Content uploaded by Mohammed Khalid
Author content
All content in this area was uploaded by Mohammed Khalid on Mar 12, 2015
Content may be subject to copyright.
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 199
OXIDATIVE KINETICS OF AMINO ACIDS BY
PEROXYDISULFATE:
EFFECT OF DIELECTRIC CONSTANT
Mohammed A. A. Khalid*
Department of Chemistry, Faculty of Science, University of Khartoum, Khartoum,
P. O. Box 321 Sudan
ﺻﻼﺨﻟاـﺔ:
ﺪﻘﻟ ﺖﻤﺗ– ﺚﺤﺒﻟا اﺬه ﻲﻓ - ﺔﻌﺒﺳ ةﺪﺴآأ ﻞﻋﺎﻔﺗ ﺔﻴﻜﻴﻧﺎﻜﻴﻣو ﺔﻴآﺮﺣ ﺔﺳاردأ ضﺎﻤﺣأعﻮﻧ ﻦﻣ ﺔﻴﻨﻴﻣ- α
ﺔﻃﺎﺳﻮﺑ أﺋﺎﻨﺛﻮﺴآاﺮﻴﺑ تﺎﻧﻮﻳﻲﺋﺎﻣ ﻂﺳو ﻰﻓ تﺎﺘﻳﺮﺒﻜﻟا ﻲ ةراﺮﺣ ﺔﺟرد ﺪﻨﻋ ﻚﻴﺗﺮﺒﻜﻟا ﺾﻤﺤﺑ ﺾﻤﺤﻣ 60 ﻰﻟإ 80
ﺔﻳﻮﺌﻣ ﺔﺟرد .و ﺰﻴآﺮﺘﻟ ﺔﺒﺴﻨﻟﺎﺑ ﻰﻟوﻷا ﺔﺒﺗﺮﻟا ﻦﻣ ﻞﻋﺎﻔﺘﻟا نأ تﺎﺑﺎﺴﺤﻟا تﺪآأأﻮﺴآاﺮﻴﺑ تﺎﻧﻮﻳ-ﺘﻳﺮﺒﻜﻟا ﻰﺋﺎﻨﺛ و تﺎ
ﺾﻤﺤﻟا ﺰﻴآﺮﺘﻟ ﺔﺒﺴﻨﻟﺎﺑ ﺔﻳﺮﻔﺼﻟا ﺔﺒﺗﺮﻟا ﻦﻣﻲﻨﻴﻣﻷا ،ﻮه ﺪهﺎﺸﻤﻟا ﺔﻋﺮﺴﻟا لﺪﻌﻣ نﻮﻧﺎﻗ نأ ﺖﺒﺛ ﺎﻤآ
–d[S2O82-]/dt = Kobs [S2O82-][Amino acid]0
ﺪﻗوا ضﺎﻤﺣأ ةﺪﻌﻟ ثﺪﺣ ﺪﻗ ﻞﻋﺎﻔﺘﻠﻟ ﺎﻴﺗاذ اﺰﻴﻔﺤﺗ نأ ﺢﻀﺗأ ﺔﻴﻨﻴﻣ ، ﺎﻌﺒﺗ ﻚﻟذو نّﻮﻜﺘﻟ ﻒﻴﺷ ﺪﻋاﻮﻗ ﻲﻓ ءﺎﻨﺛأ
ﻟا ﺪﻴهﺪﻟﻻا ﻦﻴﺑ ﻞﻋﺎﻔﺘﻟا ﺾﻤﺤﻟاو ﺞﺗﺎﻨ ﻲﻨﻴﻣﻷا ﻲﻘﺒﺘﻤﻟا نأو ، ﻂﺳﻮﻟ ﺔﺒﺴﻨﻟﺎﺑ ﻲﺑﺮﻬﻜﻟا لﺰﻌﻟا ﺖﺑﺎﺛ ﻢﻴﻗ صﺎﻘﻧإ
دﺆﻳ ﻞﺨﻟا ﺾﻤﺣ ﻦﻣ ﺮﻳدﺎﻘﻣ ﺔﻓﺎﺿإ ﺪﻨﻋ ﻞﻋﺎﻔﺘﻟايﺒﺴﻨﻟﺎﺑ ﻞﻋﺎﻔﺘﻟا ﺔﻋﺮﺳ لﺪﻌﻣ صﺎﻘﻧإ ﻰﻟإ ﺔ ضﺎﻤﺣﻷا ﻞﻜﻟ ﺔﻴﻨﻴﻣﻷا
ﺎﻬﺘﺳارد ﺖﻤﺗ ﻲﺘﻟا . ﺪﻗو ﻦﻣ ةراﺮﺤﻟا تﺎﺟرد ﺪﻨﻋ ﻞﻋﺎﻔﺘﻟا ﺔﻴآﺮﺣ ﺔﺳارد ﺖﻤﺗ60 ﻰﻟإ 80ﺔﺟرد ﺔﻳﻮﺌﻣ ،ﺁو ﻦﻜﻣ
ىﺮﺧﻷا ﺔﻴﻜﻴﻣﺎﻨﻳدﻮﻣﺮﻴﺜﻟا ﻢﻴﻘﻟا ﺔﻴﻘﺑو ﻂﻴﺸﻨﺘﻟا ﺔﻗﺎﻃ بﺎﺴﺣ ) ةﺮﺤﻟا ﺔﻗﺎﻄﻟا ﻲﻓ ﺮﻴﻐﺘﻟا، و ىﻮﺘﺤﻤﻟا ﻲﻓ ﺮﻴﻐﺘﻟا
يراﺮﺤﻟا ، ﺎﻴﺑوﺮﺘﻧﻻا ﺔﻤﻴﻗ ﻲﻓ ﺮﻴﻐﺘﻟاو.( و ﺔﺒﺴﻨﻟﺎﺑ ﻰﻟإ
ﻟﺎﻋ ﺰﻴآﺮﺗﻲ تﺎﺘﻳﺮﺒآ ﻦﻣ لدﺎﻌﺘﻣ ﺢﻠﻣ لﻮﻠﺤﻣ ﻦﻣ
ﻞﻋﺎﻔﺘﻟا ﻂﻴﻠﺧ ﻲﻓ مﻮﻴﺳﺎﺗﻮﺒﻟا، ا ﺖﺑﺎﺛ ﻢﺜﻳرﺎﻏﻮﻟ نﺎﻓ ﺔﻴﻄﺧ ﺔﻗﻼﻋ ﻪﻨﻋ ﺞﺘﻨﺗ لﺪﻌﻤﻟ رﺬﺠﻟا ﻞﺑﺎﻘﻣ لﺪﻌﻤﻟا ﺖﺑﺎﺜﻟ
ةﻮﻘﻠﻟ ﻰﻌﻴﺑﺮﺘﻟاﺔﻴﻧﻮﻳﻷا .
* E. mail: yasinawad@gmail.com
Tel.: +249912628112
Paper Received 24 July 2007; Revised 19 September 2007; Accepted 28 November 2007
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
200
ABSTRACT
The kinetics and mechanism of oxidation of alanine, asparagine, cysteine,
glutamic acid, lysine, phenylalanine, and serine by peroxydisulfate ion have been
studied in aqueous acidic (sulfuric acid) medium at the temperature range 60–80°C.
The rate shows first order dependence on peroxydisulfate concentration, and zero
order dependence on amino acid concentration.
The rate law observed is: –d[S2O82-]/dt = Kobs [S2O82-][Amino acid]0. An
autocatalytic effect has been observed in amino acids oxidation due to formation of
Schiff base between the formed aldehyde and parent amino acid. A decrease in
dielectric constant of the medium – adding acetic acid (5–15% v/v) results in a
decrease in the rate in all cases studied. Reactions were carried out at different
temperatures (60–80°C) and the thermodynamics parameters have been calculated.
The logarithm of the rate constant is linearly interrelated to the square root of the
ionic strength.
Key words: peroxydisulfate, kinetics, amino acids.
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 201
OXIDATIVE KINETICS OF AMINO ACIDS BY PEROXYDISULFATE:
EFFECT OF DIELECTRIC CONSTANT
1. INTRODUCTION
The kinetics and mechanism of the oxidation of inorganic and organic substrates by peroxydisulfate under both
catalyzed and uncatalyzed conditions have received considerable attention [1, 2]. The first-order uncatalyzed
peroxydisulfate oxidation is well studied in aqueous solution; Bartlett and Cotman [3] have suggested that radicals
produced in peroxydisulfate decomposition in aqueous solution cannot induce the peroxydisulfate decomposition, and
based their statement on the fact that autocatalysis is not observed in the thermal decomposition and that the reaction is
first order in peroxydisulfate concentration. However, to explain the increased rate on addition of an oxidizable substrate,
it necessary to postulate that radicals produced from the reducing agent can induce peroxydisulfate decomposition. Chen
[4] found that the reaction of [Fe(CN)5L]3- (L = 4-amino pyridine, 4,4′- bipyridine, and pyrazine) with peroxydisulfate,
follow an outer-sphere electron transfer mechanism. The rate constants of oxidation for the corresponding [Ru(NH3)5L]2+
complexes were also measured and were found to be faster than those of [Fe(CN)5L]3- complexes by a factor of ~ 102
even after the corrections for differences in reduction potentials and in the charges of the complexes, Chen referred the
difference in reactivity to hydrogen bonding between peroxydisulfate and the ammonia ligands of [Ru(NH3)5 L]2+ and
nonadiabaticity observed in the [Fe(CN)5L]3-complexes. Almaraz, [5] studied the mixed-valence binuclear complexes,
[(CN) 5
Fe(PyCN)Ru (NH3)5]n; which are prepared as solid compounds through stoichiometric oxidation of the fully
reduced (II) binuclear complexes with peroxydisulfate. It was found that by analysis of IR spectra, the solids were
observed to be a mixture of the predominant electronic isomers with FeII I, RuII isomers. Dua [6] studied five compounds
formed by peroxydisulfate oxidation of primaquine, and these compounds were isolated using chromatographic methods
and evaluated for antimalaria activity in vitro. It was found that one compound, 6-methoxy-5,8bis(4`-amino-1-
methylbutylamino) quinoline, had good gamitocytocidal activity against plasmodium yoelli infected mice at a 10 mg/ kg
dose in vivo. The radical anion SO4–. can be generated by the photolysis [7] or thermolysis of the peroxydisulfate as well
as by the one-electron reduction of peroxydisulfate [8]. Gilbert [9] found that the reaction of SO4-. with α–D-glucose is
selective towards the C2, C5, and C6 positions. This reflects the activating effect of a β-oxygen substituent where the
radical orbital can eclipse the β–C–O bond, providing a SOMO-δ* interaction which stabilizes the developing radical
centre. The oxidation of amino acids is of the utmost importance, both from a purely chemical point and in view of its
bearing on the mechanism of amino acid metabolism. It has been observed that there is not enough information in the
literature on the kinetics and mechanism of oxidation of all essential amino acids by peroxydisulfate, and the present
investigation is a part of our broad program of studying mechanistic aspects of the oxidation of amino acids by
peroxydisulfate in acidic aqueous media.
2. MATERIALS AND METHODS
Potassium peroxydisulfate, sodium thiosulfate, potassium iodide, sodium bicarbonate, sulfuric acid, potassium
sulfate, starch, and 2,4-dinitrophenylhydrazine used were all A. R. Grade. Amino acids, L-alanine, l-asparagine, l-
cysteine, l(+)-glutamic acid, lysine, dl-phenylalanine, and l-serine, were all chromatographically pure and obtained from
Philip Harris Biological. Deionized water was used throughout the course of the experiments. The initial concentration of
peroxydisulfate were varied from (2.5×10-3–12.5×10-3 mol.dm-3) where the concentration of amino acid were varied from
(1.0×10-3–5.0×10-3 mol.dm-3). The ionic strength of the reaction mixtures was kept constant at 0.25 mol.dm-3 by addition
of potassium sulfate solution. The dielectric constant of the medium were varied by changing the proportion of water–
acetic acid mixture, in the range of 5–15%v/v acetic acid, the respective values of dielectric constant were taken from
Akerlof [10,11] and also calculated from the average value of dielectric constant equation for mixtures of solvent
[12,13]. The reaction was carried out in glass-stoppered conical flasks. Appropriate amount of solution of amino acid,
potassium sulfate, and water (to keep the total volume constant for all runs) were taken in the conical flask and
thermostatted at 60°C for thermal equilibrium. A measured amount of peroxydisulfate solution (usually not preheated)
was rapidly added to the reaction mixture in the flask. The progress of the reaction was monitored by iodometric
determination of unreacted peroxydisulfate at different intervals of time. The course of the reaction was followed for at
least 70% of the reaction. The rate constants, Kobs, were evaluated from plots of logarithm of peroxydisulfate
concentration against time, the data were collected and analyzed using excel program.
3. STOICHIOMETRY AND PRODUCT ANALYSIS
Stoichiometry of the peroxydisulfate–amino acids reactions have been extensively studied by several workers
[14–17] who found that the reaction followed a 1:1 stoichiometry. A modification method to determine the remaining
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
202
amino acids in the reaction mixture using ninhydrin was applied. In this modification the concentration of amino acid
must be greater than that of peroxydisulfate. In all cases studied the results confirmed the 1:1 stoichiometry. The
products analyses were carried out under the kinetic conditions. A mixture of 5.0 ml of each amino acid solution (1.0%
w/v) with 10 ml of peroxydisulfate solution 0.1 mol.dm-3, was made up to 50 ml with deionized water. The mixture was
kept stoppered in a thermostat bath at 60°C for 10 hours. Ammonia was identified by Nessler’s reagent; in all cases a
brownish color was observed indicating a deamination reaction. Carbon dioxide was identified by freshly prepared lime
water; and the solution turned milky indicating a decarboxilation reaction. The aldehydes were characterized by
preparing 2,4-dinitrophenylhydrazine and by their spectral data in comparison with authentic aldehyde samples.
+−+− +++−⎯⎯→⎯−−+ 442
OH
3
2
82 NH2HSOCOCHORCOOH)CH(NHROS 2
4. RESULTS
4.1. Dependence of the Rate on Peroxydisulfate Concentration
Kinetic runs were measured by varying the concentration of peroxydisulfate from 2.5×10-3 to 12.5×10-3 mol.dm-3at a
constant temperature of 60°C, amino acid at 3.0×10-3 mol.dm-3, fixed hydrogen ion concentration, and ionic strength at
0.25 mol.dm-3 K2SO4. Plots of log[S2O82-] vs. time were linear for each initial peroxydisulfate concentration. Slopes of
these lines were used to evaluate Kob s. The reaction showed a first order dependence of rate on peroxydisulfate
concentration, Table 1.
Table 1. Effect of Varying Peroxydisulfate Concentration on the Rate of Oxidation of α–Amino Acids at Ionic
Strength = 0.25 mol.dm-3, Amino Acid Concentration = 0.003 mol.dm-3, and Temperature 60°C
103 [S2O8
2-]
mol.dm-3 pH
–d[S2O8
2-]/dt
mol.dm-3.min-1
103 [S2O8
2-]
mol.dm-3 pH
–d[S2O8
2-]/dt
mol.dm-3.min-1
Alanine Asparagine
2.5 2.0 1.53 2.5 1.91 3.095238
5.0 2.0 3.912 5.0 1.91 4.285714
7.5 2.0 4.02 7.5 1.91 7.857143
10.0 2.0 10.02 10.0 1.91 9.047619
12.5 2.0 13.12 12.5 1.91 11.90476
Cysteine Glutamic acid
2.5 1.00 5 2.5 2.02 2.1
5.0 1.00 9.52381 5.0 2.02 13.55
7.5 1.00 7.45 7.5 2.02 22.3262
10.0 1.00 12.8571 10.0 2.02 19.886
12.5 1.00 19.5238 12.5 2.02 24.3191
Lysine Phenylalanine
2.5 1.92 6.934286 2.5 1.88 4.05
5.0 1.92 10.94286 5.0 1.88 8.65
7.5 1.92 18.32 7.5 1.88 14.8
10.0 1.92 22.00476 10.0 1.88 18.4
12.5 1.92 25.78095 12.5 1.88 22.6
Serine
2.5 2.08 4.4 10.0 2.08 6.511905
5.0 2.08 5.30381
7.5 2.08 5.335714
12.5 2.08
6.5
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 203
Figure 1. Plots of logKobs vs. log[Amino acid]0 at [S2O8
2-] = 5.0×10-3 mol.dm-3, pH = 2.00 for Alanine, 1.91 for asparagine,
1.00 for cysteine, 2.02 for glutamic acid, 1.92 for lysine, 1.88 for phenylalanine, and 2.08 for Serine, temperature = 60°C, and
ionic strength = 0.25 mol.dm-3.
4.2. Dependence of the Rate on Amino Acid Concentration
Reactions were carried out keeping all experimental conditions constant and by varying initial concentration of amino
acid from 1.0×10-3 to 5.0×10-3 mol.dm-3and peroxydisulfate at 5.0×10-3 mol.dm-3. Plots of log Kobs vs. log[amino acid]0
gave straight lines parallel to the log [amino acid]0 axis indicating zero order dependence with respect to amino acid
concentration, Figure 1.
4.3. Dependence of the Rate on Dielectric Consnat and Ionic Strength
The dielectric constant of the solvent medium was varied by the addition of acetic acid to the reaction mixture (5–
25% v/v). At amino acid concentration 3.0×10-3 mol.dm-3and peroxydisulfate at 5.0x10-3 mol.dm-3, the rate decrease
with acetic acid content of solvent (Table 2), for the plot of log Kobs Vs. 1/D was found to be linear with a negative slope
(Figure 2).
The reactions were studied at varying ionic strength (0.25 to 0.45 mol.dm-3) by adding potassium sulfate solution,
with the amino acid concentration at 3.0x10-3 mol.dm-3 and peroxydisulfate at 5.0x10-3 mol.dm-3, keeping all other
experimental conditions constant. The rate was found to increase with increase in ionic strength indicating a positive salt
effect (Table 2).
4.4. Dependence of the Rate on Temperature
The reactions were studied at different temperatures (60–80°C), with the amino acid concentration at 3.0×10-3
mol.dm-3 and peroxydisulfate at 5.0×10-3 mol.dm-3, keeping all other experimental conditions constant. From the linear
Arrhenius plots of log Kobs vs. 1/T, the activation energies Ea were calculated. Values of the other activation parameters,
∆H# and ∆S#, were computed at 60°C from the Ea values (Table 3).
0
0.5
1
1.5
2
1 1.2 1.4 1.6 1.8
4+Log[AA]
0
4+LogK
obs
■-Asparagine
▲-Cystiene&Alanine
◊-Glut amic acid
x-Lysine
●-Phenylalanine
∆-Serine
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
204
4.5. Test for Free Radicals
The reaction mixture initiated polymerization of acrylonitrile, indicating an in situ formation of free radicals. Proper
control experiments were also performed.
-3.1
-2.7
-2.3
-1.9
-1.5
0.0155 0.0165 0.0175 0.0185 0.0195
1/D
LogKobs
x- Lysine
■-Asparagine
-- Serine
●-Phenylalanine
∆-Glut amic ac id
□-Alanine
▲-Cystiene
Figure 2. plot of log Kobs vs. 1/D, [S2O8
2-] = 5.0×10-3 mol.dm-3, pH = 2.00 for Alanine, 2.02 for Glutamic acid, 1.92 for lysine,
1.88 for phenylalanine, 1.91 for asparagine, 1.00 for cystine, and 2.08 for serine, temperature = 60°C, and ionic strength =
0.25 mol.dm-3.
5. DISCUSSION
The progress of the reaction was followed by examining the concentration of peroxydisulfate in the reaction mixture
at different time intervals iodometrically. At fixed concentration of amino acid (0.003 mol.dm-3) and peroxydisulfate
concentrations ranged from 2.5×10-3 to 12.5×10-3 mol.dm-3. The plot of log-d[S2O82-]/dt against log [S2O82-]0 was linear
and the slope of this line give the value of the order with respect to peroxydisulfate, which is almost equal to one for each
amino acid studied. On the other hand, at fixed concentration of peroxydisulfate (0.005 mol.dm-3) and amino acid
concentration ranged from 1.0×10-3 to 5.0×10-3 mol.dm-3, the plot of log –d[S2O82-]/dt against log [amino acid]0 was
linear (Figure 1) and the slope of this line give the value of the order with respect to amino acid which is almost equal to
zero. The rate equation can be written as the following form:
nm
Kt [AA]]O[S]/dOd[S 2
82obs
2
82
−− =− n = zero for each amino acid studied.
The variation of Kobs values with the initial concentrations of amino acid or the initial concentrations of
peroxydisulfate is not linearly interrelated, and this can be explained by the fact that the resultant product (aldehyde) of
the oxidation of amino acid by peroxydisulfate forms an additional compound with the parent amino acid in the reaction
mixture. This new compound is a Schiff base [18], and the increasing of Kobs values with increasing peroxydisulfate
concentration at fixed concentration of amino acid is attributed to an autocatalytic effect of this Schiff base. This effect
arise from the initial value of the peroxydisulfate concentration, by which a considerable amount of amino acid is
converted to aldehyde at the beginning of the reaction. This resultant product will combine with the parent amino acid to
form a Schiff base and the latter is most highly oxidizable than the parent amino acid. Increasing Kobs values with
increasing concentration of amino acid at fixed concentration of peroxydisulfate are attributed to the autocatalytic effect;
in this case it arise from the initial value of the amino acid concentration which gave a higher concentration of the
resultant product at the beginning of the reaction, this resulting in formation of a Schiff base.
.
1/D
Cysteine
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 205
Table 2. Effect of Varying Dielectric Constant and Ionic Strength (µ) on the Rate of Oxidation of α–Amino Acids
by Peroxydisulfate at Temperature 60°C
% of
Acetic
acid (v/v)
103 Kobs
min-1
µ
mol.dm-3
103 Kobs
min-1
% of
Acetic
acid (v/v)
103 Kobs
min-1
µ
mol.dm-3
103 Kobs
min-1
Alanine Asparagine
5 2.99 0.25 0.69 5 3.68 0.25 0.92
10 2.53 0.30 1.30 10 2.99 0.30 2.07
15 2.76 0.35 2.07 15 2.32 0.35 2.76
20 2.51 0.40
2.70 20 1.74 0.40
3.76
25 2.23 0.45
3.42 25 1.27 0.45
4.68
Cysteine Glutamic acid
5 5.53 0.25 2.35 5 3.22 0.25 3.92
10 5.30 0.30 3.91 10 3.22 0.30 4.32
15 5.29 0.35 7.14 15 2.53 0.35 4.61
20 5.09 0.40
9.24 20 1.78 0.40
4.96
25 4.87 0.45
11.65 25 1.23 0.45
5.30
Lysine Phenylalanine
5 5.53 0.25 2.53 5 14.51 0.25 2.07
10 4.38 0.30 2.76 10 14.51 0.30 2.14
15 2.76 0.35 2.99 15 11.28 0.35 2.22
20 1.93 0.40
3.22 20 9.51 0.40
2.28
25 1.02 0.45
3.45 25 7.43 0.45
2.36
5 5.53 0.25 2.53 5 14.51 0.25 2.07
Serine
5 8.75 0.25 0.92 20 6.53 0.40
4.30
10 8.75 0.30 2.07
15 7.60 0.35 3.15
25 5.51 0.45
5.39
In general the autocatalytic effect of the resulting Schiff base is due to the fact that the Schiff base is more highly
oxidizable than the parent amino acids. The autocatalytic effect is absent in some of amino acids, this may be due to the
following two assumptions:
1. The resulting Schiff base may be formed in all cases between the formed aldehyde and parent amino acid; the
autocatalytic effect is absent in this group of amino acids because the rate of oxidation of the formed Schiff base by
peroxydisulfate is quite close to the rate of oxidation of the amino acid itself.
2. The resulting Schiff base may not be able to form, due to steric hindrance between the resulting aldehyde and the
parent amino acid.
There are a large number of references to the reaction of aldehyde with amino acid to form a Schiff base [18], in fact
the catalytic effect of pyridoxal phosphate in the amino acid metabolism by enzymes is attributed to the Schiff base
formed between the amino acid and pyridoxal phosphate [19–21].
The reaction amino acid– peroxydisulfate was studied over the temperatures range (60–80°C). The observed rate
constants increase with increasing in temperature Table 3. The natural logarithm of the observed rate constant was
plotted against 1/T in K-1. The slope of the graph is equal to –E/Ř; from these graphs the activation energy was
calculated. From Table 3, the free energy change ∆G# for all cases under study is approximately the same. These values
of free energy change are the same as Chandraju’s values [22] for the oxidation of serine by manganese(III) in three
different media. acetic acid, pyrophosphate, and sulfuric acid respectively. This equality in the values of free energy
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
206
Table 3. Activation Parameters for the Oxidation of Amino Acids by Peroxydisulfate
Amino acid Temp.
K
103Kobs
min-1 ln A Ea
kJ.mol-1
∆S
J.K-1 at
333 K
∆H
kJ.mol-1 at
333 K
∆G
kJ.mol-1
at 333 K
333 1.612
338 6.448
343 11.515
Alanine
348 13.588
42.681 135.11 74.97 135.11 110.14
333 1.612
338 3.224
343 4.145
348 8.291
Asparagine
353 10.364
26.644 91.35 –58.36 91.346 110.78
333 2.533
338 6.448
Cysteine
343 11.285
45.416 142.13 97.709 142.13 109.59
333 0.921
338 3.224
343 4.145
348 8.061
Glutamic acid
353 14.739
38.95 126.56 43.95 126.56 111.92
333 4.145
338 9.442
Lysine
343 17.733
44.453 138.18 89.7 138.18 108.31
333 2.303
338 8.521
Phenylalanine
343 18.424
65.462 197.82 264.37 197.82 109.78
333 0.921
338 1.382
343 1.612
348 3.915
Serine
353 15.43
39.591 129.8 49.23 129.82 113.41
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 207
change, to some extent, suggests that the dependence of ∆G# on solvent or substituents is much smaller than that of
Ea and ∆H#. The values of ∆H# are approximately equal to the values of Ea. The data in Table 3 show that the energy of
activation is highest for the slowest reaction, indicate that the reaction is enthalpy and entropy controlled, the highly
negative values of ∆S# in the case of asparagine indicate the formation of more rigid transition state and this is
extensively solvated than the reactants, while the relatively higher values of ∆S# in the other amino acids indicated a
moderately rigid transition state. Values of ∆S# and ∆H# were linearly interrelated (Figure 4) with R2 = 0.9975, resulting
in an isokinetic relation.
Figure 3. Logarithm of -d[S2O8
2-]/dt against log[S2O8
2-]0.
y = 0.3268x + 111.06
R
2
= 0.9975
0
50
100
150
200
250
-100 0 100 200 300
∆S J.K
-1
at 333K
∆H kJ.mol
-1
Figure 4. Plot of enthalpy of activation against entropy of activation for the oxidation of amino acids by peroxydisulfate.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.35 1.55 1.75 1.95 2.15
4+Log[S
2
O
8
2-
]
0
-d[S
2
O
82-
]/dt
◊-Alanine
■-Asparagine
▲-Cystiene
♦-
Phenylalanine
x-Glutamic acid
Si
Cysteine
Phenylalanine
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
208
The actual iso-kinetic temperature computed from the plot of ∆H# versus ∆S# is 326.8 K, which is quite near to the
experimental temperature 333 K, the linear correlation implies that all the amino acids under study are oxidized by the
same mechanism and the changes in the rate are governed by the changes in enthalpy and entropy.
According to Brönsted and Bjerrum theory for activated complex applied to the charged particles, the reaction
between peroxydisulfate and amino acid will considered to proceed through an activated complex. The complex is
considered to be in equilibrium with reactants, and the equilibrium constant is K‡ in activities is expressed as
)/).(./(./ ‡‡‡‡
SASASA CCCaaaK
γγγ
==
where a‡ is the activity of the activated complex, aA is the activity of amino acid in acidic form, aS is the activity of
peroxydisulfate ion, γ‡ is the activity coefficient of the activated complex, γA is the activity coefficient of amino acid in
acidic form, γS is the activity coefficient of peroxydisulfate ion, C‡ is the concentration of the activated complex, CA is
the concentration of amino acid in acidic form, CS is the concentration of peroxydisulfate ion, and K‡ is the equilibrium
constant. The concentration of the activated complex is C‡ = k‡ CA.CS (γA. γS) / γ‡.
Introducing the reaction rate -dCS/dt, equilibrium constant Kobs , and taking the logarithm,
log Kobs = log(KT/h) + logK‡ +log γA + log γS –log γ‡
In dilute aqueous solution the activity coefficient term can be estimated from the Debye-Hϋckel theory
µγ
2
509.0log ii Z−=
Where µ is the ionic strength of the aqueous solution. Substituting the Debye–Hϋckel expression in the above equation
µ
]018.1[)/log(log ‡
SAobs ZZhkTKK +=
This equation predicts that the plot of log Kobs against the square root of ionic strength expressed straight lines
relationship with all cases studied, and Figure 5 express a primary kinetic salt effect because the slope of each case is
positive value.
-3.2
-3
-2.8
-2.6
-2.4
-2.2
-2
-1.8
0.45 0.5 0.55 0.6 0.65 0.7
(Ionic Strength)
1/2
Log k
obs
♦-Alanine
▲-Asparagine
■-Glutamic acid
∆-Lysine
x-Phenylalanine
◊-Cysteine
○-Serine
Figure 5. Logarithm of rate constant against square root of ionic strength
The rate decreased with decrease in dielectric constant (D) of the medium using variety of acetic acid– water
percentages, (Table 2). The plot of logarithm of Kobs versus 1/D is almost linear in all cases studied, with a negative
slope; the effect of dielectric constant on the rate for a reaction involving two ions was given by the standard
relationship, as in the following equation [23]:
Mohammed A. A. Khalid
July 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 2A 209
ASSAobs DkTdeZZKK /loglog 2
0−=
where Ko is the rate constant in a medium of infinite dielectric constant, ZAe , ZSe are the charges of the two ions, dAS
is the activated complex size, k is the Boltzman constant, and T is absolute temperature. The slope is equal to –ZAZSe2 /
KTdAS. The values of dAS (the activated complex size) and Ko (the rate constant in a medium of infinite dielectric
constant) were computed in Table 4.
Table 4. Activated complex size and the rate constant in a medium of infinite dielectric constant
Amino acid Alanine Asparagine Cysteine Glutamic
acid Lysine Phenylalanine Serine
dAS Å 4.66 0.74 7.98 1.41 0.50 1.35 2.41
K0×10-2 0.63 55.97 0.872 4.63 988 23.2 4.2
Amis [23] showed that in a straight line plot of logarithm Kobs versus 1/D, a positive slope indicates a positive ion–
dipole reaction, while a negative slope indicates the involvement of two dipoles or a negative ion–dipole reaction. In this
investigation a plot of logarithm Kobs versus 1/D, (Figure 2) give straight lines with negative slopes; these results clearly
support the involvement of two dipoles, a negative ion–dipole interaction, or a negative ion dissociation in the rate
determining step.
REFERENCES
[1] W. K. Wilmarth, and A. Haim, Mechanism of Oxidation by Peroxydisulfate ion, ed. J. O. Edwards. London:
Interscience, 1961, p. 175.
[2] D. A. House, “Kinetics and Mechanism of Oxidations by Peroxydisulfate”, Chem. Rev., 62(1962), p. 185
[3] P. D. Bartlett, and J. D. Cotman, “The Kinetics of the Decomposition of Potassium Persulfate in Aqueous Solutions of
Methanol”, J. Amer. Chem. Soc., 71(1949), p. 1419.
[4] M. H. Chen, S. Lee, S. Liu, and A. Yeh, “Kinetic Studies of the Reactions of Pentacyanoferrate(II) Complexes with
Peroxydisulfate”, Inorg. Chem., 35(9)(1996), pp. 2627–2629.
[5] A. E. Almaraz, L. A. Gentil, L. M. Baraldo, and J. A. Otable, “Mixed-Valence Cyanopyridine-Bridged Complexes of
Pentacyanoferrate and Pentaammineruthenium: Electronic Structure, Stability, and Redox Reactivity”, Inorg. Chem.,
35(26) (1996), pp. 7718–7717.
[6] V. K. Dua, S. N. Sinha, S. Biewas, N. Valecha, S. K. Puri, and V. P. Sharma, “Isolation and Antimalarial Activity of
Peroxydisulfate Oxidation Products of Primaquine”, Bioorg. Med. Chem. Lett., 12(24), (2002), pp. 3587–3589.
[7] C. L. Clifton and R. E. Huie, “Rate Constants for Hydrogen Abstraction Reactions of the Sulfate Radical, SO4-.
Alcohols”, Int. J. Chem. Kinet., 21, (1989), p. 677.
[8] B. C. Gilbert and J. K. Stell, “Mechanisms of Peroxide Decomposition. An ESR Study of the Reactions of the
Peroxomonosulphate Anion (HOOSO3–) with Ti III, Fe II, and a-Oxygen-Substituted Radicals”, J. Chem. Soc., Perkin
Trans., 2(1990), p. 1281.
[9] B. C. Gilbert, J. R. L. Smith, P. Taylor, W. Steven, and A. C. Whitwood, “The Interplay of Electronic, Steric and
Stereoelectronic Effects in Hydrogen-Atom Abstraction Reactions of SO4–, Revealed by EPR Spectroscopy”, J. Chem.
Soc., Perkin Trans., 2(1999), pp. 1631–1637.
[10] G. Akerlof, “Dielectric Constants of Some Organic Solvent–Water Mixtures at Various Temperatures”, J. Amer. Chem.
Soc., 54(1939), pp. 4126–4139.
[11] P. S. RadaKhishnamorti, and P. C. Patro, “Consecutive First Order Reaction: Part II. Hydrolysis of Dicarboxylic
Esters”, J. Indian Chem. Soc., 48(1971), p. 811.
[12] I. E. sukovataya, and N. A. Tyuikova, Vestaik Moskovskogo Univ.. Khimiya., 41(6), (2000), P. 8.
[13] F. Oehme, J. Chem. Soc., Perkin Trans. 2(1999), pp. 1631.
[14] M. G. Ram Reddy, B. Sethuram, and T. Z. Navaneeth Rao, Physik. Chem., 256(6)(1975), pp. 880–884.
[15] F. Feigl, Spot Test in Inorganic Analysis. London: Elsevier, 1958, p. 238.
Mohammed A. A. Khalid
The Arabian Journal for Science and Engineering, Volume 33, Number 2A July 2008
210
[16] A. I. Vogel, A textbook of Macro and Semimicro Quantitative Inorganic Analysis. New Delhi: Orient Longman, 1967,
p. 326.
[17] P. D. Bragg, and L. Hough, “The Oxidation of Proline, Hydroxyproline, and N-Methylglycine with Periodate”, J. Chem.
Soc., 1958, p. 4050.
[18] M. S. Ramachndran, and T. S. Vivekanandam, “Kinetics and Mechanism of the Oxidation of Amino Acids by
Peroxomonosulphate Oxidation of β-Phenylalanine, Isoleucine...”, Tetrahedron, 40(23) (1984), pp. 4929–4935.
[19] T. C. Bruice, and S. J. Benkovic, Bioorganic Mechanisms, vol. II. New York: W. A. Benjamin, 1966, ch. 8.
[20] E. Martell, Chemical and Biological Aspects of Pyridoxal Catalysis, ed. E. E. Snell; P. M. Fasella; A. Brauntein, and R.
Fanelli, New York: Pergamon Press, 1963, p. 13.
[21] E. Braunstein, The Enzymes, ed. P. D. Boyer, H. Lard, and K. Myrbach, 2nd edn., vol. II. New York: Academic Press,
1960, P. 113.
[22] S. Chandraju, K. S. Rangappa, and N. M. M. Gowda, “Manganese(III) Oxidation of L-Serine in Aqueous Sulfuric Acid
Medium: Kinetics and Mechanism”, Int. J. of Chem. Kin., 31(1999), pp. 525–530.
[23] E. S. Amis, “Coulomb's Law and the Quantitative Interpretation of Reaction Rates (II)”, J. Chem. Educ., 30(1953),
pp. 351–353.
