Oxidation of methionine by tripropylammonium fluorochromate-a kinetic and mechanistic study

Article abstract of DOI:10.1155/2011/945236

The kinetics of oxidation of methionine (Met) by tripropylammonium fluorochromate (TriPAFC) has been studied in the presence of chloroacetic acid in aqueous acetic acid medium. The reaction is first order with respect to methionine, TriPAFC and acid. The

Full text of DOI:10.1155/2011/945236

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
2011, 8(2), 643-648
http://www.e-journals.net
Oxidation of Methionine by Tripropylammonium
Fluorochromate-A Kinetic and Mechanistic Study
S. SHEIK MANSOOR
Post Graduate and Research Department of Chemistry
C. Abdul Hakeem College, Melvisharam - 632 509, Tamilnadu, India
smansoors2000@yahoo.co.in
Received 12 September 2010; Accepted 21 November 2010
Abstract: The kinetics of oxidation of methionine (Met) by tripropylammonium
fluorochromate (TriPAFC) has been studied in the presence of chloroacetic acid in
aqueous acetic acid medium. The reaction is first order with respect to methionine,
TriPAFC and acid. The reaction rate has been determined at different temperatures
and activation parameters calculated. With an increase in the amount of acetic acid in
its aqueous mixture, the rate increases. The reaction does not induce polymerization
of acrylonitrile. A suitable mechanism has been proposed.
Keywords: Tripropylammonium fluorochromate, Methionine, Kinetics, Oxidation, Mechanism
Introduction
Oxidation kinetics has received considerable attention due to the wide range of biological
processes in which metal ions participate. Chromium(VI) acts as an efficient catalyst in redox
reactions. The search for new oxidizing agents is of interest to synthetic organic chemists. Many
such reagents have been developed in recent years with some success. In recent years, significant
improvements were achieved by the use of new oxidizing agents such as benzimidazolium
fluorochromate1,
N-methyl
benzylammonium
fluorochromate2,
tributylammonium
chlorochromate3, pyridinium fluorochromate4, imidazolium dichromate5 and isoquinolinium
bromochromate6 for the study of the kinetics and mechanism of various organic compounds.
Extensive studies on the mechanism of oxidation of methionine by several oxidants
have been reported7-9. Methionine is an intermediate in the biosynthesis of cysteine,
carnitine, taurine, lecithin, phosphatidylcholine and other phospholipids. It is a naturally
occurring sulphur containing amino acid, has three coordination sites: at the N, O and S
centers. Sulphur has been established as the most susceptible to attack by chromium(VI)10-12
,
where the formation of an intermediate (chromate ester) provides a low energy path for
electron transfer. Methionine is a methyl door and this process in the body is activated by
adenosine triphosphate (ATP) and a liver enzyme such as phosphatase or dehydrogenase13.
Oxidation of methionine by chromium(VI) has been investigated by a number of workers14-16
.

644
S. SHEIK MANSOOR
Literature survey reveals that no report is available on the kinetics of oxidation of
methionine by TriPAFC. In this article, the kinetics and mechanism of the oxidation of
methionine by TriPAFC in aqueous acetic acid mixture are reported, with the view to
understand the utility of solvent variation studies in the understanding of the mechanism of
this biologically important amino acid because it may reveal the mechanism of amino acid
metabolism.
Experimental
Tripropylammine and chromium trioxide were obtained from Fluka (Buchs, Switzerland).
DL-methionine (E Merck, Germany) was used as received. Acetic acid was purified by
o
standard method17 and the fraction distilling at 118 C was collected. All other chemicals
used were of AnalaR grade.
Preparation of tripropylammonium fluorochromate
Tripropylammonium fluorochromate was easily prepared as follows: Chromium(VI) oxide
(15.0 g, 0.150 mol) was dissolved in water in a polyethylene beaker and 40% hydrofluoric
acid (11.3 mL, 0.225 mol) was added with stirring at 0 oC. To the resultant orange solution,
tripropylammine (28.3 mL, 0.150 mol) was added drop wise with stirring to this solution
o
over a period of 0.5 h and stirring was continued for 0.5 h at 0 C. The precipitated orange
solid was isolated by filtration, washed with petroleum ether (3x60 mL) and dried in vacuum
for 2 h at room temperature18. Yield 37.5 g (95%); mp 142 o C.
CrO3 / 40% HF
N(C3H7)3
(C3H7)3NH+[CrO3F]-
(1)
0 oC
Tripropylamine
Tripropylammonium flourochromate
The bright orange crystalline reagent was stored in polyethylene containers for long
periods without decomposition. The chromium(VI) content was easily determined
iodometrically. Tripropylammonium fluorochromate was soluble in water, dimethylformamide,
acetonitrile, acetone and dichloromethane and was sparingly soluble in benzene, carbon
tetrachloride, chloroform and hexane.
Stoichiometric analysis and product study
The stoichiometry of the reaction was determined by performing the several sets of
experiments with varying amounts of TriPAFC largely in excess over methionine. The
disappearance of TriPAFC was monitored until constant titre values were obtained.
Me – S – R + O2CrFO-TriPNH+ → Me – SO – R + OCrFO- TriPNH+
(R = – CH2CH2CH(NH2)COOH)
(2)
The reaction mixture was allowed to stand for a few hours. Then, sodium bicarbonate
was added and stirred vigorously, followed by drop wise addition of benzoyl chloride
solution. The precipitate N - benzoyl methionine sulphoxide was confirmed19 by its m.p 183 oC.
The procedure is similar to the one employed in the oxidation of L–methionine20 by
chromium(VI). Acetone-ethanol mixture (1:1) added to the reaction mixture resulted in the
precipitate of methionine sulphoxide, which was identified 21 by its m.p 238 oC.
Kinetic measurement
The pseudo-first-order conditions were attained by maintaining a large excess (x 15 or more)
of methionine over TriPAFC. The reactions were carried out in aqueous acetic acid medium
in the presence of chloroacetic acid. The studies were carried out in the temperature range of

Oxidation of Methionine by Tripropylammonium Fluorochromate
645
298 – 313 K. All the solutions were kept in a thermostat at constant temperature which was
o
controlled using a thermostat of 0.1 C accuracy. The required volumes of these solutions
for each run were mixed and 2 mL aliquots of the reaction mixture were pipetted out at
convenient time intervals and quenched in 10 mL 2% KI solution and the liberated iodine
was titrated against thiosulphate using starch as indicator. The pseudo-first-order rate
constants were evaluated from log titre versus time plots. All the rate constants reported are
in an average of two or more determinations. The second order rate constant k2, was
obtained from the relation k2 = kobs / [Met].
Data analysis
Data analysis were performed using microcal origin (version 6.0) computer software. The
goodness of the fit is discussed using the correlation coefficients and standard deviations.
Results and Discussion
Order of reaction
The oxidation of methionine with TriPAFC in aqueous acetic acid medium in the presence
of chloroacetic acid yields sulphoxide. The rate of oxidation was found to be first order in
[Met]. Linear plots of log k1 versus log [Met] with unit slope demonstrate the first-order
dependence of the rate on [Met]. The near constancy in the values of k1 irrespective of the
concentration of the TriPAFC confirms the first order dependence on TriPAFC (Table 1).
The dependence of the reaction rate on the hydrogen ion concentration has been
investigated at different initial concentrations of chloroacetic acid while keeping the
concentrations of the other reactants constant. It may be seen that the rate of reaction increases
linearly with an increase in the hydrogen ion concentration (Table 1). A plot of log k1 versus
log [H+] is linear with a unit slope. The reaction proceeds completely through an acid-caalysed
pathway 22. The acid catalysis may well be attributed to the protonated ion on TriPAFC to give
a stronger oxidant and electrophile. Therefore the rate law can be represented as:
– d[TriPAFC] / dt = k [Met] [TriPAFC] [H+]
Table 1. Effect of varying the concentration of [Met], [TriPAFC] and [H+] on the rate of
reaction at 303 K, Solvent composition = 30% AcOH - 70% H2O (v/v)
103[TriPAFC]
102 [Met]
mol.dm-3
2.0
[H+]
mol.dm-3
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.16
0.48
0.64
0.80
0.32
0.32
104 k1 s-1
mol.dm-3
0.5
1.0
5.56 0.10
5.52 0.08
5.48 0.16
5.58 0.04
5.50 0.06
5.52 0.08
4.08 0.08
6.82 0.12
8.22 0.12
2.72 0.8
8.18 0.10
11.00 0.12
13.68 0.14
4.84 0.04a
2.64 0.05b
2.0
2.0
1.5
2.0
2.0
2.0
2.0
1.5
2.5
1.0
1.0
1.0
2.5
3.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
1.0
1.0
2.0
2.0
1.0
a Contained 0.001 mol dm-3 acrylonitrile. b In the presence of 0.003 mol dm-3 Mn (II)

646
S. SHEIK MANSOOR
Induced polymerisation
The oxidation of Met in an atmosphere of nitrogen failed to induce the polymerization of
acrylonitrile. Furthermore, the rate of oxidation decreased with the addition of Mn(II) (Table 1).
Therefore, a one-electron oxidation giving rise to free radicals is unlikely.
Effect of solvent composition
The effect from solvent composition on the reaction rate was studied by varying the
concentration of acetic acid from 30% to 70%. The reaction rate is increases markedly with
the increase in the proportion of acetic acid in the medium (Table 2). When the acid content
increases in the medium, the acidity of the medium is increased whereas the dielectric
constant of the medium is decreased. These two effects cause the rate of the oxidation to
increase markedly. The enhancement of the reaction rate with an increase in the amount of
acetic acid generally may be attributed to two factors, viz, (i) the increase in acidity
occurring at constant [H+] and (ii) the decrease in the dielectric constant with an increase in
the acetic acid content. The plots of log k1 against the inverse of the dielectric constant are
linear with positive slopes, indicating that an interaction between a positive ion and a dipolar
molecule23.
Table 2. Effect of varying solvent polarity on the rate of reaction at 303 K [Met] = 2.0x10-2
mol.dm-3; [TriPAFC] = 1.0x10-3 mol.dm-3 ; [H+] = 0.32 mol.dm-3
%Water
70
60
%Acetic acid
Dielectric constant
104k1 s-1
5.52 0.08
6.52 0.04
7.76 0.12
8.60 0.06
9.84 0.11
30
40
50
60
70
72.0
63.3
56.0
45.5
38.5
50
40
30
Mechanism of oxidation
Based on the above kinetic observations, i.e. first order dependence on [Met], [TriPAFC]
and [H+], the following mechanism is proposed. Under the present experimental conditions,
methionine is oxidized to the sulphoxide stage only. The linear increase in the rate with
acidity suggests the involvement of protonated TriPAFC in the rate determining step. In the
first step TriPAFC becomes protonated. The protonated TriPAFC attacks the substrate, in a
slow step, to form a complex, which subsequently decomposes to give the products in a fast
step. The proposed scheme envisages an oxygen atom transfer from the oxidant and that is in
agreement with the earlier observations. The electrophile attack on the sulphide - sulphur
can be viewed as an SN2 reaction14.
Rate law
The above mechanism leads to the following rate law:
Rate
=
=
=
=
=
k3 [Complex]
k2 [Met] [TriPAFCH+]
k1[TriPAFC][H+]
k1k2k3[Met][ TriPAFC][H+]
k1k2k3 [Met] [TriPAFC] [H+]
[Complex]
[TriPAFCH+]
[Complex]
– d[TriPAFC] / dt
The rate law in its final form accounts for the observed kinetics. The negative entropy
of activation suggests complex formation in the transition state. The linear increase in rate
with acidity suggests the involvement of protonated TriPAFC in the rate-determining step.

Oxidation of Methionine by Tripropylammonium Fluorochromate
647
Thermodynamic parameters
The kinetics of oxidation of methionine was studied at four different temperatures viz., 298,
303, 308 and 313 K in acetic acid - water medium in presence of chloroacetic acid. The
second order rate constants were calculated (Table 3). The Arrhenius plot of log k2 versus
1/T is found to be linear. The enthalpy of activation, entropy of activation and free energy
of activation were calculated from k2 at 298, 303, 308 and 313 K using the eyring
relationship by the method of least square and presented in Table 3. The least square method
gives the values and standard errors of enthalpy and entropy of activation respectively.
Statistical analysis of the Eyring equation clearly confirms that the standard errors of ∆H#
and ∆S# correlate24. The entropy of activation is negative for methionine. The negative
entropy of activation in conjunction with other experimental data supports the mechanism
outlined in (Scheme 1).
Table 3. Activation parameters and second order rate constants for the oxidation of
methionine by TriPAFC [Met] = 2.0x10-2 M; [TriPAFC] = 1.0x10-3 M; [H+] = 0.32 M
Solvent composition = 30% AcOH- 70% H2O (v/v)
102 x k2,
- ∆S#
Ea
kJmol-1 kJmol-1
∆H#
∆G# kJmol-1
(at 303K)
JK-1mol-
Substrate
298K 303K 308K 313K
1
1.74
0.12
2.76
0.04
4.16
0.06
6.16
0.10
65.2
0.9
62.6
0.7
68.2
2.0
4 83.3
1.4
Methionine
HO
O
O
O
k1
k-1
Cr+
+
H+
Cr
F
O-TriPNH+
(3)
(3)
F
O-TriPNH+
TriPNH+
=
(C3H7)3NH+
HO
F
O
HO
O
+
CH3
CH3
R
k2
Cr+
+
S
Cr
S
(4)
(4)
R
O-TriPNH+
F
O-TriPNH+
R = ( - CH2-CH2-CH(NH2)COOH )
k3
O
CH3
O-TriPNH+
O = S
+
Cr
(5)
(5)
R
F
Cr (IV)
Scheme 1

648
S. SHEIK MANSOOR
Conclusion
In conclusion the oxidation of methionine by TriPAFC is first order with respect to [Met],
[TriPAFC] and [H+]. Methionine is oxidized to sulphoxide stage only. The entropy of
activation is negative, suggesting the formation of a complex in a rate-determining step.
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