Oxidation of L-amino acids by metal ion (Mn3+) in sulfuric acid medium: Effect of nucleophilicity and hydrophobicity on reaction rate

Article abstract of DOI:10.1002/kin.20572

The kinetics of oxidation of L-amino acids (AAs) glycine(1a), alanine(1b), valine(1c), isoleucine(1d), leucine(1e), proline(1f), and phenylalanine(1g) by a transition metal ion (Mn³⁺) was studied in the presence of sulfuric acid medium at 26°C by a spectrophotometrical (λ_max = 500 nm) method. In all cases, the kinetics of reactions was of first-order with respect to each [AA] and [Mn³⁺]. Increased [H⁺], [Mn ²⁺] (the reduction product of Mn³⁺), sulfate, and chloride had no effect on the reaction rate. However, the reaction rate increased with increased dielectric constant of the medium. Oxidation rate increased for 1a-gand an apparent correlation was observed between the rate of oxidation and nucleophilicty of AAs except for 1a and 1g. The reaction rate also linearly depended on the hydrophobicity of AAs except for 1f and 1g. The thermodynamic parameters for AA-metal ion complex formation and activation parameters for the rate-limiting steps have been evaluated. Analysis of the oxidation products indicated that the AAs underwent oxidative deamination and decarboxylation to form corresponding aldehydes. Based on these data, plausible mechanisms involved in the oxidation of AAs are proposed.

Full text of DOI:10.1002/kin.20572

Oxidation of L-Amino Acids
by Metal Ion (Mn3+) in
Sulfuric Acid Medium:
Effect of Nucleophilicity
and Hydrophobicity on
Reaction Rate
M. N. KUMARA,1 K. MANTELINGU,2 D. G. BHADREGOWDA,1 K. S. RANGAPPA2
1Department of Chemistry, Yuvaraja’s College, University of Mysore, Mysore 570 005, India
2Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India
Received 22 January 2011; revised 11 April 2011; accepted 11 May 2011
DOI 10.1002/kin.20572
Published online 2 September 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The kinetics of oxidation of L-amino acids (AAs) glycine(1a), alanine(1b), valine(1c),
isoleucine(1d), leucine(1e), proline(1f), and phenylalanine(1g) by a transition metal ion (Mn3+
)
was studied in the presence of sulfuric acid medium at 26◦C by a spectrophotometrical (λmax
= 500 nm) method. In all cases, the kinetics of reactions was of first-order with respect to each
[AA] and [Mn3+]. Increased [H+], [Mn2+] (the reduction product of Mn3+), sulfate, and chloride
had no effect on the reaction rate. However, the reaction rate increased with increased dielectric
constant of the medium. Oxidation rate increased for 1a–g and an apparent correlation was
observed between the rate of oxidation and nucleophilicty of AAs except for 1a and 1g. The
reaction rate also linearly depended on the hydrophobicity of AAs except for 1f and 1g. The
thermodynamic parameters for AA-metal ion complex formation and activation parameters
for the rate-limiting steps have been evaluated. Analysis of the oxidation products indicated
that the AAs underwent oxidative deamination and decarboxylation to form corresponding
aldehydes. Based on these data, plausible mechanisms involved in the oxidation of AAs are
proposed. ꢀ 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 599–607, 2011
C
INTRODUCTION
Oxidative reactions play an important role in a va-
riety of biochemical events ranging from normal
metabolism to aging and disease process [1,2]. Pep-
tides and proteins represent major targets for mod-
ification in these reactions and the identification
Correspondence to: M. N. Kumara; e-mail: kumaramanikya@
yahoo.com.
2011 Wiley Periodicals, Inc.
c
ꢀ

600
KUMARA ET AL.
of sites and structures of modifications may lead
to a mechanistic understanding and approaches
for their prevention. In this context, oxidation of
L-amino acids (L-AAs) is one of the well-documented
biochemical processes. Several studies have been re-
ported on the kinetics of oxidation of AAs by var-
ious oxidants in different media [3,4]. Most of the
previous kinetic investigations provide information
about reaction mechanism, thermodynamic parame-
ters, activation parameters [5], and oxidative reaction
products.
Previously, we reported the kinetics of oxidation of
peptides (di, tri, tetra, and penta) by Mn3+ [6,7]. We
also reported reaction mechanism and the compara-
tive studies on the kinetics of oxidation of AAs and
peptides (di, tri, and tetra) by NBS [8–10]. In all the
above studies, an apparent correlation was observed be-
tween the rate of oxidation and the hydrophobicity of
peptide sequences. These results made us to elucidate
linear relation between reaction rate and hydropho-
bicity. Recently, Brotzel et al. reported that kinetic
investigations provide a good comparison of nucle-
ophilic reactivities of AAs and small peptides and also
allocate AAs in a general scheme of nucleopholicity
[11]. A survey of literature indicated that the studies
were limited only to mechanistic investigation of AAs,
whereas linear correlation of reaction of L-AAs with
nucleophilicity and hydrophobicity has so for not been
studied.
Depending on the polarity of the side chain, AAs
vary in their hydrophilic or hydrophobic charac-
ter. These properties are important in protein and
protein–protein interactions. The importance of phys-
ical properties of the side chains comes from the
influence of AA residues interactions with other
structures, both within a single protein and between
proteins. The distribution of hydrophobic and hy-
drophilic AAs determines the tertiary structure of the
protein whereas their physical location on the out-
side structure of the proteins influences their quater-
nary structure [12,13]. We have already reported the
dependence of hydrophobicity of AAs with oxida-
tion rate. This program is further extended to inves-
tigate the apparent correlation between nucleophilic-
ity and reaction rate of biomolecules such as L-AAs
glycine(1a), alanine(1b), valine(1c), isoleucine(1d),
leucine(1e), proline(1f), and phenylalanine(1g). This
is not only intended as a guide for their use in
synthesis but also for predicting rates of individual
steps in biotransformations in the absence of enzymes.
This study has been attempted to correlate oxidative
reactivity of L-AAs with their nucleopholicity and
hydrophobicity.
EXPERIMENTAL
Preparation of Manganese(III) Sulfate
A 0.05 M solution of manganese(III) sulfate was pre-
pared [14] using 0.2 M solution of manganese(II) sul-
fate in 5.0 M sulfuric acid performed in an undivided
cell with a thin platinum foil anode (generation area
4.0 cm2) and a thin platinum spiral cathode (effec-
tive area 0.2 cm2). The manganese(III) sulfate solution
contained an excess but known concentration of man-
ganese(II) sulfate to suppress the disproportionate ion
reaction. Though, the solution appeared to be stable
for more than a month at [H+] > 5.0 M, solution of
manganese(III) sulfate prepared afresh daily was used
in the experiments.
Amino Acids
All AAs used except glycine are of L-configuration
unless otherwise specified. All AAs were purchased
from Advanced Chem. Tech. (Louisville, KY). All
other reagents were prepared from AR grade chemi-
cals. Triple distilled water was used for preparing aque-
ous solutions.
Kinetic Procedure
The kinetic studies were carried out in glass-stoppered
boiling tubes under pseudo-first-order conditions with
[AAs] ꢁ [Mn3+]. The reactions were initiated by the
rapid addition of known amounts of oxidant solution,
preequilibrated to the desired temperature, to mixtures
containing the required amount of AAs, sulfuric acid,
MnSO4, and water in the boiling tube, thermostated
at the same temperature. The progress of the reaction
was monitored for at least two half-lives by measuring
the absorbance of unreacted oxidant at 500 nm using
a spectrochem Elico SL 150 UV–vis spectrophotome-
ter. The reaction mixture was quenched appropriately
[15]. The pseudo-first-order rate constants (kobs) were
calculated by graphical methods and the values were
reproducible within 3% error.
Stoichiometry and Product Analysis
Reaction mixtures containing AA (0.001 M), sulfuric
acid (0.1 M), and excess Mn3+ (0.01 M) were kept
for 24 h at 25◦C. The unconsumed Mn3+ was then
determined to calculate the stoichiometry ratios. Two
moles of oxidant were sufficient to oxidize one mole
of AA leading to products such as aldehydes, carbon
dioxide, ammonia, and Mn2+. Based on these results,
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OXIDATION OF L-AMINO ACIDS BY METAL ION (Mn3+) IN ACID MEDIUM
601
the stoichiometric equations (1) and (2) are suggested.
The stoichiometric equations for glycine (Gly), ala-
nine (Ala), valine (Val), lucine (Lue), isolucine (Ile),
proline, and phenylalanine (Phe) are as follows:
ear with slopes of 1.01, 1.02, 0.99, 0.98, 0.99, 1.01,
and 0.99 for Gly, Ala, Val, Ile, Leu, Pro, and Phe,
respectively.
O
+
+
2 Mn2+
+ H+
Mn(OH)2+ + Mn3+
+
OH
RCHO
+ NH4
+ CO2
+
H3N
CH
R
C
(1)
The stoichiometric equation for proline (Pro):
O
C
OH
+
2 Mn(OH)2+
+
+
2 Mn2+
+
H2N
NH4 +
CH3-CH2-CH2-CHO
CO2
+
(2)
where R = H for glycine; R = CH3 for alanine;
R = CH(CH3)2 for valine; R = CH2 CH(CH3)2
for leucine; R = CHCH3(C2H5) for isoleucine; and
R = CH2C6H5 for phenylalanine.
Dependence of the Reaction Rate on [Acid]
Kinetic measurements were performed in H2SO4-
NaClO4 solutions of different [H+]. The effective [H+]
used was evaluated using the calibration curve of
[H2SO4] versus [H+] [17]. An increase in [H+] (from
0.01 to 1.0 M) had no effect on the rate.
After the reaction was completed, the reaction
products were extracted with diethyl ether and sub-
jected to column chromatography on silica gel (60–
200 mesh) using gradient elution (dichloromethane
to chloroform). R1f (0.76): Chloroform-methanol-acetic
acid (92:5:3); R2f (0.65): Chloroform-methanol-acetic
acid (87:10:3); R3f (0.53): Chloroform-methanol-
acetic acid (82:15:3). Corresponding aldehydes were
determined qualitatively by gas chromatography,
which were shown to be identical with those of au-
thentic samples. Ammonia and CO2 were detected by
the conventional tests [16].
Dependence of Rate on Product and
Added Salts
The effect of varying concentrations of MnSO4 (which
is the reduction product of the oxidant, Mn3+) on
the rate of reaction was investigated. An increase in
[MnSO4] (from 0.01 to 0.5 M) had no effect on the
rate. This indicated that the product (MnSO4) is not
involved in a preequilibrium with the oxidant. Simi-
larly, the effect of anions Cl− (from 0.01 to 0.5 M) and
SO24− (from 0.01 to 1.0 M) on the reaction rate was
insignificant.
RESULTS
Dependence of the Reaction Rate on
[Mn3+] and [AAs]
All kinetic runs were performed under pseudo-first-
order conditions with [AA]o ꢁ [Mn3+]o. Plots of
log [Mn3+] versus time were linear (0.998) even be-
yond 75% of the reaction, showing a first-order depen-
dence of the rate on [Mn3+] (Table I, Fig. 1). At con-
stant [Mn3+]o, [Mn2+]o, [H2SO4]o, and temperature,
the rate increased with increase in [AA]o (Table I).
Plots of log kobs versus log [AA]o (Fig. 2) were lin-
Effect of Varying Solvent Composition
The dielectric constant (D) of the reaction medium
was varied by changing the solvent composition with
added methanol (0.0–40%). The reaction rate increased
with increase in methanol content (Table II). Plots of
log kobs versus 1/D were linear (r > 0.999, s ≤ 0.01)
with positive slopes. Measurements of reaction rate
International Journal of Chemical Kinetics DOI 10.1002/kin

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KUMARA ET AL.
Table I Effect of Varying Reactant Concentration on the Rate of Oxidation of AAs with [ H+ ] = 0.1 M; [Mn(II)] =
0.01 M; T = 26◦C
kobs× 104 (s−1
)
[Mn(III)] ×
[AA] ×
102 (mol dm−3
)
103 (mol dm−3
)
Gly
Ala
Val
Ile
Leu
Pro
Phe
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
1.05
1.15
1.30
1.35
1.40
1.38
1.43
0.79
1.04
1.30
1.58
1.82
2.08
2.34
2.21
2.35
2.41
2.47
2.46
2.51
2.56
1.25
1.77
2.41
2.75
3.23
3.71
3.98
3.20
3.46
3.53
3.67
3.64
3.56
3.68
1.90
2.63
3.53
4.07
4.78
5.37
6.02
4.62
4.75
4.87
4.77
4.83
4.81
4.85
3.01
3.71
4.87
5.88
6.76
7.76
8.91
5.14
5.19
5.22
5.25
5.24
5.30
5.33
3.31
3.98
5.22
6.60
7.58
8.91
5.90
6.05
6.10
6.13
6.15
6.16
6.18
3.80
4.89
6.10
7.41
8.91
10.23
12.02
7.61
7.62
7.63
7.65
7.68
7.61
7.64
4.46
5.88
7.63
9.12
10.71
12.58
14.12
10.00
Gly
Ala
Vla
Ile
2.4
2.2
2
Lue
pro
phe
1.8
1.6
1.4
1.2
1
0
10
20
30
40
50
60
Time (Min)
Figure 1 Effect of [Mn3+] on the rate at 26◦C. Mn[III]0 = 0.001 M, [AA]0 = 0.01 M, [H+] = 0.01 M, Mn[II] = 0.1 M.
constants for the oxidation by Mn3+ were done both
in presence and absence of each substrate (AAs) and
the rate constants were taken for the calculation of
effective kobs. The rate of oxidation of methanol in the
absence of AAs was negligible under the conditions
used.
Table III). Arrhenius plots of log kobs versus 1/T
(Fig. 3) were linear. The activation energies (Ea) were
calculated from the slopes of the plots. Based on
these values, the activation parameters ꢀH‡, ꢀS‡,
and ꢀG‡ along with the frequency factor (log A)
were evaluated (Table IV). The moderate values of
ꢀH‡, large negative values of ꢀS‡, and the high val-
ues of ꢀG‡ support the proposed reaction mecha-
nism. The near constant values of ꢀG‡ indicate a
similar reaction mechanism for the oxidation of all
the AAs.
Activation Parameters
To determine the activation parameters, the reactions
were carried out at different temperatures (20–40◦C,
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OXIDATION OF L-AMINO ACIDS BY METAL ION (Mn3+) IN ACID MEDIUM
603
Table II Effect of Varying Dielectric Constant on the Reaction Rate, with [Mn (III)] = 0.001 M; [AA] = 0.01 M; [Mn(II)]
= 0.01 M; [H+] = 0.1 M; T = 26◦C
kobs × 104 (s−1
)
MeOH
(% v/v)
Dielectric
Constant (D)
Gly
Ala
Val
Ile
Leu
Pro
Phe
00
10
20
30
40
76.73
72.37
67.48
62.71
58.06
1.30
1.42
1.56
1.68
1.73
2.41
2.76
2.94
3.05
3.23
3.53
3.74
3.93
4.15
4.27
4.87
4.97
5.32
5.55
5.76
5.22
5.45
5.62
5.84
5.99
6.1
7.63
7.79
7.94
8.24
8.45
6.34
6.57
6.78
6.85
Table III Temperature Dependence of the Oxidation of Substrate with [Mn(III)] = 0.001 M; [AA] = 0.01 M; [Mn(II)] =
0.01 M; [H+] = 0.1 M
kobs × 104 (s−1
)
1/T × 10−3
Temperature (K)
(K−1
)
Gly
Ala
Val
Ile
Leu
Pro
Phe
295
298
301
304
307
3.389
3.355
3.322
3.289
3.257
0.02
1.11
1.33
1.92
2.45
0.24
1.38
1.62
2.20
2.94
0.40
1.54
1.78
2.36
2.94
0.540
1.68
1.92
2.54
3.10
0.57
1.71
1.95
2.56
3.13
0.64
1.78
2.03
2.60
3.17
0.72
1.88
2.12
2.69
3.28
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
Gly
Ala
Vla
Ile
leu
pro
phe
0.75
0.85
0.95
1.05
1.15
1.25
3 + log [S]
Figure 2 Effect of [AA] on the rate at 26◦C. Mn[III]0 = 0.001 M, [AA]0 = 0.01 M, [H+] = 0.1 M, Mn[II] = 0.01 M.
Table IV Activation Parameters for the Oxidation of AAs by Mn(III): [Mn (III)] = 0.001 M; [AA] = 0.01 M; [Mn(II)] =
0.01 M; [H+] = 0.1 M
AA
Ea (kJ mol−1
)
ꢀH‡ (kJ mol−1
)
ꢀS‡ (J mol−1K−1
)
ꢀG‡ (kJ mol−1
)
logA
Gly
Ala
Val
Ile
Leu
Pro
Phe
46.43
48.18
48.33
48.93
48.43
48.18
48.22
43.91
45.64
45.84
46.44
45.94
45.70
45.73
–134.22
–141.27
–157.64
–139.64
–134.14
–153.35
–151.40
84.04
87.88
92.84
88.19
86.00
88.90
91.00
4.21
5.02
4.99
5.10
4.74
5.23
5.31
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604
KUMARA ET AL.
3.4
2.9
2.4
1.9
1.4
0.9
0.4
–0.1
Gly
Ala
Vla
Ile
leu
pro
phe
0.00317
0.00322
0.00327
0.00332
0.00337
0.00342
1/T (K –1)
Figure 3 Effect of temperature. Mn[III]0 = 0.001 M, [AA]0 = 0.01 M, [H+] = 0.1 M, Mn[II] = 0.01 M.
Formation of dihydroxo species Mn(OH)+(aq), pro-
Test for Free Radicals
duced by further hydrolysis of Mn(OH)+2(aq) is another
possibility [22].
Addition of the reaction mixture to aqueous acrylamide
monomer (3%) did not initiate polymerization, indi-
cating the absence of in situ formation of free radical
species in the reaction sequence.
Mn(OH)2 (aq) + H+
+
Mn(OH)2+
+
H2O
(aq)
(4)
However, a fresh solution of manganese(III) sulfate is
always prepared and used, immediately after cessation
of the el+ectrolysis, to eliminate any reaction due to
Mn(OH)2(aq). Molar absorption coefficient, “ε” ranges
DISCUSSION
Data published by Diebler and Sutin [18], Packler and
Chawla [19], and Well and Davies [20] have shown that
in the presence of F− ion, aqueous solution of man-
ganese(III) sulfate consists of hexaquomanganese(III),
between 131 and 110 dm3 mol−1 cm−1 at [H+] = 1.20–
2.50 M. The high value of ε has been attributed to the
2+
presence of hydrolyzed species of Mn(OH)
Since
(aq).
Mn(III)(aq) Mn(OH)+(aq2), and MnF
Along the same
2+
(aq).
there is no hydrogen ion dependence on the rate, this
suggests that Mn(OH)+2(aq) is not the reactive species
[23]. The absence of+a sulfate effect on the reaction rate
indicates that MnSO4(aq) is not the active species in the
line, it can be assumed with justification that Mn(III)
species present in sulfuric acid medium are Mn
Mn(OH)+(aq2), and MnSO+4 . Therefore, it was shown
[21] that manganesen(III) sulfate in aqueous sulfu-
ric acid contains Mn
species.
3+
,
(aq)
3+
+2
present condition. Therefore, Mn
and Mn(OH)
(aq)
(aq)
3+
+2
and Mn(OH)
as reactive
(aq)
(aq)
are the reactive species as shown in the mechanism.
This accounts for the observed experimental results:
rate = k1Mn(OH)2(a+q) [AA]
(5)
kh1
Mn3+
+
H2O
(aq)
Mn(OH)2+(aq) + H+
Plots of log (kobs) versus 1/D gave a straight line
with a positive slope for positive ion-dipole interac-
tion [24]. The positive dielectric effect in the present
investigation shows charge dispersal in the transition
state, pointing toward a positive ion–dipole reaction
and hence supports Schemes 1 and 2.
(3)
The hydrolysis constant of manganese(III) sulfate cal-
culated was Kh = 0.93 0.03 at 25◦C. The absorption
3+
+2
spectra of both Mn
and Mn(OH)
have been re-
(aq)
(aq)
ported to be similar in both visible and UV-region.
Our observation of the electronic absorption spectra is
consistent with the reported values.
When log (kobs) for the reactions of the AAs
1a–g with metal ion (Mn3+) were plotted against
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF L-AMINO ACIDS BY METAL ION (Mn3+) IN ACID MEDIUM
605
3+
R
O
+
k1
HO
C
O
CH
Mn(OH)2+
Oxident
H3N
CH
R
C
OH
+
- H2O
NH2
AA
Slow step
Mn
- Mn2+
+ Mn3+
-CO2, - Mn2+, - H+
+
+ H2O
+
RCHO
+
NH4
R
CH NH2
R
CH
C
OH
NH2
O
+
R= -H for 1a; -CH3 for 1b; -CH(CH3)2 for 1c; -CH(CH3)CH2CH3 for 1d;
-CH2-CH(CH3)2 for 1e ; -CH2-C6H5 for 1g
Scheme 1
3+
O
C
OH
k1
HO
O
+ Mn(OH)2+
Oxident
C
+
- H2O
N
H2N
H
Mn
AA
- Mn2+
,
+ Mn(OH)2+, H2O
+
CH3-CH2-CH2-CHO + NH4 + CO2 + Mn2+
N
COOH
Scheme 2
Table V Oxidation Rate Constants (kobs) for the
Reactions of AAs by Metal Ion Mn3+, Nucleopholicity
Parameter [11] and Hydrophobicity Parameter [13]
hydrophobicity parameters (Table V) of AAs [13], lin-
ear correlations were observed (Fig. 4) except for 1f
and 1g. These results suggest the possibility of in-
fluence of hydrophobicity on reaction rate. The prob-
able reason is the presence of carboxylic groups in
AAs. Carboxylic groups were more destabilized and
enhance the rate of formation of a transition state with
metal ion.
In contrast, 1f and 1g oxidation rates were not
linearly correlated with hydrophobicity. The benzyl
group present in 1g and heterocyclic five-membered
ring present in 1f have small nonsignificant localized
electrical effect [12]. But they have a small electron
donor delocalized (resonance) effect, steric, and polar-
izability effect. The steric and polarizability effects are
Rate Constant
AA kobs × 104 (s−1
Nucleophilicity Hydrophobicity
)
Parametera
Valuesb
Gly
Ala
Val
Ile
Leu
Pro
Phe
1.30
2.41
3.53
4.87
5.22
6.10
7.63
13.51
13.01
13.65
–
14.01
18.08
14.12
0.74
0.11
–0.31
–0.60
–0.55
2.33
–0.32
aIncreasing nucleopholicity parameter reflects greater nucle-
ophilicity.
bMore negative values reflect greater hydrophobicity.
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606
KUMARA ET AL.
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
1f
1e
1d
1g
1c
1b
1a
–1
–0.5
0
0.5
1
1.5
2
2.5
Hydrophobicity
Figure 4 Plots of hydrophobicity with rate constants (kobs) of AAs.
2
1.9
1.8
1.7
1.6
1.5
1.4
1f
1g
1e
1c
1b
1.3
1.2
1.1
1
1a
12
13
14
15
Nucleophilicity parameter
Figure 5 Plots of rate constants (kobs) with nucleopholicity parameter of AAs.
16
17
18
19
parameterized by υ and α constants respectively using
regression equation [25]. Correlation of the oxidation
rates of 1g and 1f with regression equation clearly
shows that υ and α are strongly collinear. This clearly
indicates that there may be significant dependence of
steric effect but some dependence of polarizability on
reaction rate cannot be ruled out in 1g and 1f.
When log (kobs) for the reactions of the AAs 1a–g
with metal ion (Mn3+) were plotted against their nu-
cleopholicity parameters N[11] (Table V), linear cor-
relation was observed (Fig. 5) except for 1a and 1f. All
AAs including 1a and 1f have relativities with one or-
der in magnitude (Table I), but their basicities is in the
order of four in magnitude. This discrepancy could be
explained on the basis of the intrinsic properties like
differential salvation of protonated and deprotonated
forms of the AAs. This explanation suggests that 1a
and 1f deviate from linear correlation.
Furthermore, it was shown in the previous report
[11], introduction of neutral electron acceptor groups
( H) in amines causes a reduction of nucleophilic
reactivity. In contrast, a CO−2 group causes a slight
increase in nucleophilic reactivity. This result shows
that the Coulombic attraction between the negatively
charged AAs and the positively charged oxidant does
not cause a more significant rate enhancement. There-
fore CO−2 substituent causes only a slight increase of
nucleopholicity. Hence, reduction of nucleophilic re-
activity cannot be ruled out in 1a.
CONCLUSION
The rates of oxidation of AAs by metal ion (Mn3+
)
were obtained under identical experimental conditions,
and it was found that the rates of oxidation of AAs
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF L-AMINO ACIDS BY METAL ION (Mn3+) IN ACID MEDIUM
607
increased with hydrophobicity and nucleophilicity. An
apparent correlation was observed between the rate
of oxidation and nucleophilicty of AAs except for 1a
and 1g. This discrepancy could be explained on the
basis of the intrinsic properties like differential salva-
tion of protonated and deprotonated forms of the AAs.
The oxidation rate also linearly depends on the hy-
drophobicity of AAs except for 1f and 1g. This clearly
indicates that there may be significant dependence of
steric effect but some dependence of polarizability on
reaction rate cannot be ruled out in the case of 1f
and 1g.
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International Journal of Chemical Kinetics DOI 10.1002/kin