Oxidation of Amino acids by Manganese (III) in aqueous Sulphuric acid

Abstract

The Kinetics of Oxidation of amino acids glycine, alanine and valine by manganese (III) acetate in aqueous sulphuric acid has been studies. The reaction is found to proceed through the formation of intermediate complex. This shows an inverse dependence on [H 2 SO 4 ]. The reaction has second order dependence with respect to Mn (III) and first order dependence with respect to substrate. The mechanism consistent with rate data has been proposed.

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Journal of Chemical and Pharmaceutical Research
__________________________________________________
ISSN No: 0975-7384
CODEN(USA): JCPRC5
J. Chem. Pharm. Res., 2011, 3(1):529-535
529
Oxidation of Amino acids by Manganese (III) in aqueous Sulphuric acid
Ritu Singh, D. K. Tamta, S. K. Joshi
*
, N. Chandra and N. D. Kandpal
Physical Chemistry Laboratory, Department of Chemistry, Kumaun University, S.S.J. Campus,
Almora, Uttarakhand, India
_____________________________________________________________________________
ABSTRACT
The Kinetics of Oxidation of amino acids glycine, alanine and valine by manganese (III) acetate
in aqueous sulphuric acid has been studies. The reaction is found to proceed through the
formation of intermediate complex. This shows an inverse dependence on [H
2
SO
4
]. The reaction
has second order dependence with respect to Mn (III) and first order dependence with respect to
substrate. The mechanism consistent with rate data has been proposed.
Key Words: Amino acid, Oxidation, Kinetics, Mn(III), first order, Sulphuric acid.
______________________________________________________________________________
INTRODUCTION
Manganese compounds have attracted much attention with regards to the oxidation of various
biological substrates. In this respect Mn(III) oxidation of amino acids, their derivatives, protein-
based polymers and peptides are gaining special importance owing to their biological relevance.
Kinetics of oxidation of these compounds has been studied using Mn(III) as an oxidant in
different media [1,2,3,4,5,6]. The kinetic studies on Mn(III) oxidation of organic or inorganic
substrate in general and medicinal compounds in particular have been reported in perchlorate,
sulphate, acetate and pyrophosphate medium [7, 8, 9, 10]. A medium can influence the reaction
rate, due to its polarisability, hydrogen bond acceptor ability, electrophilicty, nucleophilicity and
specific orientation including associative or dissociative nature. It is a specific character of
manganese (III) that it can form different reactive species in presence of different acids.
Generally the manganese (III) can be obtained by the processes of electrolysis [9] and it has a
tendency to undergo hydrolysis. Manganese(III) sulphate has been scarcely used [11-14] in
redox studies due to the difficulty in obtaining it in the pure and stable form. The manganese (III)
acetate has been obtained in pure and stable form.
In the oxidation studies by manganese (III) species in sulphuric, perchloric, pyrophosphate and
acetic acid media have different nature. To our knowledge there is no any report on the

S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
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530
manganese(III) acetate oxidation in sulphuric acid. Manganese(III) acetate forms Mn(OAc)
3-
in
acetic acid which form Mn(OAc)
4-
by addition of acetate ion. In this study it is sort to understand
how the manganese (III) acetate changes in the solution environment by taking sulphuric acid in
place of acetic acid and how these species can bring about changes in the oxidation mechanism
of amino acids namely Glycine, Alanine and Valine. Various studies have been reported on the
oxidation of amino acids by various oxidants including manganese(III) [8,9,11,13,15,16,]. In
sulphuric acid media the Mn(III) species are Mn(III)
aq
, MnOH
2+ aq
and MnSO
4+
and MnOH
2+ aq
has been reported [9]. In oxidation the combine role of Mn(III)
aq
and MnOH
2+ aq
has been
observed as reactive species. In presence of acetic acid the reactive species of manganese (III)
acetate is different from the sulphuric acid medium. In this media Mn(OAc)
4-
has been reported
[17]. We have used manganese(III) acetate in sulphuric acid to study the kinetics of oxidation of
amino acids to investigate the mechanism of the reaction and reactive manganese(III) species.
Secondly this study is essential in order to investigate the micellar effect on oxidation of amino
acid under similar conditions.
EXPERIMENTAL SECTION
The kinetics studies were carried out in Pyrex conical flask under pseudo first order reaction
condition. The reaction where initiated by reaction of measured amount of substrate solution to
the reaction mixture containing requisite quantities of other reactants. The progress of the
reaction was monitored by the spectrophotometric estimation of Mn(III) at 340 nm. The second
order rate constant were obtained from the slop of the plots between concentration of Mn(III)
against time. The reproducibility of rate constant from replicate run was always higher than
98.5%. The amount of the manganese(III) used per mole of organic substrate was estimated by
taking Mn(III) in large excess under experimental condition. The following stochiometric
reactions obtained from experimental results are given below-
2Mn(III) + RCH(NH
2
)COOH + H
2
O → 2Mn(II) + RCHO + NH
3
+ CO
2
+ 2H
+
Where R = H (glycine), CH
3
(alanine), (CH
3
)
2
CH (Valine)
RESULT AND DISCUSSION
Stochiometric determination indicated the following overall reaction for the oxidation of amino
acids.
2Mn(III) + RCH(NH
2
)COOH + H
2
O → 2Mn(II) + RCHO + NH
3
+ CO
2
+ 2H
+
....1
Where R = H (glycine), CH
3
(alanine), (CH
3
)
2
CH (Valine)
Table 1: Variation of rate with initial [Mn (III)] at 301 K
[Amino acid] = 0.045 mol dm
-3
, [Mn (II) acetate] = 0.10 mol dm
-3
, [H
2
SO
4
] = 1.5 mol dm
-3
S.No. [Mn (II)] mol dm
-3
10
2
k
obs
(mol
-
1
dm
3
sec
-
1
)
Glycine Alanine Valine
1 0.0015 1.29 1.21 0.77
2 0.0025 1.30 1.22 0.78
3 0.0030 1.31 1.24 0.79
4 0.0035 1.32 1.24 0.79
5 0.0040 1.33 1.25 0.80

S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
______________________________________________________________________________
531
Effect of Oxidant:
When amino acids were in excess, the rate at which Mn (III) disappears followed the pseudo
second order rate law. The plot between time and concentration of manganese(III) was linear.
The value of gradient of these plots gives the value of specific rate constant. The rate constants
obtained with the variation of initial concentration of the Mn(III) in the range 0.0015 mol dm
-3
to
0.0040 mol dm
-3
are given in Table 1.
Effect of substrate: At constant Mn(III) concentration the reaction rate was observed with the
variation of initial concentration of amino acids from 0.02 mol dm
-3
to 0.07 mol dm
-3
.The values
of rate constant are given in Table 2.
Table 2 : Variation of rate with initial [substrate] at 301 K
[Amino acid] = 0.0040 mol dm
-3
, [Mn (II) acetate] = 0.10 mol dm
-3
, [H
2
SO
4
]= 1.5 mol dm
-3
S.No. [Substrate] mol dm
-3
10
2
k
obs
(mol
-
1
dm
3
sec
-
1
)
Glycine Alanine Valine
1 0.02 0.63 0.59 0.38
2 0.03 0.90 0.85 0.54
3 0.04 1.23 1.16 0.74
4 0.05 1.53 1.44 0.92
5 0.06 1.83 1.72 1.10
6 0.07 2.13 2.00 1.28
The rate increased with increase in initial concentration of the substrate. A plot of [Substrate]
versus K
obs
is linear as illustrated in Fig1.
y = 0.0329x - 0.0003
R
2
= 0.9996
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 0.5 1 1.5 2 2.5
[Glycine] (mol dm
-3
)
10
2
kobs (mol
-1
dm
3
sec
-1
)
Fig 1: Plot of [Glycine] versus K
obs
at 301K
Effect of H
2
SO
4
: At constant concentration of amino acids and Mn(III) the rate constants were
measured with the variation of initial concentration of sulphuric acid in the range from 1.0 mol
dm
-3
to 3.0 mol dm
-3
.The rate constants obtained are listed in Table 3

S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
______________________________________________________________________________
532
Table 3 : Variation of rate with initial [H
2
SO
4
] at 301 K
[Amino acid] = 0.045 mol dm
-3
, [Mn (III) acetate] = 0.0040 mol dm
-3
, [Mn (II) acetate] = 0.10 mol dm
-3
S.No. [H
2
SO
4
] mol dm
-3
10
2
k
obs
(mol
-
1
dm
3
sec
-
1
)
Glycine Alanine Valine
1 1.0 2.01 1.89 1.21
2 1.5 1.33 1.25 0.80
3 2.0 1.08 1.01 0.64
4 2.5 0.88 0.82 0.52
5 3.0 0.71 0.67 0.42
Increase in [H
2
SO
4
], decreased the rate and the plots of 1/[H
2
SO
4
] versus K
obs
were linear for
each amino acids. An illustrative plot is given in Fig. 2.
y = -0.61x + 2.422
R2 = 0.9033
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5
1/[H2SO4]
102 kobs (mol-1 dm3 sec-1)
Fig 2: Plot of 1/[H
2
SO
4
] versus k
obs
for glycine at 301K
Effect on initial [Mn (II)]: We have always used [Mn(II)] = 0.10 mol dm
-3
. At lower
concentration the variation of initial concentration of Mn(II) 0.04 mol dm
-3
to 0.10 mol dm
-3
.The
experimental results are given in Table 4
Table 4 : Variation of rate with initial [Mn (II)] at 301 K
[Amino acid] = 0.045 mol dm
-3
, [Mn (III) acetate] = 0.004 mol dm
-3
,
[H
2
SO
4
] = 1.5 mol dm
-3
S.No. [Mn (II)] mol dm
-3
10
2
k
obs
(mol
-
1
dm
3
sec
-
1
)
Glycine Alanine Valine
1 0.04 1.37 1.29 0.82
2 0.05 1.35 1.26 0.80
3 0.06 1.32 1.23 0.82
4 0.08 1.31 1.23 0.79
5 0.10 1.33 1.25 0.80
The experimental data of Table 4 indicates that the addition of Mn(II) has no effect on reaction
rate. It is used to stabilize the Mn(III) in aqueous sulphuric acid. In the lower concentration range
up to 0.1 dm
-3
it has no inhibitory effect on reaction rate as reported in literature [16].

S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
______________________________________________________________________________
533
The results of the study of oxidation of amino acids by manganese (III) acetate in sulphuric acid
could be summarized as below.
i. The reactions have second order dependence with respect to manganese (III) because the
time versus [manganese (III)] plots is linear in nature. The value of linearity always found
greater than 0.95.
ii. The order with respect to the substrate concentration is always first as confirmed by a linear
plot between [substrate] versus k
obs
values, which passed through origin.
iii. The reaction rate is retarded by H
2
SO
4
. The reaction shows an inverse dependence on
[H
2
SO
4
] and the plot of 1/[ H
2
SO
4
] versus k
obs
is linear which didn’t pass through the origin.
This highlights the point that it is the unprotonated species, which takes part into the reaction.
iv. The concentration of Mn (II) in the reaction mixture is 0.1 mol dm
-3
. Below this
concentration the variation of Mn (III) has no effect on reaction rate. In the study the Mn (II)
promotes the stability of Mn(III) in the solution through the established equilibrium.
Mn (III) + Mn (III) Mn (IV) + Mn (II) …2
v. The absence of polymerization in the reaction mixture in presence of acrylonitrile indicates
the absence of free radical formation in the oxidation process. It reveals that the reaction
takes place through the formation of intermediate complex.
The mechanism for the oxidation of amino acids in sulphuric acid medium has been proposed by
considering that the manganese (III) and substrate molecule interacts with each other to yield the
product as given in scheme 1.
S
+
H
+
k
1
SH
+
(Fast)
…(i)
S
+
Mn(III) K
2
k
2
X
(Fast)
…(ii)
X
+
Mn(III) k
3
→
S’
+
H
2
O
…(iii)
S’ → Product (Intermediate) …(iv)
Scheme 1
Where, S = Substrate or Amino acid
S’= Intermediate
SH
+
= Protonated substrate or protonated amino acid

S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
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534
Amino acid in the solution in presence of acid can have protonated and unprotonated form. The
total concentration of the amino acid can be given as
[S]
T
= [S] + [SH
+
] …(3)
According to proposed scheme1 the concentration of unprotonated amino acid can be calculated
as
K
1
= [SH
+
]
[S] [H
+
]
According to the proposed scheme 1 the rate of oxidation may be represented as
- d[Mn(III)] =
K
2
k
3
[S]
t
[Mn(III)]
2
…(5)
dt {1+ K
1
[H]}
K
2
k
3
[S]
t
[Mn(III)]
2
…(6)
k
obs
=
{1+K
1
[H]}
The rate law equation 6 is consistent with the experimental data, first order in substrate, second
order in Mn(III) and inverse first order in H
2
SO
4
. The second order in Mn (III) is in agreement
with the observed kinetics and earlier reported results
[15, 18]. The addition of Mn(II) has no
effect on the rate which is in agreement with the previous report on oxidation of amino acids by
manganese(III) acetate in sulphuric acid [19]. In sulphuric acid, it was shown that the manganese
(III) solution in aqueous sulphuric acid contains Mn(III)
aqua
and MnOH
2+aqua
as reactive spices.
Both the species remains in equilibrium
[18].
CONCLUSION
In our study we have taken Mn(III) as reactive species which is consistent with the report in
which Mn(III) has been considered more reactive [20]. The decrease in reaction rate with
increase in sulphuric acid concentration may attribute to the formation of protonated species of
the substrate which is non reactive in oxidation process. In case of amino acids the decrease was
found in the reaction rate with increase of [H
+
/H
2
SO
4
] has been reported which also supports our
observations [21]. Finally, the observed kinetic data and others results discussed earlier are
supportive of the derived rate law equation 6, the proposed mechanism in scheme 1 and
kinetically considered active species Mn(III) involve in the oxidation.
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S. K. Joshi et al J. Chem. Pharm. Res., 2011, 3(1):529-535
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