPC]
dt
(1)
(2)
Rate =
=
,
kK1K 2K 3[LꢀCYS][OH −]
[H2IO36−] + K1K 2K 3[OH −][LꢀCYS] + K1K 2[OH −] + K1[H2IO36−][OH −]
Rate
[DPC]
= kobs
=
.
KINETICS AND CATALYSIS Vol. 50
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2009

536
HOSAMANI et al.
Table 2. Thermodynamic activation parameters for the oxꢀ
idation of Lꢀcystine by DPC in aqueous alkaline medium
with respect to the slow step of Scheme
This explains all the observed kinetic orders of differꢀ
ent species. The rate law (2) can be rearranged in to the
following form, which is suitable for verification:
A. Effect of temperature
[H2IO36−]
kK1K2K3[OH−][LꢀCYS]
Temperature, K
k
×
102, s–1
1
kobs
=
(3)
293
298
303
308
1.66
2.50
5.64
8.14
[H2IO36−]
kK2K3[LꢀCYS] kK3[LꢀCYS] k
1
1
+
+
+ .
According to equation (3), other conditions being
constant, plots of 1/kobs versus 1/[OH] ( 0.996,
0.012
r
≥
S
≤
B. Activation parameters (Scheme)
Parameters
0.014), 1/kobs versus 1/[LꢀCYS] (
r
≥
0.992,
S
≤
)
Values
2−
and 1/kobs versus
(r
≥
0.987,
S
≤
0.011) should
[H3IO6 ]
Ea
83 1 kJ/mol
81 2 kJ/mol
82 3 kJ/mol
13.0 0.2
be linear and are found to be so (Fig. 4). The slopes and
intercepts of such plots lead to the values of K1 K2 K3
and as (1.27 0.04) mol/l, (0.50 0.02)
10–2 mol/l,
(8.15 0.12)
102 1/mol, and (2.50 0.01) 10–2 s–1
#
,
,
Δ
H
k
×
Δ
G#
×
log
A
respectively. The equilibrium constant K1 is far
greater than K2. This may be attributed to the greater
tendency of DPC to undergo hydrolysis compared to
the dissociation of hydrolyzed species in alkaline
medium. The value of k1 is good agreement with earꢀ
lier literature [25].
C. Effect of temperature to calculate K1, K2, and K3 for the
oxidation of Lꢀcystine by diperiodatocuprate(III) in alkaꢀ
line medium
K2
×
102,
K3
l/mol
×
10–2,
Temperature, K
K1, mol/l
mol/l
293
298
303
308
0.12 0.01 0.76 0.03
1.27 0.04 0.50 0.02
1.98 0.04 0.24 0.05
3.38 0.05 0.17 0.05
8.5 0.2
8.1 0.1
6.6 0.3
5.7 0.3
The thermodynamic quantities for the first, second
and third equilibrium steps of Scheme can be evaluꢀ
ated as follows. The
2− , [LꢀCYS], and [OH–
]
[H3IO6 ]
(as in Table 1) were varied at four different temperaꢀ
tures. The plots of 1/kobs versus 1/[OH–], 1/kobs versus
D. Thermodynamic quantities using K1
, K2, and K3
2−
1/[LꢀCYS] and 1/kobs versus
should be linear
[H3IO6 ]
Thermodynamic Values from Values from Values from
K2
(Fig. 4). From the slopes and intercepts, the values of
K1 K2, and K3 were calculated at different temperatures
and these values are given in Table 2. The vant Hoff’s
plots were made for variation of K1 K2, and K3 with
temperature (logK1 versus 1/ 0.985, S 0.004)
logK2 versus 1/ 0.973, 0.007), and log K3 verꢀ
sus 1/ 0.992,
enthalpy of reaction
quantities
, kJ/mol
K1
K3
,
Δ
Δ
Δ
H
154
520 10 –303
–0.6 0.3 13.1 0.8 –16.6 0.8
3
–77.4 0.8 –20.5 0.8
S
, J K–1 mol–1
5
–13.1 0.8
,
T
(r
≥
≤
,
G
298, kJ/mol
T
(r
≥
S
≤
T
(r
≥
S
≤
0.008)) and the values of
Table 3. The rate constant of the slow steps at different temꢀ
peratures of some acids (for isokinetic temperature)
ΔH
and free energy of reaction
ΔG were calculated for the first, second and third equiꢀ
k
1 at 298 K,
s
k
2 at 303 K,
s
Amino acids
Reference
l mol–1 –1
l mol–1 –1
librium steps. These values are given in Table 2.
A comparison of the thermodynamic quantities of first
step of Scheme with those obtained for the slow step of
the reaction shows that these values mainly refer to the
rate limiting step, supporting the fact that the reaction
before rate determining step is fairly fast and involves
low activation energy [27].
LꢀTryptophan
LꢀAspartic acid
LꢀLysine
0.014
0.012
0.021
0.025
0.019
0.016
0.041
0.056
[30]
[31]
[32]
LꢀCystine
Present work
KINETICS AND CATALYSIS Vol. 50
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2009

A KINETIC AND MECHANISTIC STUDY ON THE OXIDATION OF LꢀCYSTINE
537
1/[OH–], l/mol (
3)
3 + log
1.8
k
2 [s–1]
60
300
40
20
0
250
200
150
100
50
0
4
250
200
150
100
50
1.6
1.4
1.2
50
1
3
100
150
200
250
2
2
3
1
1.0
1.0
0
1000 2000 3000 4000
1/[LꢀCYS], l/mol (
1.1
1.2
1.3
1.4
3 + log
1.5
k
1 [s–1]
2
)
0
Fig. 5. Plot of log
k at 303 K versus logk at 298 K for isoꢀ
2 1
0
0.0002
0.0004
0.0006
[H2IO63–], mol/l (
1
)
kinetic temperature (Table 3): (
1) Lꢀtryptophan, (2) Lꢀ
aspartic acid, ( ) Lꢀlysine, ( ) Lꢀcystine.
3
4
Fig. 4. Verification of rate law (1) for the of oxidation of
Lꢀcystine by diperiodatocuprate(III) at 25°C
.
rate is governed by the enthalpy of activation [29]. The
linearity and the slope of the plot obtained may conꢀ
firm that the kinetics of this reaction follows a similar
mechanism, as previously suggested.
The rate constants of the slow step at different temꢀ
perature of some acids by DPC are summarized in
Table 3. According to Exner [28], if the rates of several
reactions in a series have been measured at two temꢀ
peratures and logk2 (at T2) is linearly related to logk1
Among various species of DPC in alkaline
medium, monoperiodatocuprate(III) is considered as
active species for the title reaction. The results indicate
that pH in the reaction medium is crucial. Rate conꢀ
stant of slow step and other equilibrium constants
involved in the mechanism are evaluated and activaꢀ
tion parameters with respect to slow step of reaction
were computed. The overall mechanistic sequence
described here is consistent with product studies,
(at T1), i.e., logk2
=
a
+
b
logk1, he proposes that
β
can
be evaluated from the equation
β
=
T1T2 – 1)/T2b
(
b
–
T1.
(V)
We have calculated the isokinetic temperature to be
287 K by plotting logk2 at 303 K versus logk1 at 298 K
(Fig. 5). The value of
β
(287 K) is lower than experiꢀ
mental temperature (298 K). This indicates that the mechanistic and kinetic studies.
APPENDIX
According to Scheme,
kK1K2K3[LꢀCYS][OH−][Cu(OH)2(H3IO6)2]3−
−d[DPC]
(A.1)
Rate =
= k[C] =
,
[H2IO36−]
dt
[DPC]t = [DPC]f + [Cu(OH)2(H3IO6)(H2IO6)]4− + [Cu(OH)2(H3IO6)]− + [C]
3−
3−
−
−
−
(A.2)
⎡
⎤
[H2IO6 ] + K1[H2IO6 ][OH ] + K1K2[OH ] + K1K2K3[OH ][LꢀCYS]
= [DPC]f
,
[H2IO36−]
⎢
⎣
⎥
⎦
where [DPC]t and [DPC]f refer to total and free DPC concentrations respectively. The free [DPC] is given by
[DPC]t[H2IO36−]
[DPC]f =
.
(A.3)
[H2IO36−] + K1[H2IO36−][OH−] + K1K2[OH−] + K1K2K3[OH−][LꢀCYS]
Similarly total [OH–] can be calculated as
KINETICS AND CATALYSIS Vol. 50
No. 4
2009

538
HOSAMANI et al.
[OH−]t = [OH−]f + [Cu(OH)2(H3IO6)(H2IO6)]4− + [Cu(OH)2(H3IO6)]− + [C]
K1K2[Cu(OH)2(H3IO6)2]3−[OH−]
(A.4)
(A.5)
= [OH−]f + K1[OH−][Cu(OH)2(H3IO6)2]3−
+
[H2IO36−]
K1K2K3[Cu(OH)2(H3IO6)2]3−[OH−][LꢀCYS]
[H2IO36−]
+
.
The view of low concentrations of DPC used, the second, third and fourth terms in the above equation are
neglected, therefore,
[OH]t = [OH]f ,
[LꢀCYS]t = [LꢀCYS]f .
(A.6)
(A.7)
Substituting equitation’s (A.3), (A.6), and (A.7) in (A.1) we get
kK1K2K3[LꢀCYS][OH−][DPC]
[H2IO36−] + K1K2K3[OH−][LꢀCYS] + K1K2[OH−] + K1[H2IO36−][OH−]
Rate =
.
13. Kitajima, K.N. and Moroꢀoka, Y., Chem. Rev., 1994,
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2009