Mechanism of oxidation of L-Cysteine by Tetraoxoiodate(VII) in aqueous acid medium

Article abstract of DOI:10.14233/ajchem.2015.18976

The kinetics and mechanism of the oxidation of L-cysteine by tetraoxoiodate(VII) ion in aqueous acid medium has been studied at 0.03 δ[H+] δ0.1 mol dm^-3- under pseudo-first order conditions of an excess of tetraoxoiodate(VII) concentration at 1 = 0.11 mol dm^-3- (NaClO₄). The reaction obeys the rate expression:-d [IO₄-]/dt = {k₃K₁K₂ [H+] + k₅} [RSH][IO4 -] Addition of AcO- and NO₃ - had no effect on the reaction but the rate of reaction decreased with increase in ionic strength of the medium. Increase in dielectric constant decreased the rate of reaction. The rates are consistent with a mechanism which involves the formation of free radicals which subsequently dimerized into disulfides. The reaction has been rationalized on the basis of the inner-sphere electron transfer mechanism.

Full text of DOI:10.14233/ajchem.2015.18976

Asian Journal of Chemistry; Vol. 27, No. 10 (2015), 3777-3780
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18976
Mechanism of Oxidation of L-Cysteine by Tetraoxoiodate(VII) in Aqueous Acid Medium
1
3
4
PIUS O. UKOHA , OGUEJIOFO T. UJAM1,2,*, JOHNSON F. IYUN and SOLOMON E.O. OKEREKE
1Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka, Enugu State, Nigeria
2Department of Chemistry, University of Victoria, Victoria, BC, V8W 3V6, Canada
3Department of Chemistry, Ahmadu Bello University, Zaria, Kaduna State, Nigeria
4Department of Pure and Industrial Chemistry, Abia State University, Uturu, Abia State, Nigeria
*Corresponding author: Tel: +234 8062573097; E-mail: oguejiofo.ujam@unn.edu.ng
Received: 1 January 2015;
Accepted: 27 February 2015;
Published online: 22 June 2015;
AJC-17345
The kinetics and mechanism of the oxidation of L-cysteine by tetraoxoiodate(VII) ion in aqueous acid medium has been studied at 0.03
≤ [H+] ≤ 0.1 mol dm-3 under pseudo-first order conditions of an excess of tetraoxoiodate(VII) concentration at 1 = 0.11 mol dm-3 (NaClO4).
The reaction obeys the rate expression:
-d [IO4–]/dt= {k3K1K2[H+] + k5} [RSH][IO4–]
Addition of AcO– and NO3– had no effect on the reaction but the rate of reaction decreased with increase in ionic strength of the medium.
Increase in dielectric constant decreased the rate of reaction. The rates are consistent with a mechanism which involves the formation of
free radicals which subsequently dimerized into disulfides. The reaction has been rationalized on the basis of the inner-sphere electron
transfer mechanism.
Keywords: L-Cysteine, Oxidation, Mechanism, Tetraoxoiodate(VII).
oxygen atom transfer reactions11,12. Kinetic information has
been reported on the oxidation of L-cysteine and closely related
INTRODUCTION
L-Cysteine, is a naturally occurring sulfur-containing non-
essential amino acid found in living cells1 and an important
structural and functional component of many proteins and
enzymes. It is unique among the twenty amino acids required
by human because it contains a sulfhydryl (-SH) group which is
involved in the formation of disulfide bonds that are crucial in
defining the structures of many proteins. The sulfhydryl group
is a nucleophilic center and therefore can undergo addition and
substitution reactions. It is an important precursor in the intra-
cellular manufacture of the antioxidant, glutathione, which
protects cells from toxins such as free radicals2 through its ability
to regenerate and prolong the activity of vitamin E3. In human
biological system L-cysteine is often involved in electron transfer
reactions, helps in enzyme-catalyzed reactions and is also oxi-
dized by oxygen to give cysteine sulfonic acid4. L-Cysteine is
also able to bind to heavy metal ions in biological systems thus
acting as a scavenger in the removal of toxic metal ions from the
human body5. L-cysteine and its derivatives form stable metal
complexes6-8 with 3d metal fragments and therefore can poten-
thiols by metal complexes13-15. These reports indicate that
oxidation of the thiols to disulphide may involve binuclear
complex formation prior to the electron transfer step. There
were also cases where the involvement of binuclear complexes
were not established in the redox process16,17. Iyun et al.18
reported a 2:1 stoichiometry for the reaction of L-cysteine with
oxobridged ruthenium ion (RuORu)4+. Also the reaction
showed inverse acid dependence and negative Bronsted-Debye
salt effect. The reaction operated by an outer-sphere mechanism.
However, oxidation of L-cysteine and DL-penicillamine by
Cr(VI) in both high and low pH medium showed a sulphur-
linked chromate ester as possible intermediate and followed
innersphere-pathway19,20
.
In this report we have for the first time followed the course
of the electron transfer reaction of L-cysteine, a biologically
important thiol with an oxyanion. We are interested in its reaction
with oxygen donors outside the body to ascertain whether the
reaction will conform to the mechanism assumed when reacting
with pure oxygen in living cells leading to formation of cystine
sulphonic acid or otherwise.Also we hope this study will extend
the frontier of knowledge in understanding the role assumed by
tially be employed as new agents for metal chelation therapy9,10
.
The thiol group undergoes nucleophilic attack, electron
transfer, hydride transfer, hydrogen radical transfers and
thiols as electron transfer enzymes21,22
.

3778 Ukoha et al.
Asian J. Chem.
TABLE-1
EXPERIMENTAL
SECOND ORDER RATE CONSTANTS FOR THE OXIDATION OF
L-CYSTEINE BY PERIODATE ION IO4– IN AQUEOUS ACID
MEDIUM AT 23.5 0.5 °C AT (RSH) = 0.001 mol dm-3
L-Cysteine (Koch-light Lab. MP 220 °C) hereafter denoted
as RSH was ascertained pure by melting point determination.
KIO4 (East Anglia Chemicals, 99.8 %) was standardized
iodimetrically. All other reagents wereAnalaR grade are used
without further purification unless otherwise stated.
Kinetic studies were conducted via spectrophotometry by
monitoring the increase in absorbance of the reaction mixture
at 480 nm (λmax of aqueous iodine)23 as a function of time
using Milton Roy Spectronic 2ID Spectrophotometer. All
measurements were made under pseudo-first order conditions
of excess periodate and at constant ionic strength of. 0.11 mol
dm-3 (NaClO4) unless otherwise stated. Under such conditions,
kinetic curves were exponential and rate constants were
obtained from logarithmic plots of absorbance differences
against reaction time. Specific rates from replicate runs were
in close agreement (std dev. = 0.01). Second order rate constants,
k2, were derived as ratios of kobs/[IO4-] for each run.
103 [IO4–]
[H+]
I (NaClO4)
(mol dm-3)
103 kobs
k2
(mol dm-3)
(mol dm-3)
(s-1)
(dm3 mol-1 s-1)
8.0
8.4
8.8
9.0
10.0
0.10
0.10
0.10
0.10
0.10
0.11
0.11
0.11
0.11
0.11
6.20
6.49
6.74
6.95
7.74
7.75
7.73
7.66
7.72
7.74
log [IO4– ]
-1.10
-2.12
-2.10
-2.08
-2.06
-2.04
-2.02
-2.00
-1.98
-1.12
y = 0.9975x + 0.8825
R2 = 0.997
-1.14
-1.16
-1.18
-1.20
-1.22
RESULTS AND DISCUSSION
The stoichiometry of the reaction was determined by spec-
trophotmeteric titration. The [H+], [RSH] and ionic strength
were kept constant at 0.1 mol dm-3, 1.0 × 10 3 mol dm-3 and
0.11 mol dm-3 (NaClO4) and 0.1 mol dm-3 respectively at 23.5
0.5 °C. The concentration of periodate was varied from 1 ×
10-4 to 1 × 10-2 mol dm-3. Plot of absorbance at completion of
reaction versus [RSH]/[IO4-] gave a sharp break at 6.80 0.03.
This result indicates a stoichiometry of one mole of 1O4- reacting
with about seven moles of RSH as represented with eqn. 1.
Fig. 1. A plot of log kobs versus log [IO4–] for the oxidation of RSH by IO4
–
-d[IO4 ]/dt = k2[IO4-][RSH]
(2)
-
Pseudo-first order rate constants have a linear dependence
on [IO4-] at constant [H+] (Table-1). It is from these results
that second order rate constants, k2, were evaluated as the ratio
kobs/[IO4-]. The k2 values are fairly constant (7.72 0.035 dm3
2IO4- + 2H+ + 14RSH → 7RSSR + 8H2O + I2
(1)
One of the products of the reaction, RSSR, a disulphide,
has no effect on the rate of reaction in excess IO4-. However,
when RSH is used in excess the rate of reaction is retarded.
This could be attributed to product accumulation. Production
of disulphide, RSSR, as a by-product of oxidation of RSH has
been documented by other workers17,18,23. In this reaction
evidence for disulphide being the organic product of reaction
and not sulphoxide was confirmed by the method reported by
McAuley et al.24,25. Evidence for iodine being of one of the
products of the reaction was indicated by the sudden change
of the reaction mixture from colourless to yellowish brown.
The UV-visible spectra of this solution showed wavelength
maxima of 480 nm which is very similar to the absorption
maxima of aqueous iodine26,27. Test with starch solution was
positive.
mol s-1). Within the range 0.03 ≤ [H+] ≤ 0.1 mol dm-3 and
-1
other parameters kept constant, the rate of reaction increased
with [H+] as shown in Table-2.
TABLE-2
EFFECT OF ACID CONCENTRATION ON RATE
OF OXIDATION OF L-CYSTEINE BY IO4– IN
AQUEOUS ACID MEDIUM AT 23.5 0.5 °C
103 (IO4–)
[H+]
I (NaClO4)
(mol dm-3)
k2
(mol dm-3)
(mol dm-3)
(dm3 mol-1 s-1)
8.0
8.0
8.0
8.0
8.0
0.03
0.05
0.08
0.11
0.10
0.11
0.11
0.11
0.11
0.11
5.12
5.90
7.02
8.00
7.75
Under pseudo-first order conditions with [IO4-] at least
20-fold excess and isolation of L-cysteine at [H+] = 0.1 mol
dm-3, plots of log (A∞–At) were linear to about 85 % extent of
reaction indicating unit order with respect to RSH. Also,
varying of [IO4-] from 8.0-10. 0 × 10-3 mol dm-3 at constant
[RSH] at [H+] = 0.1 mol dm-3, the results (Table-1) indicate
increase of pseudo-first order rate constants, kobs, with [IO4-].
A plot of log kobs versus log [IO4-] (Fig. 1), was linear (R2
= 0.997) with a slope of 0.997 indicating first order with respect
[IO4-]. Also, a plot of kobs vs. [IO4-] was linear passing through
the origin thereby confirming first order. This gives the rate
law at 0.1 mol dm-3 [H+] to be.
A plot of log k2 against I1/2 (Fig. 2) was linear (R2 = 0.9552)
and gave a slope of -1.05 suggesting that the product of the
charges at the rate determining step is negative. Plot of rate
constant against [H+] in this acid range was linear and fitted
eqn. 3:
k2 = a + b [H+]
(3)
The values of a and b as determined from Fig. 3 are 4.08
dm3 mol-1 s-1 and 36.05 dm6 mol-2 s-1 respectively. Enhancement
of rate of reaction on increasing [H+] may be resulting from
the various equilibria established by IO4- in aqueous solution28
as shown below.

Vol. 27, No. 10 (2015)
Mechanism of Oxidation of L-Cysteine by Tetraoxoiodate(VII) in Aqueous Acid Medium 3779
0.88
Within the acid range of this reaction (pH < 1.0),
L-cysteine (RSH) instead of being in the Zwitterionic form
will exist as the cationic form HRSH+. This proposition was
also made in the reaction between RSH and trisoxalatoco-
baltate(III) ion23. It was suggested as the reason for the
dependence of the reaction on [H+]. Electrostatically, HRSH+
is favoured over RSH as the reactive intermediate since the
oxidant is negatively charged. It is our opinion that the reactive
species in the title reaction is HRSH+.
Variation of ionic strength of the medium from 0.11-0.18
mol dm-3 (NaClO4) (Table-3) resulted in negative Brønsted-
Debye primary salt effect. The rate of reaction decreased as
ionic strength increased. Least mean square plot of log k2
0.87
0.86
0.85
0.84
0.83
0.82
0.81
y = -1.0454x + 1.2341
R2 = 0.9552
0.34
0.35
0.36
0.37
0.38
0.39
0.40
0.41
against I1/2 gave a slope of -1.05 suggesting that the product of
charges at the rate determining step is negative. This indicates
that oppositely charged species with unit charges respectively
are interacting30. Also, the rate of reaction decreased as the
dielectric constant (acetone-H2O mixture) of the medium
increased. Change in dielectric constant from 69.74 to 77.71
caused a reduction in the second order rate constant from 5.25-
1.05 dm3 mol-1 s-1. At dielectric constant of 78.94 there was no
noticeable reaction for up to 2 h. This corroborates operation
of anion-cation interaction at the rate determining step31.
Addition of varying concentrations (0.01-0.5 mol dm-3) of
acetate and nitrate ions in the reacting system did not affect
the rate of the reaction. Second order rate constant k2 values
were within the range of 7.69 to 7.75 dm3 mol-1 s 1. This is
suggestive of an inner-sphere mechanism32. However, no
detectable intermediates were observed. This seems to be in
agreement with the observation of Abdel-Khalek33 in other
reactions of IO4- where inner-sphere pathway was reported
although with no detectable intermediates. Based on the acid
½
I
Fig. 2. log k2 versus I1/2
9
8
7
6
5
4
3
2
1
0
y = 36.054x + 4.0763
R2 = 0.9985
0
0.02
0.04
0.06
mol dm–3
0.08
0.10
0.12
Fig. 3. k2 versus [H+]
IO4- +2H2O → H4IO6-
H4IO6- +H+ → H5IO6
(4)
(5)
TABLE-3
EFFECT OF IONIC STRENGTH ON THE RATE OF
OXIDANTION OF L-CYSTEINE BY IO4– IN AQUEOUS
ACID MEDIUM AT 23.5 0.5 °C AT (RSH) = 0.001 mol dm-3
These protonated species contributed meaningfully to the
overall reaction rate since they are likely to be more reactive
than IO4-. Also under acidic conditions (pH < 7) amino acids
are known to possess charged groups resulting from the proto-
nation of the amino group29 to –NH3+. Scheme-I shows the
proposed sequence of protonation of L-cysteine, reaction with
IO4- and formation of L-cystine.
103 [IO4–]
[H+]
I (NaClO4)
(mol dm-3)
k2
(mol dm-3)
(mol dm-3)
(dm3 mol-1 s-1)
8.0
8.0
8.0
8.0
0.10
0.10
0.10
0.10
0.12
0.14
0.15
0.16
7.41
7.08
6.67
6.55
-
IO4
+
HS
O
HS
H N
O
HS
H N
O
K1
K2
+
+
IO4
H
H N
OH
OH
3
OH
2
2
k
3
HS
H N
O*
O
S
S
O
+ IO3* + OH-
OH
HO
NH
2
H N
OH
2
2
Scheme-I: Sequence of protonation of L-cysteine, reaction with IO4- and formation of L-cystine

3780 Ukoha et al.
Asian J. Chem.
dependence path of the reaction and substituting eqn. 3 into
eqn. 2 we have the rate law to be
This suggests the possibility of intermediates or precursor
complexes taking part in the reaction18. Above data strongly
suggest that this reaction very likely follows the inner-sphere
mechanism according to the proposed reaction scheme.
-d[IO4-]/dt = {a + b [H+]}[IO4-][RSH]
(6)
Eqn. 6 results from the following steps:
K1
7RSH + 7H+
HRSH+ + IO4-
7HRSH+
(7)
(8)
(9)
ACKNOWLEDGEMENTS
K2
One of the authors, Pius O. Ukoha thank the Federal College
of Chemical and Leather Technology, Zaria for award of study
fellowship to carry out this work andAhmadu Bello University,
Zaria for providing some of the instruments.
[HRSH-IO4]
k3
[HRSH-IO4]
RS* + IO3* + H2O
6RS* + 3H2O* + I* + 6H+ (10)
k4
6HRSH+ + IO3*
k5
k6
IO4- + RSH
6RSH + IO3*
RS* + IO3* + OH-
6RS* + I* + 3H2O
(11)
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I* + I* → I2
OH- + H+ → H2O
I2 + I- → I3
14RS* → 7RSSR
(14)
(15)
(16)
(17)
-
Rate = k3[RSH2–IO4] + k5[RSH] [IO4-]
From eqn. 7:
[RSH2+] = K1[RSH][H+]
(18)
(19)
(20)
From eqn. 8:
[RSH2- IO4] = K2[HRSH+][IO4-]
Substituting eqn. 18 into eqn. 19 gives
[RSH2 – IO4] = K1K2[RSH][H+][IO4-]
Therefore
Rate = k3 K1K2[RSH][H+] + k5[RSH][IO4-]
(21)
(22)
= k3K1K2[H+] + k5[RSH][IO4-]
Eqn. 21 is similar to eqn. 3, where a = k5 and b = k3 K1K2
The presence of free radicals was indicated by the positive
polymerization test on addition of acrylamide to partially
oxidized mixture of the reactants in excess methanol. The
formation of the intermediate as suggested in eqn. 8 is thought
to be fast whereas its decomposition is the rate determining
step. The reaction between IO4 and VO2 showed similar
characteristics34. Michaelis-Menten plot of 1/kobs versus 1/(IO4-)
was linear with positive intercept on the 1/kobs axis (Fig. 4).
-
+
0.18
y = 1.2864x + 0.001
R2 = 0.9968
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
33. A.A. Abdel-khalek, Polyhedron, 8, 51 (1989).
34. D.J.B. Galliford and J.M. Ottaway, Analyst, 97, 412 (1972).
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
1/[IO4– ]
Fig. 4. Michaelis-Menten-type plot of 1/kobs versus 1/[IO4–] for the title reaction