Mechanistic study on the oxidation of L-phenylalanine by copper(III) in aqueous alkaline medium: A kinetic approach

Article abstract of DOI:10.1007/s00706-010-0377-2

Oxidation of the amino acid L-phenylalanine by diperiodatocuprate(III) in alkaline medium at constant ionic strength of 0.25 mol dm^-3 was studied spectrophotometrically at different temperatures (298-313 K). The reaction between diperiodatocuprate(III) and L-phenylalanine in alkaline medium exhibits 1:2 stoichiometry. Intervention of free radicals was observed in the reaction. Based on the observed orders and experimental evidence, a mechanism involving monoperiodatocuprate(III) as the reactive oxidant species has been proposed, proceeding through the formation of a complex and reaction of the intermediate of L-phenylalanine with monoperiodatocuprate(III) to give the products. The products were identified by spot test, infrared (IR), and gas chromatography-mass spectrometry (GC-MS). The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to the slow step of the mechanism were computed and are discussed. The thermodynamic quantities were determined for different equilibrium steps. The isokinetic temperature was also calculated and found to be 331 K. Springer-Verlag 2010.

Full text of DOI:10.1007/s00706-010-0377-2

Monatsh Chem (2010) 141:1069–1076
DOI 10.1007/s00706-010-0377-2
ORIGINAL PAPER
Mechanistic study on the oxidation of L-phenylalanine
by copper(III) in aqueous alkaline medium: a kinetic approach
Ragunatharaddi R. Hosamani
Sharanappa T. Nandibewoor
•
Rajesh N. Hegde
•
Received: 1 September 2009 / Accepted: 5 August 2010 / Published online: 1 September 2010
Ó Springer-Verlag 2010
Abstract Oxidation of the amino acid L-phenylalanine by
diperiodatocuprate(III) in alkaline medium at constant
They have been oxidized by a variety of oxidizing agents
[1]. The study of oxidation of amino acids is of interest
because of their biological significance and selectivity
towards the oxidant to yield different products [2–5].
L-Phenylalanine (L-PA) is an essential amino acid. It
forms active sites of enzymes and helps in maintaining
their proper conformation by keeping them in proper ionic
states. So, oxidation of L-PA may help in understanding
some aspects of enzyme kinetics (Appendix). L-Phenylal-
anine supplementation helps in the suppression of pain and
aids weight loss through the suppression of appetite. It is
converted to tyrosine, which is a precursor to dopamine,
having a definite function in sexual desire.
-
3
ionic strength of 0.25 mol dm was studied spectropho-
tometrically at different temperatures (298–313 K). The
reaction between diperiodatocuprate(III) and L-phenylala-
nine in alkaline medium exhibits 1:2 stoichiometry.
Intervention of free radicals was observed in the reaction.
Based on the observed orders and experimental evidence,
a mechanism involving monoperiodatocuprate(III) as the
reactive oxidant species has been proposed, proceeding
through the formation of a complex and reaction of the
intermediate of L-phenylalanine with monoperiodatocup-
rate(III) to give the products. The products were identified
by spot test, infrared (IR), and gas chromatography-mass
spectrometry (GC-MS). The reaction constants involved in
the different steps of the mechanism were calculated. The
activation parameters with respect to the slow step of the
mechanism were computed and are discussed. The ther-
modynamic quantities were determined for different
equilibrium steps. The isokinetic temperature was also
calculated and found to be 331 K.
In recent years, the study of the highest oxidation states of
transition metals has intrigued many researchers. Transition
metals in a higher oxidation state can be stabilized by che-
lation with suitable polydentate ligands. Metal chelates such
as diperiodatocuprate(III) [6], diperiodatoargentate(III)
[7–9], and diperiodatonickelate(IV) [10] are good oxidants
in a medium with appropriate pH value. Periodate and tel-
lurate complexes of copper in its trivalent state have been
extensively used in the analysis of several organic com-
pounds [11]. The kinetics of self-decomposition of these
complexes was studied in some detail [12]. Copper(III) is
shown to be an intermediate in the copper(II)-catalyzed
oxidation of amino acids by peroxydisulfate [13]. The oxi-
dation reaction usually involves the copper(II)–copper(I)
couple, and such aspects are detailed in different reviews
[14, 15]. The use of diperiodatocuprate(III) (DPC) as an
oxidant in alkaline medium is new and restricted to a few
cases due to its limited solubility and stability in aqueous
medium. DPC is a versatile one-electron oxidant for various
organic compounds in alkaline medium, and its use as an
analytical reagent is now well recognized [16, 17]. Copper
complexes have occupied a major place in oxidation
Keywords Kinetics ꢀ Mechanism ꢀ L-Phenylalanine ꢀ
Oxidation ꢀ Diperiodatocuprate(III)
Introduction
Amino acids not only act as building blocks in protein
synthesis but also play a significant role in metabolism.
R. R. Hosamani ꢀ R. N. Hegde ꢀ S. T. Nandibewoor (&)
Post-Graduate Department of Studies in Chemistry,
Karnatak University, Dharwad 580003, Karnataka, India
e-mail: stnandibewoor@yahoo.com
123

1
070
R. R. Hosamani et al.
-
Table 1 Effect of [DPC], [L-PA], [OH ], and [IO ] on oxidation of
L-phenylalanine by DPC in alkaline medium at 25 °C
-
4
chemistry due to their abundance and relevance in biological
chemistry [18, 19]. Copper(III) is involved in many bio-
logical electron-transfer reactions [20]. It has also been used
5
4
0 [DPC] 10 [L-PA] [OH ]
-
4
10 [IO
-
3
3
1
4
]
10
10
-3
-3
-3
-3
[
21] in the differential titration of organic mixtures, in the
(mol dm ) (mol dm ) (mol dm ) (mol dm ) kobs
k
calc
-1
(s )
-1
(
s
)
estimation of chromium, calcium, and magnesium from their
ores, and antimony, arsenic, and tin from their alloys. Since
multiple equilibria between different copper(III) species are
involved, it would be interesting to know which of the spe-
cies is the active oxidant.
1
3
5
8
1
5
5
5
5
5
5
5
5
5
5
5
5
.0
.0
.0
.0
0.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
5.0
5.0
5.0
5.0
5.0
3.0
5.0
10.0
20.0
30.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.02
0.05
0.08
0.15
0.20
0.15
0.15
0.15
0.15
0.15
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
1.0
0.5
0.8
1.0
3.0
5.0
7.06
7.09
7.08
7.12
7.11
6.58
7.08
7.63
8.36
9.80
4.57
5.88
6.38
7.08
7.84
7.75
7.47
7.08
5.81
4.94
7.14
7.14
7.14
7.14
7.14
6.28
7.14
8.01
8.58
9.37
4.52
6.06
6.63
7.14
7.51
7.62
7.33
7.14
5.71
4.76
A literature survey reveals that there are no reports on
oxidation of L-phenylalanine (L-PA) by diperiodatocup-
rate(III). The present study deals with the title reaction to
investigate the redox chemistry of DPC in alkaline media
and to arrive at a suitable mechanism on the basis of kinetic
and spectral results and to calculate the thermodynamic
quantities of various steps.
Results and discussion
Reaction orders
Reaction orders were determined from the slope of lg kobs
versus lg(concentration) plots by varying the concentra-
tions of L-phenylalanine, alkali, and periodate in turn while
keeping all other concentrations and conditions constant.
5.0
5.0
5.0
Effect of [diperiodatocuprate(III)]
3
2
1
0
.5
.5
.5
.5
The oxidant DPC concentration was varied in the range of
-
5
-4
-3
.0 9 10 to 1.0 9 10 mol dm , and the fairly con-
1
stant kobs values indicate that order with respect to [DPC]
was unity (Table 1). This was also confirmed by linearity
of the plots of lg(absorbance) versus time (r C 0.986,
S B 0.014) up to 80% completion of the reaction, as shown
in Fig. 1.
5
4
3
2
1
Effect of [L-phenylalanine]
0
50
100
150
200
250
300
t / s
The effect of L-phenylalanine on the rate of reaction was
studied at constant concentrations of alkali, DPC, and per-
Fig. 1 First-order plots for oxidation of L-phenylalanine by diperio-
datocuprate(III) in aqueous alkaline medium at 25 °C.
-
iodate at constant ionic strength of 0.25 mol dm . The
3
5 -3
Diperiodatocuprate(III)] 9 10 (mol dm ): 1 1.0; 2 3.0; 3 5.0; 4
[
8.0; 5 10.0
-
4
substrate L-PA was varied in the range of 3.0 9 10 to
-
.0 9 10 mol dm . The kobs values increased with
3
-3
3
increase in concentration of L-phenylalanine (Table 1). The
increase with increase in alkali concentration, and order
was found to be less than unity (Table 1) (r C 0.988,
S B 0.007).
order was found to be less than unity (r C 0.995, S B 0.009).
Effect of [alkali]
Effect of [periodate]
The effect of alkali on the reaction has been studied in the
-
3
range of 0.02–0.20 mol dm at constant concentrations of
L-phenylalanine, DPC, and periodate at constant ionic
The effect of increasing concentration of periodate was
studied by varying the periodate concentration from
-
3
-5
-4 -3
to 5.0 9 10 mol dm , keeping all other
strength of 0.25 mol dm at 25 °C. The rate constants
5.0 9 10
1
23

Mechanistic study on the oxidation of L-phenylalanine by copper(III)
1071
reactant concentrations constant. It was found that the added
periodate had a retarding effect on the rate of reaction. The
order with respect to periodate concentration was negative
less than unity (r C 0.984, S B 0.004) (Table 1).
Effect of temperature (T)
The kinetics were studied at four different temperatures
(298, 303, 308, and 313 K) under varying concentrations of
L-phenylalanine, alkali, and periodate, keeping other con-
ditions constant. The rate constants were found to increase
with increase in temperature. The rate constants (k) of the
slow step of Scheme 1 were obtained from the slopes and
Effect of ionic strength and dielectric constant
of the medium
-
2-
It was found that ionic strength and dielectric constant of
the medium had no significant effect on the rate of the
reaction.
intercepts of 1/kobs versus 1/[L-PA], 1/[OH ], and [H IO ]
3 6
plots at four different temperatures and were used to cal-
culate the activation parameters. The energy of activation
corresponding to these constants was evaluated from the
Arrhenius plot of lg k versus 1/T (r C 0.976, S B 0.009).
Other activation parameters obtained are tabulated in
Table 2.
Effect of initially added products
The externally added products, phenylacetaldehyde and
copper(II) (CuSO ), did not have any significant effect on
4
The water-soluble copper(III) periodate complex is
7
-
the rate of the reaction.
reported [25] to be [Cu(HIO ) (OH) ] . However, in an
6 2 2
aqueous alkaline medium and at the high pH range
employed in the study, periodate is unlikely to exist as
Polymerization study
4
-
HIO (as present in the complex), as is evident from its
6
The intervention of free radicals in the reaction was exam-
ined as follows. The reaction mixture, to which a known
quantity of acrylonitrile monomer was initially added, was
kept for 4 h in an inert atmosphere. On diluting the reaction
with methanol, a white precipitate was formed, indicating
the intervention of free radicals in the reaction [22]. The
blank experiments of either DPC or L-phenylalanine alone
with acrylonitrile did not induce any polymerization under
the same conditions as those induced for the reaction mix-
ture. Initially added acrylonitrile decreased the rate of
reaction, indicating free radical intervention, as was the case
in earlier work [23, 24].
involvement in the multiple equilibria (1–3) [26],
depending on the pH of the solution.
ꢁ
þ
H5lO6 ꢀ H5lO þ H
ð1Þ
ð2Þ
ð3Þ
6
H4lO6 ꢀ H3lO6 þ Hþ
ꢁ
2ꢁ
H lO ꢀ H lO þ Hþ
2ꢁ
3ꢁ
3
6
2
6
Periodic acid exists as H IO in acid medium and as
6
5
-
H IO near pH 7. Hence, under alkaline conditions as
4
6
employed in this study, the main species are expected to be
2
-
H IO and H IO . Thus, at the pH employed in this
3-
6
3
6
2
study, the soluble copper(III) periodate complex might be
K1
3-
-
4-
[
Cu(OH) (H IO ) ] + OH
[Cu(OH) (H IO )(H IO )] + H O
2
3
6 2
2
3
6
2
6
2
K2
-
3-
4
-
[Cu(OH) (H IO )] + H IO
2 3 6 2 6
[
Cu(OH) (H IO )(H IO )]
2
3
6
2
6
-
CH2 CH COO
K3
-
[
Cu(OH) (H IO )]
Complex (C)
2
3
6
+
NH2
.
CH2 CH
2-
+
Cu(OH) + H IO
6
+ CO2
slow
2
3
Complex (C)
NH2
k
.
CH2 CHO
+ NH + Cu(OH) + H IO
3 2 3 6
CH2 CH
fast
-
-
2-
+
[Cu(OH) (H IO )] + OH
2
3
6
NH2
Scheme 1
123

1
072
R. R. Hosamani et al.
Table 2 Thermodynamic activation parameters for oxidation of L-
phenylalanine by DPC in aqueous alkaline medium with respect to the
slow step of Scheme 1
in alkalinity (Table 1) and also decrease in rate with
2
-
increase in [H IO ] (Table 1) suggest an equilibrium of
3
6
the copper(III) periodate complex to form a monoperio-
datocuptrate(III) (MPC) species. Such types of results have
been well noticed in literature [28]. It may be expected that
a lower periodate complex such as monoperiodatocup-
trate(III) is more important in the reaction than the DPC.
2
10 k (s
-1
)
T (K)
(
a) Effect of temperature (K)
2
3
3
3
98
03
08
13
0.92
2.34
3.09
4.04
2-
The inverse fractional order in [H IO ] might also be due
3
6
to this reason. Therefore, MPC might be the main reactive
form of the oxidant.
Parameters
Values
The less than unit order in [L-PA] presumably results
from formation of a complex (C) between the MPC species
and L-phenylalanine prior to the formation of the products.
This complex (C) decomposes in a slow step to form a free
radical derived from L-phenylalanine. This free radical
species further reacts with another molecule of MPC in a
fast step to form products as given Scheme 1.
(
b) Activation parameters (Scheme 1)
-
1
E
a
(kJ mol
à
)
-1
73 ± 3
71 ± 4
DH (kJ mol
)
-1
à
-1
DS (J K mol
)
-46 ± 2
85 ± 2
à
-1
DG (kJ mol
)
lg A
10 ± 0.6
Since Scheme 1 is in accordance with the generally
well-accepted principle of noncomplementary oxidations
taking place in a sequence of one-electron steps, the
reaction between the substrate and oxidant would afford a
radical intermediate. A free radical scavenging experiment
revealed such a possibility (see below). This type of radical
intermediate has also been observed in earlier work [29].
The probable structure of the complex is given in
Scheme 2.
4
10 K
(mol dm
-4
10 K
3
(dm mol
T (K)
K
(
1
2
3
dm mol
-1
)
-3
)
3
-1
)
(
1 2 3
c) Effect of temperature to calculate K , K , and K for oxidation of
L-phenylalanine by DPC in alkaline medium
2
3
3
3
98
03
08
13
2.66
2.72
2.79
2.88
2.73
4.29
6.17
7.44
1.59
0.75
0.24
0.19
Spectroscopic evidence for the complex formation
between oxidant and substrate was obtained from UV–Vis
Thermodynamic quantities Values from Values from Values from
K
-
spectra of L-phenylalanine (5.0 9 10 ), DPC (5.0 9
4
K
1
2
K
3
-
5
-
), [OH ] = 0.15 mol dm
-3
10
and mixture of both.
(
d) Thermodynamic quantities using K
1
, K , and K
2
3
A bathochromic shift of about 5 nm from 259 to 264 nm in
the spectra of DPC was observed. The Michaelis–Menten
plot also proved the complex formation between DPC and
L-phenylalanine, which explains the less than unit order
dependence on [L-PA]. Such a type of complex between a
substrate and an oxidant has been observed in other studies
[29].
-1
)
DH (kJ mol
4.2 ± 0.2
52 ± 2
-115 ± 4
-308 ± 6
-24 ± 0.2
-
1
-1
DS (J K mol
DG298 (kJ mol
)
22 ± 0.4 108 ± 5
-2.5 ± 0.1 20 ± 0.2
-1
)
3
-
[
Cu(OH) (H IO ) ] , a conclusion also supported by
2 3 6 2
Scheme 1 leads to the rate law (5).
earlier work [4, 5].
The reaction between the diperiodatocuprate(III) com-
plex and L-phenylalanine in alkaline medium has
stoichiometry of 1:2 (L-PA:DPC) with a first-order depen-
dence on [DPC], apparent less than unit order in [substrate]
and [alkali], and a negative fractional order in [periodate].
No effect of added products was observed. Based on the
experimental results, a mechanism is proposed for which
all the observed orders in each constituent such as [oxi-
ꢁd½DPCꢂ
Rate ¼
dt
ꢀ
ꢁ
3
ꢁ
ꢁ
kK1K2K3½l-PAꢂ½OH ꢂ CuðOHÞ ðH3IO6Þ
2
2
¼
ꢀ
ꢁ
;
3ꢁ
6
H IO
2
ð4Þ
Rate
¼
kobs
½
DPCꢂ
ꢁ
-
-
kK1K2K3½l-PAꢂ½OH ꢂ
dant], [reductant], [OH ], and [IO ] may be well
¼ ꢀ
ꢁꢁ
ꢀ
ꢁ
:
4
H2IO6 þ K1K2K3½OH ꢂ½l-PAꢂ þ K1K2½OH ꢂ þ K1 H2IO ½OHꢁꢂ
3
ꢁ
ꢁ
3ꢁ
6
accommodated. It is known that L-phenylalanine exists in
the form of a zwitterion [27] in aqueous medium. In highly
acidic medium, it exists in the protonated form, whereas in
highly basic medium it is in the fully deprotonated form
ð5Þ
This explains all the observed kinetic orders of different
species. The rate law (5) can be rearranged into the following
form, which is suitable for verification:
[
27]. The results of increase in reaction rate with increase
1
23

Mechanistic study on the oxidation of L-phenylalanine by copper(III)
1073
2-
Table 3 The rate constant of the slow steps at different temperatures
of some acids (for isokinetic temperature)
O
I
OH
OH
O
O
OH
OH
Amino
acids
k
(dm mol
1
at 298 K
3
k
(dm mol
2
at 303 K
3
Reference
-1 -1
s
-1 -1
s )
Cu
)
L-Lysine
L-Cystine
0.021
0.041
Hiremath
et al. [44]
HO
0.025
0.056
Hosamani
et al. [28]
NH2
O
CH2 CH
C
O
L-Phenylalanine 0.0092
0.023
Present work
species in alkaline medium. The effect of ionic strength
and dielectric constant of the medium on the rate
qualitatively explains the reaction between two
negatively charged ions, as seen in Scheme 1.
Scheme 2
-
-1
3
-1
-
The thermodynamic quantities for the first, second, and
third equilibrium steps of Scheme 1 can be evaluated as
[
OH ] / dm mol (3)
2
2
1
1
1
1
.2
.0
.8
.6
.4
.2
0
10
20
30
40
50
60
0
.2
0.3
0.2
0.1
0.0
2-
6
-
follows. [H IO ], [L-PA], and [OH ] (as in Table 1) were
3
0
0
0
0
.18
.16
.14
.12
0.1
.08
varied at four different temperatures. The plots of 1/k
obs
-
versus 1/[OH ], 1/k versus 1/[L-PA], and 1/kobs versus
obs
(3)
2
-
H IO ] should be linear (Fig. 2). From the slopes and
[
3 6
intercepts, the values of K , K , and K were calculated at
1
(
2)
2
3
different temperatures and are given in Table 2. van’t Hoff
(
1)
plots were made for variation of K , K , and K with tem-
3
1
2
perature [lg K versus 1/T (r C 0.985, S B 0.004), lg K
1
2
0
0
0
1
1
2
3
4
versus 1/T (r C 0.973, S B 0.007), and lg K3 versus
/T (r C 0.992, S B 0.008)], and the values of enthalpy of
3
-1
3
-1
0
[L-PA] / dm mol (2)
1
1
2
3
4
5
6
reaction DH, entropy of reaction DS, and free energy of
reaction DG were calculated for the first, second, and third
equilibrium step. These values are given in Table 2. Com-
parison of the thermodynamic quantities of the first step of
Scheme 1 with those obtained for the slow step of the
reaction shows that these values mainly refer to the rate-
limiting step, supporting the fact that the reaction before the
rate-determining step is fairly slow and involves high acti-
vation energy [30, 31].
4
3-
-3
1
2 6
0 [H IO ] / mol dm (1)
Fig. 2 Verification of rate law (6) for oxidation of L-phenylalanine
by diperiodatocuprate(III) at 25 °C
ꢀ
ꢁ
ꢀ
ꢁ
3
ꢁ
ꢁ
3ꢁ
H2IO6
1
H2IO6
¼
þ
kobs
kK1K2K3½OH ꢂ½l-PAꢂ kK2K3½l-PAꢂ
1
1
þ :
k
þ
ð6Þ
kK3½l-PAꢂ
à
à
The moderate values of DH and DS are both favorable
à
According to Eq. 6, other conditions being constant,
-
plots of 1/kobs versus 1/[OH ] (r C 0.995, S B 0.014),
for electron-transfer processes. The negative value of DS
suggests that the intermediate complex is more ordered
than the reactants [32]. The observed modest enthalpy of
activation and the higher rate constant of the slow step
indicate that the oxidation presumably occurs via an inner-
sphere mechanism. This conclusion is supported by earlier
observations [21, 33, 34].
1
/kobs versus 1/[L-PA] (r C 0.999, S B 0.012), and 1/k
obs
-
versus [H IO ] (r C 0.999, S B 0.011) should be linear,
2
6
3
and are indeed found to be so (Fig. 2). The slopes and
intercepts of such plots lead to the values of K , K , K ,
1
2
3
-4
3
and k as (2.6 ± 0.1) dm mol , (2.7 ± 0.1) 9 10
-1
-
3
4
3
-1
mol dm , (1.6 ± 0.05) 9 10 dm mol , and (0.92 ±
The activation parameters for oxidation of some acids
by DPC are summarized in Table 3. The entropy of the
activation for the title reaction falls within the observed
range. Variation in the rate within a reaction series may be
caused by change in the enthalpy or entropy of activation.
-
2 -1
0
.02) 9 10
s , respectively. The value of K is in good
1
agreement with earlier literature [28]. These constants were
used to calculate the rate constants for comparison with
experimental values, which were found to be in reasonable
agreement (Table 1), which fortifies the Scheme 1. The
equilibrium constant K is far larger than K . This may be
à
Changes in rate are caused by changes in both DH and
à
DS , but these quantities vary extensively in a parallel
1
2
à
à
attributed to the greater tendency of DPC to undergo
hydrolysis compared with the dissociation of hydrolyzed
fashion. A plot of DH versus DS is linear according to the
following equation:
123

1
074
R. R. Hosamani et al.
z
z
Experimental
DH ¼ b DS þ constant:
b is called the isokinetic temperature; it has been asserted
Materials
à
à
that apparently linear correlations of DH with DS are
sometimes misleading and the evaluation of b by means of
the above equation lacks statistical validity [35]. Exner
All chemicals used were of reagent grade, and double-
distilled water was used throughout the work. The cop-
per(III) periodate complex was prepared [38, 39] and
standardized by standard procedures [40]. The UV–Vis
spectrum with maximum absorption at 415 nm verified the
existence of a copper(III) complex. A solution of L-phenyl-
alanine (HiMedia Laboratories) was prepared by dissolving
an appropriate amount of recrystallized sample in double-
distilled water. The purity of L-PA was checked by its m.p. of
[
36] advocates an alternative method for the treatment of
experimental data. If the rates of several reactions in a
series have been measured at two temperatures and lg k2
(
at T ) is linearly related to lg k (at T ), i.e., lg k =
2 1 1 2
a ? blg k , he proposes that b can be evaluated from the
equation
1
b ¼ T1T2ðb ꢁ 1Þ=ðT2b ꢁ T1Þ:
We have calculated the isokinetic temperature to be
273 °C [Lit. m.p. 275 °C]. The IR spectrum agreed with
literature. The required concentration of L-PA was used from
its stock solution, and copper sulfate (BDH) was prepared by
dissolving known amounts of the samples in distilled water.
Periodate solution was prepared and standardized iodomet-
rically [41]. Required alkalinity and ionic strength were
3
31 K by plotting lg k at 303 K versus lg k at 298 K
2 1
(
r C 0.997, S B 0.006) (Fig. 3). The value of b (331 K) is
higher than the experimental temperature (298 K). This
indicates that the rate is governed by the enthalpy of
activation [37]. The linearity and the slope of the plot
obtained may confirm that the kinetics of these reactions
follows a similar mechanism, as previously suggested.
maintained by KOH (BDH) and KNO (Analar), respec-
3
tively, in the reaction solutions.
Kinetic measurements
Conclusions
Since the initial rate of reaction is fast, kinetic measurements
were performed using a Varian CARY 50 Bio UV–Vis
spectrophotometer. The kinetics was followed under
pseudo-first-order condition, where [L-PA] [ [DPC] at
Among various species of DPC in alkaline medium,
-
monoperiodatocuptrate(III) [Cu(OH) (H IO )] is consid-
2
3
6
ered as active species for the title reaction. The results
indicated that the role of pH in the reaction medium is
crucial. The rate constant of the slow step and other
equilibrium constants involved in the mechanism were
evaluated, and activation parameters with respect to the
slow step of the reaction were computed. The overall
mechanistic sequence described herein is consistent with
product studies, and mechanistic and kinetic studies.
2
98 ± 0.1 K, unless specified. The reaction was initiated by
mixing the DPC into L-PA solution which also contained the
required concentration of KNO , KOH, and KIO . Reaction
3
4
progress was followed spectrophotometrically at 415 nm
by monitoring the decrease in absorbance due to DPC
with the molar absorbancy index e to be 6,235 ±
3
-1
-1
1
00 dm mol cm . It was verified that there is negligible
interference from other species present in the reaction mix-
ture at this wavelength.
The pseudo-first-order rate constants kobs were deter-
mined from lg(absorbance) versus time plots. The plots
were linear up to 80% completion of reaction under the
2
1
1
1
1
1
0
.0
.8
.6
.4
.2
.0
.8
-
range of [OH ] used. The orders for various species were
determined from the slopes of plots of lg kobs versus
respective concentration of species, except for [DPC] in
which no variation of kobs was observed as expected for the
reaction condition. During the kinetics, a constant con-
3
2
1
-
4
-3
centration of 1.0 9 10 mol dm
of KIO4 was used
throughout the study unless otherwise stated. Since perio-
date is present in excess in DPC, the possibility of
oxidation of L-phenylalanine by periodate in alkaline
medium at 298 K was tested. The progress of the reaction
was followed iodometrically. However, it was found that
there was no significant reaction under the experimental
conditions employed compared with the DPC oxidation of
0.8
1.0
1.2
1.4
1.6
3
+ lg k
1
Fig. 3 Plot of lg k
temperature (Table 3). 1 L-phenylalanine; 2 L-lysine; 3 L-cystine
2
at 303 K versus lg k
1
at 298 K for isokinetic
1
23

Mechanistic study on the oxidation of L-phenylalanine by copper(III)
1075
Scheme 3
-
CH2 CH COO
2[Cu(OH) (H IO )] OH-
-
+
2 3 6
+
NH2
CH2 CHO
+
2-
NH + 2Cu(OH) + 2H IO + CO2
3
2
3
6
L-phenylalanine. The total concentration of periodate and
-
OH was calculated by considering the amount present in
peak at m/z = 119 (m-1) amu, confirming the presence of
phenylacetaldehyde (Fig. 4). All other peaks observed in
GC-MS can be interpreted in accordance with the observed
structure of phenylacetaldehyde.
the DPC solution and that additionally added. Kinetics runs
were also carried out in N atmosphere in order to under-
2
stand the effect of dissolved oxygen on the rate of reaction.
No significant difference in the results was obtained under
The by-products were identified as ammonia by Nessler’s
reagent [43], and the CO was qualitatively detected by
2
a N atmosphere and in the presence of air. In view of the
2
bubbling nitrogen gas through the acidified reaction mixture
and passing the liberated gas through a tube containing
ubiquitous contamination of carbonate in the basic med-
ium, the effect of carbonate was also studied. Added
carbonate had no effect on the reaction rates.
limewater. The presence of Cu(OH) was also tested.
2
Regression analysis of experimental data to obtain the
regression coefficient r and standard deviation S of points
from the regression line was performed using Microsoft
Excel 2003.
Stoichiometry and product analysis
Different sets of reaction mixtures containing varying
ratios of DPC to L-phenylalanine in presence of constant
Appendix
-
amounts of OH , KNO , and KIO were kept for 5 h in a
3
4
closed vessel under nitrogen atmosphere. The remaining
concentration of DPC was estimated spectrophotometri-
cally at 415 nm. The results indicate 1:2 stoichiometry as
given in Scheme 3.
According to Scheme 1
ꢁ
d½DPCꢂ
dt
Rate ¼
¼ k½Cꢂ
ꢀ
ꢁ
3
ꢁ
ꢁ
kK1K2K3½l-PAꢂ½OH ꢂ CuðOHÞ ðH3IO6Þ
The main oxidation product was identified as aldehyde
2
2
¼
ꢀ
ꢁ
;
ð7Þ
ð8Þ
3
6
ꢁ
(
phenylacetaldehyde) by a spot test [42]. Phenylacetalde-
H2IO
ꢀ
ꢁ
ꢀ
ꢁ
4
ꢁ
ꢁ
½
DPCꢂ ¼ ½DPCꢂ þ CuðOHÞ ðH IO ÞðH IO Þ þ CuðOHÞ ðH IO Þ þComplex ðCÞ
T
f
2
3
6
3
6
2
3
6
"
#
ꢀ
ꢁ
ꢀ
ꢁ
3
ꢁ
3ꢁ
ꢁ
ꢁ
ꢁ
;
H2IO6 þ K1 H2IO6 ½OH ꢂ þ K1K2½OH ꢂ þ K1K2K3½OH ꢂ½l-PAꢂ
¼
½DPCꢂf
ꢀ
ꢁ
3ꢁ
H2IO
6
hyde was confirmed by its IR spectrum, which showed
where [DPC]
T
and [DPC] refer to total and free
f
-
C=O stretching at 1,720 cm , indicating the presence of
1
DPC concentrations, respectively. The free [DPC] is
given by
-
1
aldehydic C=O, and C–H stretching at 2,845 cm . Fur-
thermore, phenylacetaldehyde was subjected to GC-mass
spectral analysis. GC-mass data were obtained using a 17A
Shimadzu gas chromatography with a QP-5050A Shima-
dzu mass spectrometer using the electrospray ionization
½
DPCꢂf
¼ ꢀ
ꢀ
ꢁ
3ꢁ
½
DPCꢂ H2IO6
T
ꢁꢁ
ꢀ
ꢁ
:
3
3ꢁ
ꢁ
ꢁ
ꢁ
H2IO6 þ K1 H2IO6 ½OH ꢂ þ K1K2½OH ꢂ þ K1K2K3½OH ꢂ½l-PAꢂ
ð9Þ
(
EI) technique. The mass spectrum showed a molecular ion
123

1
076
R. R. Hosamani et al.
9
. Kumar A, Vaishali A, Ramamurthy P (2000) Int J Chem Kinet
3
2:286
0. Shan H, Qian J, Gao MZ, Shen SG, Sun HW (2004) Turk J Chem
8:9
1. Niu W, Zhu Y, Hu K, Tong C, Yang H (1996) Int J Chem Kinet
8:899
2. Rozovoskii GI, Misyavichyus AK, Prokopchik AY (1975) Kinet
Catal 16:337
3. Reddy B, Sethuram B, Navaneeth Rao T (1978) Indian J Chem A
1
1
1
1
1
2
2
1
6:313
4. Karlin KD, Gultneh Y (1997) In: Lipard SJ (ed) Progress in
inorganic chemistry, vol 35. Wiley, New York, p 220
5. Tolman WB (1997) Acc Chem Res 30:227
6. Kovat Z (1959) Acta Chim Hung 21:247
7. Kovat Z (1960) Acta Chim Hung 22:313
8. Kitajima KN, Moro-oka Y (1994) Chem Rev 94:737
9. Halcrow MA (2001) Angew Chem Int Ed 40:816
0. Peisach J, Alsen P, Blumberg WE (1966) The biochemistry of
copper. Academic, New York, p 49
1
1
1
1
1
2
Fig. 4 GC-mass spectrum of phenylacetaldehyde with its molecular
ion peak at m/z = 119
-
Similarly, total [OH ] can be calculated as
2
1. Sethuram B (2003) Some aspects of electron transfer reactions
involving organic molecules. Allied, New Delhi, p 73
ꢀ
ꢁ
4ꢁ
ꢁ
ꢁ
f
½
OH ꢂ ¼ ½OH ꢂ þ CuðOHÞ ðH3IO6ÞðH2IO6Þ
T
2
22. Hiremath GC, Mulla RM, Nandibewoor ST (2005) J Chem Res
97
2
ꢀ
ꢁ
ꢁ
1
3. Kolthoff IM, Meehan EJ, Carr EM (1953) J Am Chem Soc
75:1439
4. Bhattacharya S, Banerjee P (1996) Bull Chem Soc Jpn 69:3475
5. Reddy KB, Sethuram B, Navaneeth Rao T (1987) Z Phys Chem
268:706
þ CuðOHÞ ðH3IO6Þ þ½C];
ð10Þ
ð11Þ
2
ꢀ
ꢁ
3
ꢁ
ꢁ
ꢁ
¼
½OH ꢂ þK1½OH ꢂ CuðOHÞ ðH3IO6Þ
f
2
2
2
2
ꢀ
ꢁ
3
ꢁ
ꢁ
K1K2 CuðOHÞ ðH3IO6Þ
½OH ꢂ
2
2
þ
þ
ꢀ
ꢁ
3
ꢁ
H2IO6
26. Bailar JC Jr, Emeleus HJ, Nyholm SR, Trotman-Dickenson AF
(1975) Comprehensive inorganic chemistry, vol 2. Pergamon,
Oxford, p 1456
27. Chang R (1981) Physical chemistry with applications to biolog-
ical systems. McMillan, New York, p 326
28. Hosamani RR, Shetti NP, Nandibewoor ST (2009) Kinet Catal
ꢀ
ꢁ
3
ꢁ
ꢁ
K K K CuðOHÞ ðH IO Þ
½OH ꢂ½l-PAꢂ:
1
2
3
2
3
6 2
ꢀ
ꢁ
3
ꢁ
H IO
2
6
In view of the low concentrations of DPC used, the second,
third, and fourth term in the above equation are neglected;
therefore,
5
0:530
9. Kiran TS, Hiremath DC, Nandibewoor ST (2007) Z Phys Chem
21:501
2
2
½
similarly
OHꢂ ¼ ½OHꢂ ;
ð12Þ
30. Rangappa KS, Raghavendra MP, Mahadevappa DS, Channego-
uda D (1998) J Org Chem 63:531
31. Bilehal DC, Kulkarni RM, Nandibewoor ST (2001) Can J Chem
T
f
7
9:1926
½
l-PAꢂ ¼ ½l-PAꢂ :
Substituting Eqs. 9, 10, and 12 into Eq. 7, we get
ð13Þ
T
f
32. Weissberger A (1974) In: Lewis ES (ed) Investigations of rates
and mechanism of reactions in techniques of chemistry, vol 4.
Wiley, New York, p 421
3
3. Moore FM, Hicks KW (1976) J Inorg Nucl Chem 38:379
Rate
ꢁ
34. Hiremath DC, Sirsalmath KT, Nandibewoor ST (2008) Catal Lett
122:144
kK1K2K3½l-PAꢂ½OH ꢂ½DPCꢂ
¼
ꢀ
ꢁꢁ
ꢀ
ꢁ
:
3
3ꢁ
ꢁ
ꢁ
ꢁ
H2IO6 þ K1 H2IO6 ½OH ꢂ þ K1K2½OH ꢂ þ K1K2K3½OH ꢂ½l-PAꢂ
3
5. Lewis ES (1974) Investigations of rates and mechanisms of
reactions, 3rd edn. Wiley, New York, p 415
6. Exner O (1972) Collect Czech Chem Commun 37:1425
3
References
37. Leffler JE (1955) J Org Chem 20:1202
8. Jaiswal PK, Yadava KL (1973) Indian J Chem 11:83
3
1
. Mahadevappa DS, Rangappa KS, Gouda NM, Thimmegowda B
1982) Int J Chem Kinet 14:1183
39. Murthy CP, Sethuram B, Navaneeth Rao T (1981) Z Phys Chem
262:336
(
2
3
. Mahanti MK, Laloo D (1990) J Chem Soc Dalton Trans 311
. Kulkarni RM, Bilehal DC, Nandibewoor ST (2003) Transition
Met Chem 28:199
40. Jeffery GH, Bassett J, Mendham J, Denny RC (1996) Vogel’s text
book of quantitative chemical analysis, 5th edn. ELBS, Longman,
Essex, p 455
4
5
6
. Balreddy K, Sethuram B, Navneeth Rao T (1981) Indian J Chem
A 20:395
. Chougale RB, Hiremath GA, Nandibewoor ST (1997) Polish J
Chem 711:1471
. Reddy B, Sethuram B, Navaneeth Rao T (1984) Indian J Chem A
41. Panigrahi GP, Misro PK (1978) Indian J Chem A 16:201
42. Feigl F (1975) Spot tests in organic analysis. Elsevier, New York,
p 333
43. Jeffery GH, Bassett J, Mendham J, Denny RC (1996) Vogel’s text
book of quantitative chemical analysis, 5th edn. ELBS, Longman,
Essex, p 679
2
3:593
7
8
. Kumar A, Kumar P, Ramamurthy P (1999) Polyhedron 18:773
. Kumar A, Kumar P (1999) J Phys Org Chem 12:79
44. Hiremath DC, Kiran TS, Nandibewoor ST (2007) Int J Chem
Kinet 39:236
1
23