PERMANGANATE OXIDATION OF L-ALANINE IN ACIDIC SOLVENT: AUTOCATALYSIS BY INTERMEDIATE Mn(IV) SPECIES

Article abstract of DOI:10.1135/cccc19940528

The kinetics of L-alanine oxidation by permanganate ion in buffer solutions at pH 2.2 have been studied.Both spectrophotometric and kinetic evidence is provided for the existence of an intermediate Mn(IV) species of colloidal nature.An autocatalytic course of the oxidation is discussed.

Full text of DOI:10.1135/cccc19940528

5
28
Insausti, Mata-Perez, Alvarez-Macho:
PERMANGANATE OXIDATION OF L-ALANINE IN ACIDIC SOLVENT:
AUTOCATALYSIS BY INTERMEDIATE Mn(IV) SPECIES
Maria J. INSAUSTI, Fernando MATA-PEREZ and Maria P. ALVAREZ-MACHO
Departamento de Quimica-Física,
Facultad Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain
Received January 18, 1993
Accepted May 11, 1993
The kinetics of L-alanine oxidation by permanganate ion in buffer solutions at pH 2.2 have been stu-
died. Both spectrophotometric and kinetic evidence is provided for the existence of an intermediate
Mn(IV) species of colloidal nature. An autocatalytic course of the oxidation is discussed.
Among oxidizing agents used in the laboratory, potassium permanganate had been fre-
1
quently preferred . This gave rise to numerous studies on the behaviour of this ion as
an oxidizing agent.
In view of the important role of amino acids in metabolism, kinetic studies on per-
manganate oxidation of a few amino acids have also been carried out. Thus, it has been
2
– 5
found that the oxidation of alanine in strongly acidic solvents
is catalyzed by Mn(II)
species. On the other hand, within the range of physiological pH (that is between
pH 6 – 8), it has been shown that the same reaction is autocatalyzed by Mn(IV)
species, which is temporarily soluble, as long as phosphate ions are present in the
6
– 15
solvent
. However, opinions are not unanimous in this respect. So, for instance,
6
other workers refer to Mn(III) as the autocatalyst for the permanganate oxidation of
phenylalanine.
However, there are no reports on permanganate oxidation of L-alanine in the acid pH
region. This led us to study the oxidation at pH about 2.00. The aim was to describe
kinetics of the reaction with respect to its possible autocatalytic character, and especial-
ly to elucidate the nature of the catalyst.
EXPERIMENTAL
The reagents used were potassium permanganate, potassium chloride and L-alanine from Merck
(
p.a.), phosphoric acid and potassium dihydrogen phosphate from Panreac (p.a.).
Kinetic measurements. Kinetic runs were followed spectrophotometrically using a Spectronic 1201
instrument. The reactor was a special quartz cell (with 1 cm of optical pathway) provided with an
internal temperature control system connected to a conventional thermostat (Frigiter model), which
ensured temperature stability during the runs within 0.05 °C. The measurements were carried out at 40 °C.
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Permanganate Oxidation of L-Alanine
529
Radiometer 51 pH-meter provided with a combined electrode was used to measure pH. In all the
−4
−3
experiments pH was set to 2.2. Reagent concentrations were 4 . 10 mol dm for the permanganate
−2
−3
and 5 . 10 mol dm for L-alanine in an aqueous phosphoric acid–potassium dihydrogen phosphate
−3
buffer (ionic strength 0.05 mol dm ).
Stoichiometry. The stoichiometry of the reaction was determined by the analysis of both KMnO4
and CH CHO species. Permanganate was determined by quenching the known amount of the reaction
3
2
+
2+
mixture with the known excess of Fe , followed by titration of the unreacted Fe against the
4
+
standard Ce . The aldehyde was determined colorimetrically, using chromotropic acid. The
KMnO /CH CHO ratio was found to be about 3 : 5. Hence, the stoichiometry is as follows:
4
3
MnO4 + 5 CH3CHNH3CO2H + H+ −−−−> 3 Mn2+ + 5 NH3 +
−
3
5
CO2 + 5 CH3CHO + 7 H2O .
(A)
RESULTS AND DISCUSSION
KINETICS
UV spectra (Figs 1 and 2) of the reaction mixtures clearly show a decrease of the
absorbance at 525 nm during the reaction. Furthermore, the absorbance at 418 nm first
rises, then falls, becoming zero in later stages.
In this range of the UV-VIS spectrum, a number of different oxidation states of
1
,16,17
18,19
manganese show optical absorption
. First, proofs have been given
that the
band at 418 nm is characteristic of Mn(IV) species. It is possible that the formation of
both Mn(IV) and Mn(III) species takes place from a transient compound of manganese,
FIG. 1
FIG. 2
Sucessive scans at 8 min intervals for the re-
Sucessive scans at the end of the reduction of
−4
−3
−4
−3
duction of permanganate ion (4 . 10 mol dm )
permanganate ion (4 . 10 mol dm ) by L-alanine
−3
−3
by L-alanine (0.05 mol dm ) at 40 °C, pH 2.2
(0.05 mol dm ) at 40 °C, pH 2.2 and ionic
−3
−3
and ionic strength 0.05 mol dm
strength 0.05 mol dm
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

5
30
Insausti, Mata-Perez, Alvarez-Macho:
such as Mn(VI), which perhaps is formed during the observed induction period. In fact,
the spectra of the reaction mixtures (Fig. 1) are consistent with those of both Mn(VI)
1
and Mn(VII) species and support the above assumption. However, there is no evidence
that under our experimental conditions Mn(III) exists as an unstable reaction intermedi-
2
0
ate. This species occurs only in strongly acidic media . So, when the acidity decreases,
2
1
Mn(III) disproportionates into Mn(IV) and the product , Mn(II).
On the contrary, it is well known that the absorbance at 418 nm, A418, is a measure
of the Mn(IV) produced during the reaction. However, both the plot of ln A418 vs time
and the plot of A525 vs A418 are not linear. These findings exclude that Mn(IV) is the
reaction product. On the other hand, the plot of A525 vs time, t (Fig. 3) has a sigmoid
2
2
profile, which is characteristic of autocatalytic reactions . Besides, the values of A
4
18
vs t (Fig. 4, curve 1) first increase, reaching a maximum value, and then fall rapidly to
zero. A similar result (a bell profile curve) was found for the experimental reaction rate
vs time dependence (Fig. 4, curve 2). Moreover, the maximum values of both the reac-
tion rate and the absorbance at 418 nm, A418, correspond to the inflection point of the
A525 vs time plots (Fig. 3). All these findings are consistent with the assumption that the
reaction is autocatalyzed by Mn(IV) species. On the other hand, a detailed analysis of
curve 2 in Fig. 4 reveals the existence of an induction period. Comparison of curves 1
FIG. 3
FIG. 4
Absorbance at 525 nm vs time for the oxidation
Absorbance at 418 nm vs time (1) and reaction
−3
−3 −1
of L-alanine (0.05 mol dm ) by potassium per-
rate vs time (r, mol dm
nate concentration 4 . 10 mol dm , L-alanine
concentration 0.05 mol dm , 40 °C, pH 2.2
s ) (2). Permanga-
−
4
−3
−4
−3
manganate (4 . 10 mol dm ) at 40 °C, pH
2
−3
−3
.2 and ionic strength 0.05 mol dm
−3
and ionic strength 0.055 mol dm
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Permanganate Oxidation of L-Alanine
531
and 2 (Fig. 4) shows that for the induction period the absorption at 418 nm remains
almost constant.
The induction period is very sensitive to changes in the concentration of the equimo-
lar H PO –H KPO mixture when the other experimental conditions remain constant.
3
4
2
4
The analysis of Fig. 5 reveals that Mn(IV) is a soluble species longer to appear. Its
concentration decreases (as reflected in the drop of the maximum absorption at 418 nm)
as the concentration of H PO –H KPO increases. A similar effect of the phosphate
3
4
2
4
3,24
2
buffer concentration has been found
also for neutral solvents (pH from 6 to 8).
However, in those cases the colloidal nature of Mn(IV) species was proved.
In spite of its soluble nature, for the present acidity conditions Mn(IV) species shows
2
5,26
the characteristic behaviour of colloids
. Indeed, at the moment when Mn(IV)
species is detectable at 418 nm (Fig. 4, curve 2), the log plot of the absorbance against
wavelength gives the straight line with the slope −3.90 which is very close to the the-
oretical value of Rayleigh’s equation (1)
5
2
log A = log (128 π α c N l ⁄ 3M) − 4 log λ ,
(1)
3
where A absorbance of colloid solution, α polarizability of colloid particles in m ,
−
3
c concentration in kg m , N Avogadro’s number, l length of the cell containing the
−
1
solution in m, M statistical molar mass of colloid particles in kg mol and λ is wave-
length in nm.
FIG. 5
Absorbance at 418 nm vs time for the oxidation
−3
of L-alanine (0.5 mol dm ) by permanganate ion
−
4
−3
(
4 . 10 mol dm ) at 40 °C, pH 2.2. Ionic
−3
strength: 1 0.05, 2 0.15 and 3 0.25 mol dm
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

5
32
Insausti, Mata-Perez, Alvarez-Macho:
The value of the slope which is now obtained (3.90) is slightly lower than that
found15 for the permanganic oxidation of L-alanine at pH 6 – 9 (5.9). But in that case
the colloid flocculated, which was not detected under our experimental conditions. On
the contrary, the soluble form of Mn(IV) which was produced at pH 2.2 was finally
converted into a Mn(II) species transparent to radiation at both 418 and 525 nm. Such
2
7 – 33
a form of Mn(IV) has already been postulated
as an intermediate. On the other
3
4
hand the reduction of Mn(IV) to Mn(II) is favoured by solvent acidity . Therefore, the
fact that the colloidal Mn(IV) species is converted to Mn(II) at pH 2.2 means only that
in this solvent its reduction is fast, compared to its flocculation.
Because of the induction period (Fig. 4), the analysis of ln A525 vs time plot is not
conventional. For this reason, an alternative treatment proposed by Bazsa et al. was
3
5 – 37
used
. Two parallel reaction pathways were considered, one being autocatalytic.
For this model, time at the inflection point, t , on the absorbance vs time plots (which
i
evidently corresponds to the maximum reaction rate) is given as:
p
ti
=
,
(2)
k + k [Mn(IV)]
0
1
2
where k and k are the specific non-catalytic and catalytic reaction rate, respectively,
1
2
[
Mn(IV)] is the initial catalyst concentration and p is a mathematical parameter. From
0
−
4
our data the ratio k /k = 1.03 . 10 , which is of the same order of magnitude as that
found for neutral solvents .
1
2
1
5
The interrelation between t and Mn(IV) concentration permits to use the t parameter
i
i
to study the influence of experimental variables on the autocatalyst, in a similar way to
3
5 – 37
other systems
.
−
4
When the initial permanganate concentration was changed from 3 to 7 . 10
3
mol dm , the t values have increased, leading to the following equation (r = 0.9973)
i
xy
−
log t = 3.64 + 0.48 [MnO ] .
(3)
i
4 0
However, an increase in the initial amino acid concentration results in a decrease of
ti values. There exists a linear relation (rxy = 0.9932) between log t and log [alanine].
i
This is consistent with Eq. (2) because both k and k are actually pseudofirst-order rate
1
2
−
4
−3
constants, depending upon the amino acid concentration (2 to 8 . 10 mol dm ). In
fact, the slope found (−1.02) confirms the first order of the oxidation with respect to the
amino acid.
log ti = 0.69 − 1.02 log [alanine]
(4)
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Permanganate Oxidation of L-Alanine
533
The influence of the phosphates was also analyzed in detail (Table I). For these
purposes, experiments were carried out by changing concentrations of both KH PO
2
4
and H PO while their mutual ratio was maintained constant. Hence, pH is constant but
3
4
the ionic strength of the solvent is varied. Figure 5 shows that the induction period
increases on increasing the phosphate concentration. Nevertheless, this trend was not
observed when, for a given phosphate concentration, the ionic strength of the solvent
was controlled by the addition of KCl. This result points to that in the formation of the
intermediate Mn(IV) species, the phosphates have a specific role similar to that sug-
1
3
gested for neutral solvents . On the other hand, the results obtained by using KCl
(
Table II) prove the existence of the salt kinetic effect under the conditions used.
The values of ln t vs 1/T (31 to 46 °C) fit well the Arrhenius equation, the activation
i
−
1
energy being E = 72.2 kJ mol , (r = 0.9998, Eq. (5)):
a
xy
ln t = −23.25 + 8 636.55 1 ⁄ T .
(5)
i
TABLE I
−
−4
−3
Influence of the buffer concentrations on the inflection time t ; [MnO ] = 4 . 10 mol dm ,
alanine] = 0.05 mol dm , pH 2.2, 40 °C
i
4
−3
[
[
H PO ]
[KH PO ]
ti, min
3
4
2
4
0.043
0.087
0.130
0.174
0.05
0.10
0.15
0.20
77
85
89
101
TABLE II
−
4
Influence of the potassium chloride concentration on the inflection time t ; [MnO ] = 4 . 10
i
4
−3
−3
−3
mol dm , [alanine] = 0.05 mol dm ; µ = 0.05 mol dm , pH 2.2, 40 °C
[
KCl]
ti, min
0.0
0.1
0.2
0.3
0.4
77
85
93
97
109
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

5
34
Insausti, Mata-Perez, Alvarez-Macho:
MECHANISM
The experimental rate equation indicates that the oxidation of alanine involves two
parallel reactions, of which only one produces Mn(IV) which acts as the catalyst. Both
mechanisms should lead to the first-order dependence of the reaction rate on the alanine
concentration.
In a number of preceding investigations19,33,38, homogeneous pathway has been con-
sidered as the key for the Mn(IV)-catalyzed mechanism (heterogeneous). Under the
present experimental conditions, no findings contradicts this assumption. Hence, two
parallel mechanisms have been considered as interdependent. This assumption is the
3
8
main difference between our contribution and that by Hussain et al. who proposed
two quite independent routes.
With this in mind, the following scheme can be proposed:
K1
ACO H + ACO H
D ,
(B)
(C)
2
2
where A is CH CH(NH )
3
3
k3
D + Mn(VII)
A[Mn(VII)CO H−ACO H .
2 2
C1
We believe that the presence of the dimeric species D is justified due to the existence
of hydrogen bonding between alanine molecules. On the other hand, it has been as-
3
9
sumed that amino acid molecules form a complex with Mn(VII) species. In opinion of
3
8
Hussain et al. , the formation of such a complex (C ) should lead to the reaction pro-
1
ducts through the following steps.
Non-Catalyzed Mechanism
k4
+
−
A[Mn(VII)CO H−ACO H
A[H Mn(VII)]CO −ACO H
(D)
(E)
2
2
2
2
C1
C2
k5
+
−
2 A[HMn(VI)]CO•
2
A[H Mn(VII)]CO −ACO H + Mn(VII)
2
2
k-5
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Permanganate Oxidation of L-Alanine
535
k6
A[HMn(VI)]CO•
[HMn(VI)] + A + CO
•
(F)
2
2
slow
A•
CH CHNH + H
•
+
(G)
(H)
(I)
3
2
•
+
CH CHNH + Mn(VII)
CH CH= NH + Mn(VI)
3 2
3
2
+
+
CH CH= NH + H O
CH CHO + NH
3
2
2
3
4
k7
[
HMn(VI)] + H O
[H Mn(V)OH]
(J)
2
2
fast
fast
. . .
[
H Mn(V)OH] + ACO H
Mn(III) + . . . .
Mn(II) + . . . .
(K)
(L)
2
2
.
fast
. . . .
Mn(III) + ACO H
2
An intramolecular hydrogen transfer (Eq. (C)) takes place first, giving the zwitte-
rionic compound C . The cleavage of the compound C is explained both by an intra-
2
2
4
0
molecular electron transfer to Mn(VII) and by a simultaneous attack by the per-
manganate ion.
It has been assumed41 that the process (F) which produces radical species A and
•
.
CO is the rate-determining step. The radical A produces a neutral free radical by de-
2
protonation (Eq. (G)). Oxidation of the neutral free radical by permanganate ion
(
Eq. (E)) gives the ethyliminium ion (imines are known to be intermediates of the per-
4
2
manganate oxidation of amines ). Hydrolysis of the ethyliminium ion results in acetal-
dehyde and ammonium ion formation (Eq. (I)).
In agreement with these considerations, the rate of the reaction v = k
6
•
[
AHMn(VI)CO ]. This assumption means that k should smaller than k . On the other
2
6
−5
hand, the location of different isosbestic points allows us to suppose that the whole
reaction proceeds uniformly under the steady-state conditions. So, by applying this ap-
•
proximation to the A[HMn(VI)]CO species, their respective concentrations are found
2
to be the square root of k [C ][Mn(VII)].
3
2
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

5
36
Insausti, Mata-Perez, Alvarez-Macho:
From the steady-state approximation with respect to both C and C compounds it
1
2
follows that
k3
k4
2
[
[
C ] =
K [ACO H] [Mn(VII)]
(6)
1
1
2
and
k [C ]
k [C ]
4
1
4
1
C ] =
=
=
2
k (1 + [C ]) [Mn(VII)]
k5 [Mn(VII)]
5
2
k3 K1
k5
2
=
[ACO H] .
(7)
(8)
2
Hence, the theoretical rate equation for the non-catalyzed mechanism is
1
⁄ 2
1 ⁄2
v = k (k K )
[ACO H] [Mn(VII)]
,
6
3
1
2
in agreement with the experimental reaction orders. Alternatively to the step (J),
Mn(VI) species should be reduced also by the amino acid molecules. This process leads
to Mn(VI) species and is the key for the catalyzed mechanism.
Catalyzed Mechanism
[
HMn(VI)] + ACO H
A[H Mn(IV)]CO
2
(M)
(N)
2
2
A[H Mn(VI)]CO + H O
. . . .
Mn(IV) + . . .
2
2
2
At this moment, one should keep in mind two important features of the catalyzed
reaction. First, its reaction rate shows first-order dependence on the Mn(IV) concentra-
tion and further the reaction order with respect to alanine and Mn(VII) species is the
same as that for the non-catalyzed reaction. Furthermore, there is enough evidence for
the colloidal nature of Mn(IV) species also under the conditions used in this work.
However, in order to define the reaction rate relating to the catalyzed mechanism,
one has to make a number of other considerations. First, it is necessary to specify
Collect. Czech. Chem. Commun. (Vol. 59) (1994)

Permanganate Oxidation of L-Alanine
537
whether the Mn(VII) species, alanine, or both are adsorbed onto colloid Mn(IV) beads.
Based on the extensive literature data on the adsorption of ions onto oxyhydrates of
Mn(IV), one can conclude that the zwitterionic species C has the greater tendency to
2
be adsorbed than the Mn(VII) species. If, apart from that, one takes into account that
the former species is found in great excess in the bulk solution compared to the latter,
one can reasonably expect that the majority of active sites on the colloid surface will be
occupied by the C species. Provided that the Mn(IV) species acts as an oxidizing
2
agent, one has to decide whether the C species, once adsorbed, is oxidized by Mn(IV)
2
4
3
or by Mn(VII). If, for instance, the slow rate of N H /MNO reaction is taken into
2
4
2
account, the reaction between the C species and the adsorbed Mn(IV) can be con-
2
1
8
sidered to be negligible. Therefore, we have assumed that the heterogeneous mechan-
ism involves the reaction between the C adsorbed on colloidal Mn(IV) and Mn(VII),
2
the permanganate being supplied from the bulk solution.
In order to determine the concentration of this adsorbate, we have assumed that the
adsorption of the C species onto colloidal Mn(IV) fulfills Freundlich isotherm (9)
2
b
[
adsorbate] = M [Mn(VI)] a [C ] ,
(9)
2
where a and b (their values are one) are parameters of the isotherm and M is the mean
molar weight of the colloidal adsorbent under the conditions used in this work.
The reaction rate for the catalyzed mechanism shows the same formal dependence
(
reaction order) on both alanine and Mn(VII) species concentration as that for the non-
catalyzed mechanism. Hence, based on the adsorbate genesis one can conclude that the
catalyzed mechanism involves a similar sequence of steps as the homogeneous process.
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