Kinetic and mechanistic investigations on oxidation of L-tryptophan by diperiodatocuprate(III) in aqueous alkaline medium

Article abstract of DOI:10.1524/zpch.2009.5432

The oxidation of L-tryptophan (L-TRP) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.20 mol dm^-3 was studied spectrophotometrically at 298 K. The reaction between DPC and L-tryptophan in alkaline m

Full text of DOI:10.1524/zpch.2009.5432

Z. Phys. Chem. 223 (2009) 299–317 . DOI 10.1524.zpch.2009.5432
by Oldenbourg Wissenschaftsverlag, München
©
Kinetic and Mechanistic Investigations on
Oxidation of L-tryptophan by
Diperiodatocuprate(III) in Aqueous Alkaline
Medium
By Nagaraj P. Shetti and Sharanappa T. Nandibewoor
*
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad, Karnataka-580 003,
India.
(
Received July 14, 2008; accepted December 8, 2008)
Kinetics . Mechanism . L-tryptophan . Oxidation . Diperiodatocuprate(III)
The oxidation of L-tryptophan (L-TRP) by diperiodatocuprate(III) (DPC) in aqueous alkaline
-3
medium at a constant ionic strength of 0.20 mol dm was studied spectrophotometrically at
2
98 K. The reaction between DPC and L-tryptophan in alkaline medium exhibits 1:4 stoichiom-
etry (L-tryptophan: DPC). The reaction is of first order in [DPC] and has less than unit order
in [L-TRP] and negative fractional order in [periodate] and [alkali]. Intervention of free radicals
was observed in the reaction. The oxidation reaction in alkaline medium has been shown to
proceed via a DPC- L-tryptophan complex, which decomposes slowly in a rate determining
step followed by other fast steps to give the products. The main products were identified by
1
13
spot test, IR, H NMR, CNMR and LC-MS spectral studies. The reaction constants involved
in the different steps of the mechanism were calculated. The activation parameters with respect
to slow step of the mechanism were computed and discussed and thermodynamic quantities
were also determined.
1. Introduction
Amino acids act not only as the building block in protein synthesis but they also
play a significant role in metabolism and have been oxidized by a variety of
oxidizing agents [1]. The study of the oxidation of amino acids is of interest
because of their biological significance and selectivity towards the oxidant to
yield the different products [2–3].
L-tryptophan [L-TRP] is an essential amino acid and is needed to maintain
optimum health. It is particularly plentiful in bananas, dried dates, milk, cottage
*
Corresponding author. E-mail: stnandibewoor@yahoo.com
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00
N. P. Shetti and S. T. Nandibewoor
cheese, meat, fish, turkey, and peanuts. This amino acid is required for the pro-
duction of niacin (vitamin B ) a precursor of serotonin, a neurotransmitter, that
3
is important for normal nerve and brain function. Serotonin is one of the useful
chemicals in stabilizing emotional moods, pain control, inflammation, intestinal
peristalsis, etc. It is also important in controlling hyperactivity in children, assists
in alleviating stress, helps with weight loss and reducing appetite. A shortage of
L-tryptophan may be a contributing factor to heart artery spasms.
In recent years, the study of highest oxidation state of transition metals has
intrigued many researchers. Transition metals in a higher oxidation state can be
stabilized by chelation with suitable polydentate ligands. Metal chelates such as
diperiodatocuprate(III) [3], diperiodatoargentate(III) [4] and diperiodatonickel-
ate(IV) [5] are good oxidants in a medium with an appropriate pH value. Perio-
date and tellurate complexes of copper in its trivalent state have been extensively
used in the analysis of several organic compounds [6]. The kinetics of self-
decomposition of these complexes was studied in some detail [7]. Copper(III) is
shown to be an intermediate in the copper(II) catalyzed oxidation of amino acids
by peroxydisulphate [8]. The oxidation reaction usually involves the copper(II)-
copper(I) couple and such aspects are detailed in different reviews [9]. The use
of diperiodatocuprate(III) (DPC) as an oxidant in alkaline medium is new and
restricted to a few cases due to the fact of 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 [10]. Copper complexes have occupied a major place in oxidation
chemistry due to their abundance and relevance in biological chemistry [11].
Copper(III) is involved in many biological electron transfer reactions [12]. They
have also been used [13] in the differential titration of organic mixtures, in the
estimation of chromium, calcium and magnesium from their ores, 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
species is the active oxidant.
In earlier reports [3,14] on DPC oxidation, periodate had a retarding effect
-
and the order in [OH ] was found to be less than unity in most of the reactions.
However in the present study we have observed entirely different kinetic obser-
vations. Literature survey reveals that there are no reports on the oxidation of L-
tryptophan (L-TRP) by diperiodatocuprate(III). The present study deals with the
title reaction to investigate the redox chemistry of DPC in alkaline media, to
compute the thermodynamic quantities of various steps of Scheme 1 and to arrive
at a suitable mechanism on the basis of kinetic and spectral results.
2. Experimental
2.1 Materials and reagents
All chemicals used were of reagent grade and double distilled water was used
throughout the work. The copper(III) periodate complex was prepared [15] and
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Kinetic and Mechanistic Investigations …
301
standardized by a standard procedure [16]. The UV-vis spectrum with maximum
absorption at 415nm verified existence of copper(III) complex. The solution of
L-tryptophan (S.D. Fine Chem.) and copper sulphate (BDH) were prepared by
dissolving known amounts of the samples in distilled water. Periodate solution
was prepared and standardized iodometrically [17]. Required alkalinity and ionic
strength were maintained by KOH (BDH) and KNO (Analar) respectively in
3
reaction solutions.
2.2 Kinetic measurements
Since, the initial rate of reaction is fast, the kinetic measurements were performed
on a Varian CARY 50 Bio UV-Visible Spectrophotometer attached to a rapid ki-
netic accessory (HI-TECH SFA-12). The kinetics was followed under pseudo-first
order condition where [TRP] > [DPC] at 298±0.1 K, unless specified. The reaction
was initiated by mixing the DPC to L-trp solution which also contained required
concentration of KNO , KOH and KIO The progress of reaction was followed
3
4
spectrophotometrically at 415 nm by monitoring the decrease in absorbance due to
3
-1
-1
DPC with the molar absorbancy index, 'ε' to be 6235±100 dm mol cm
Literature ε = 6230 [18]). It was verified that there is a negligible interference
(
from other species present in the reaction mixture at this wavelength.
The pseudo-first order rate constants, 'kobs' were determined from the log(ab-
sorbance) versus time plots. The plots were linear up to 80% completion of
-
reaction under the range of [OH ] used. The orders for various species were
determined from the slopes of plots of log kobs versus respective concentration
of species except for [DPC] in which non variation of 'kobs' was observed as
expected to the reaction condition. During the kinetics a constant concentration
-
4
-3
viz. 5.0!10 mol dm of KIO was used throughout the study unless otherwise
4
stated. Since periodate is present in the excess in DPC, the possibility of oxida-
tion of L-tryptophan 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 to the DPC oxidation of L-tryptophan. The total concentration of peri-
-
odate and OH was calculated by considering the amount present in the DPC
solution and that additionally added. Kinetics runs were also carried out in N2
atmosphere in order to understand the effect of dissolved oxygen on the rate of
reaction. No significant difference in the results was obtained under a N atmos-
2
phere and in the presence of air. In view of the ubiquitous contamination of
carbonate in the basic medium, the effect of carbonate was also studied. Added
carbonate had no effect on the reaction rates.
LC-ESI-MS analysis was carried out using reverse phase high performance
chromatography (HPLC) system with a phenomena C-18 column, UV-visible
detector and series mass analyzer. 5 μL of acidified product and methanol solu-
tion was injected. The mobile phase consisted of ammonium acetate (eluent A)
and acetonitrile at a flow rate of 1 ml.min . Gradient elution was run to separate
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N. P. Shetti and S. T. Nandibewoor
Scheme 1. Stoichiometry of the reaction.
substrate and product. Gradient 0min.50% A-20min.50%A-20min.75%B-
2
0min.25%B. UV detection at 254 nm.
2.3 Stoichiometry and product analysis
Different sets of reaction mixtures containing excess of DPC to L-tryptophan in
-
presence constant amounts of OH and KNO were kept for 6 hrs in closed vessel
3
under inert atmosphere. The remaining DPC concentration was estimated by
spectrophotometrically at 415 nm. The results, 1:4 stoichiometry as given in
Scheme 1.
The main product, indole-3-acetic acid was separated [19] by TLC, using the
mixtures of methyl acetate, isopropanol and 25% ammonium hydroxide in the
ratio of 45:35:20(v.v). It is characterized by its melting point (164 °C) and IR
and NMR spectra respectively.
-
1
The IR spectrum of the product showed in Fig. 1, a sharp band at 3389 cm ,
-
1
assigned to indole -NH and a series of bands in the region of 2730 to 3127 cm
were due to the hydrogen bonded -OH stretching frequencies. An intense sharp
-1
band at 1701 cm was due to C = O stretching frequencies of carboxylic group.
-
1
The absorbance of broad bands in the region of 3086 to 3310 cm and presence
-
1
of only an intense sharp band at 3389 cm confirms the absence of free NH2
and presence of indole-NH group. The product indole-3-acetic acid was further
1
13
1
confirmed by its HNMR and CNMR in Fig. 2 respectively. HNMR
300 MHz, DMSO d ) spectra showed peaks in ppm (1H, s, δ-10.88) carboxylic
(
6
OH (D O exchanged); (2Ar H, d, δ-7.49 and 7.47); (2Ar H, t, δ-7.36 and 7.33);
2
(
1Ar H, s, 6.98); (1H, s, δ-4.48) NH D O exchanged; and (1H, s, δ-2.5). Where
2
13
as CNMR (300 MHz, DMSO d ) peaks at δ: 174.29, 136.88, 127.94, 124.75,
6
1
21.97, 119.39,119.33, 112.25, 108.46 and 31.82 ppm are assigned to carbons
designated with numbers 1,2, 3, 4, 5, 6, 7, 8, 9 and 10 respectively. LC-ESI-MS
analysis of isolated acidified product indicated the presence of Indole-3-acetic
acid as molecular ion peak of m.z 176 (m+1) (Fig. 3).
The byproducts were identified as ammonia by Nessler's reagent [20] and
the CO was qualitatively detected by bubbling nitrogen gas through the acidified
2
reaction mixture and passing the liberated gas through tube containing 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 point from the regression line was performed using
Microsoft Excel-2003 programme.
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Kinetic and Mechanistic Investigations …
303
-
1
Fig. 1. IR spectra of indole-3-acetic acid in KBr. Indole NH-band at 3389 cm , hydrogen
-1
bonded -OH stretching band between (2730 to 3127) cm and carboxylic group (C = O) stretch-
-1
ing at 1701 cm .
3. Results
3.1 Reaction orders
The reaction orders were determined from the slope of log rate constants versus
concentration plots by varying the concentrations of L-tryptophan, alkali and
periodate in turn while keeping all other concentrations and conditions constant.
3.2 Effect of [Diperiodatocuprate (III)]
-
5
The oxidant DPC concentration was varied in the range of 1.0!10 to 1.0!
-
4
-3
1
0
mol dm and the fairly constant k values indicate that order with respect
obs
to [DPC] was unity (Table 1). This was also confirmed by linearity of the plots
of log [absorbance] versus time (r ≥ 0.986, S ≤ 0.014) up to 80% completion of
the reaction as shown in Fig. 4.
3.3 Effect of [L-tryptophan]
The effect of L-tryptophan on the rate of reaction was studied at constant concen-
-
3
trations of alkali, DPC and periodate at a constant ionic strength of 0.20 mol dm .
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04
N. P. Shetti and S. T. Nandibewoor
Fig. 2. 13CNMR spectra of indole-3-acetic acid in DMSO d
27.94, (4) = 124.75, (5) = 121.97, (6) = 119.39, (7) = 119.33, (8) = 112.25, (9) = 108.46,
10) = 31.82.
. (1) = 174.29, (2) = 136.88, (3) =
6
1
(
-
4
-3
-3
The substrate, L-TRP was varied in the range of 1.0!10 to 1.0!10 mol dm .
The kobs values increased with increase in concentration of L-tryptophan (Ta-
ble 1). The order with respect to [L-TRP] was found to be less than unity (r ≥
.995, S ≤ 0.009). This is also confirmed in the plots of kobs versus [L-TRP]0
.82
0
which is linear rather than the direct plot of kobs versus [L-TRP] (Fig. 5).
3.4 Effect of [Alkali]
The effect of increase in concentration of alkali on the reaction was studied at
constant concentrations of L-tryptophan, DPC and periodate at a constant ionic
-
3
strength of 0.20 mol dm at 25 °C. The rate constants decreased with increase
in alkali concentration (Table 1), indicating negative fractional order dependence
of rate on alkali concentration (r ≥ 0.988, S ≤ 0.007).
3.5 Effect of [Periodate]
The effect of increasing concentration of periodate was studied by varying the
-
4
-3
-3
periodate concentration from 1.0!10 to 1.0!10 mol dm keeping all other
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Kinetic and Mechanistic Investigations …
305
Fig. 3. LC-ESI-MS analysis of isolated product indicated the presence of Indole-3-acetic acid
as molecular ion peak of m.z 176 (m+1).
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 con-
centration was negative less than unity (r ≥ 0.0984, S ≤ 0.004) (Table 1).
3
.6 Effect of Ionic Strength (I) and Dielectric Constant on the Medium
(
D)
-
-
The addition of KNO at constant [DPC], [L-TRP], [OH ] and [IO ], was found
3
4
that increasing ionic strength of the reaction medium increases the rate of the
reaction. Dielectric constants of the medium, D, was varied by varying t-butyl
alcohol and water percentage. The D values were calculated from the equation
D = DWVW + D V , where D and D are dielectric constants of pure water and
B
B
W
B
t-butyl alcohol, respectively, and VW and V are the volume fractions of compo-
B
nents water and t-butyl alcohol, respectively, in the total volume of the mixture.
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06
N. P. Shetti and S. T. Nandibewoor
Fig. 4. First order plots on the oxidation of L-tryptophan by DPC in aqueous alkaline medium
5
-3
at 298 K. (10 DPC (mol dm ): (1) 1.0; (2) 3.0; (3) 5 .0; (4) 7.0; (5) 10.0.
-
-
Table 1. Effect of [DPC], [L-TRP], [IO
4
] and [OH ] on diperiodatocuprate(III) oxidation of L-
-3
tryptophan in alkaline medium at T = 298 K, I = 0.2 mol dm .
5
4
-
4
-
3
-1
3
-1
[
DPC]!10 [L-TRP]!10
[IO
4
]!10
[OH ] (mol
k
obs!10 (s )
k
cal×10 (s )
-3
-3
-3
-3
(
mol dm )
(mol dm )
(mol dm )
5.0
dm )
1
3
5
7
.0
.0
.0
.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
10.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
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
0.1
0.02
0.06
0.1
0.15
0.2
5.84
5.81
5.79
5.75
5.91
1.33
3.80
5.79
6.82
8.81
11.4
8.06
5.79
4.83
3.71
8.23
6.58
5.79
5.19
4.49
5.92
5.92
5.92
5.92
5.92
1.43
3.89
5.92
7.61
9.6
5.0
5.0
5.0
10.0
5.0
5
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
5.0
5
5.0
5
5
5
5
5
5
5
5
5
5
5
5
5
5.0
5.0
5.0
1.0
11.4
7.81
5.92
4.76
3.68
7.94
6.78
5.92
5.11
4.49
3.0
5.0
7.0
10.0
5.0
5.0
5.0
5.0
5.0
The decrease in dielectric constant of the reaction medium increased the rate of
the reaction.
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Kinetic and Mechanistic Investigations …
307
Fig. 5. Plot of kobs versus [L-TRP]0.82 and kobs versus [L-TRP].
3.7 Effect of Initially Added Products
The externally added products, Indole 3-acetic acid and copper(II) (CuSO ) in
4
-
5
-4
the concentration range of 1.0!10 to 1.0!10 , did not have any significant
effect on the rate of the reaction.
3.8 Polymerization Study
The intervention of free radicals in the reaction was examined as follows. The
reaction mixture, to which a known quantity of acrylonitrile monomer was ini-
tially added, was kept for 2 hour in an inert atmosphere. On diluting the reaction
with methanol, a white precipitate was formed, indicating the intervention of
free radicals in the reaction [18]. The blank experiments of either DPC or L-
tryptophan alone with acrylonitrile did not induce any polymerization under the
same conditions as those induced for reaction mixture. Initially, added acryloni-
trile decreased the rate of reaction indicating free radical intervention, which is
the case in earlier work [21].
3.9 Effect of Temperature (T)
The kinetics was studied at four different temperatures (288, 293, 298 and 303)
K under varying concentrations of L-tryptophan, alkali and periodate, keeping
other conditions 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
2-
-
from the slopes and intercepts of 1.kobs versus 1.[L-TRP], [H IO6 ] and [OH ]
3
plots at four different temperatures and were used to calculate the activation
parameters. The energy of activation corresponding to these constants was evalu-
ated from the Arrhenius plot of log k versus 1.T (r ≥ 0.976, S ≤ 0.009) and
other activation parameters obtained are tabulated in Table 2–5.
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N. P. Shetti and S. T. Nandibewoor
Table 2. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in
aqueous alkaline medium with respect to the slow step of Scheme 2: Effect of temperature.
2
-1
Temperature (K)
k!10 (s )
1.14
2
2
2
3
88
93
98
03
1.98
2.68
3.24
Table 3. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in
aqueous alkaline medium with respect to the slow step of Scheme 2: Activation Parameters.
Parameters
Values
50.7±1.0
48±2
-1
a
E (kJ mol )
#
-1
ΔH (kJ mol )
#
-1
-1
ΔS (JK mol )
-113±10
82±4
#
-1
ΔG (kJ mol )
log A
7.3±0.2
Table 4. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in
aqueous alkaline medium with respect to the slow step of Scheme 2: Effect of temperature to
4 5 6
calculate K , K and K for the oxidation of L-tryptophan by diperiodatocuprate(III) in alkaline
medium.
2
!104
-2
Temperature (K)
K
4
!10
K
5
K
6
!10
-3
-3
3
-1
(
mol dm )
6.5±0.3
7.2±0.3
9.1±0.4
11.6±0.5
(mol dm )
6.8±0.3
3.7±0.2
3.0±0.1
2.5±0.1
(dm mol )
17.9±0.3
21.4±0.5
25.5±0.6
27.9±0.7
2
2
2
3
88
93
98
03
Table 5. Thermodynamic activation parameters for the oxidation of L-tryptophan by DPC in
aqueous alkaline medium with respect to the slow step of Scheme 2: Thermodynamic quantities
using K4,
5 6
K and K .
Thermodynamic quantities
Values from
Values from
Values from
K
4
K
5
K
6
-
1
ΔH (kJ mol )
29±2
76±4
6±0.2
-47±3
-224±10
20±1
22±1
139±6
-19±1
ΔS (JK-1 mol )
-1
-1
ΔG298 (kJ mol )
4. Discussion
The water-soluble copper(III) periodate complex is reported [22] to be
7
-
[
Cu(HIO ) (OH) ] . However, in an aqueous alkaline medium and at a high pH
6 2 2
4-
range employed in the study, periodate is unlikely to exist as HIO6 (as present
in the complex) as is evident from its involvement in the multiple equilibria [23]
(1)–(3), depending on the pH of the solution.
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Kinetic and Mechanistic Investigations …
309
(
(
(
1)
2)
3)
-
Periodic acid exists as H IO in acid medium and as H IO near pH 7.
5
6
4
6
Hence, under alkaline conditions as employed in this study, the main species are
2
-
3-
expected to be H IO6 and H IO6 . Thus, at the pH employed in this study, the
3
2
3-
soluble copper(III) periodate complex might be [Cu(OH) (H IO ) ] , a conclu-
2
3
6 2
sion also supported by earlier work [3,14].
The reaction between the diperiodatocuprate(III) complex and L-tryptophan
in alkaline medium has the stoichiometry 1:4 (L-TRP: DPC) with a first order
dependence on [DPC] and an apparent order of less than unit order in [substrate],
a negative fractional order dependence both on the [periodate] and [alkali]. 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 [oxidant], [reductant], [OH ] and [IO ] may be well accommodated. It
4
is known that L-tryptophan exists in the form of Zwitterion [24] in aqueous
medium. In highly acidic medium, it exists in the protonated form, where as in
highly basic medium it is in the fully deprotonated form [24].
In most of the reports [3] on DPC oxidation, periodate had a retarding effect
-
and OH had an increasing effect on the rate of the reaction. However in the
present kinetic study, different kinetic results have been obtained. In this study
-
both OH and periodate retarded the rate of the reaction. The result of decrease
in rate of reaction with increase in alkalinity (Table 1) can be explained in terms
3
-
of prevailing equilibrium of formation of [Cu(OH) (H IO ) ]
from
2
3
6 2
4-
[
Cu(OH) (H IO )(H IO )] hydrolysis as given in the following Eq. (4).
2 3 6 2 6
(4)
Also decrease in rate with increase in [H IO62-] (Table 1) suggests that equi-
3
librium of copper(III) periodate complex to form monoperiodatocuptrate(III)
MPC) species as given in Eq. (5) is established.
(
(5)
Such type of equilibria (4) and (5) have been well noticed in literature [3,14].
It may be expected that a lower periodate complex such as monoperiodatocup-
trate(III) (MPC) is more important in the reaction than the DPC. The inverse
2-
fractional order in [H IO6 ] might also be due to this reason. Therefore, MPC
3
might be the main reactive form of the oxidant.
The less than unit order in [L-TRP] presumably results from formation of a
complex (C) between the MPC species and L-tryptophan prior to the formation
of the products. This complex (C) decomposes in a slow step to form a free
radical derived from L-tryptophan. This free radical species further reacts with
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N. P. Shetti and S. T. Nandibewoor
Scheme 2. Detailed Scheme for the oxidation of L-tryptophan by alkaline diperiodatocuprate
III).
(
another molecule of MPC in a fast step to form indole-3-acetal intermediate. The
indole-3-acetal then reacts with two more molecules of MPC in a further fast
step to form the products such as indole-3-acetic acid, Cu(II) and periodate as
given Scheme 2.
Since Scheme 2 is in accordance with the generally well-accepted principle
of non-complementary oxidations taking place in sequence of one-electron steps,
the reaction between the substrate and oxidant would afford a radical intermedi-
ate. A free radical scavenging experiment revealed such a possibility (see infra).
This type of radical intermediate has also been observed in earlier work [14].
The direct plot of kobs versus [L-TRP] was drawn to know the parallel reaction
if any along with interaction of oxidant and reductant. However the plot of kobs
versus [L-TRP] was not linear. Thus, in Scheme 2, the parallel reaction and
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Scheme 3. The probable structure of the complex [C].
Fig. 6. Spectroscopic evidence for the complex formation between DPC and L-Tryptophan. (1)
UV-vis spectra of DPC complex (262 nm); (2) UV-vis spectra of mixture of DPC and L-
tryptophan (269 nm).
involvement of two molecules of L-TRP in the complex are excluded. The proba-
ble structure of the complex is given in Scheme 3.
Spectroscopic evidence for the complex formation between oxidant and sub-
-
4
strate was obtained from UV-Vis spectra of L-tryptophan (5.0!10 ), DPC
-
5
-
-3
(
5.0!10 ), [OH ] = 0.10 mol dm ) and mixture of both. A bathochromic shift
of about 7 nm from 259 to 266 nm in the spectra of DPC was observed in Fig. 6.
The Michaelis-Menten plot also proved the complex formation between DPC
and L-tryptophan, which explains the less than unit order dependence on [L-
TRP]. Such a complex between an oxidant has been observed in other studies
[25].
Scheme 2 leads to the rate law (6).
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N. P. Shetti and S. T. Nandibewoor
(6)
(7)
This explains all the observed kinetic orders of different species. The rate law
7) can be rearranged in to the following form, which is suitable for verification:
(
(8)
According to Eq. (8), other conditions being constant, plots of 1.k versus
obs
-
[
OH ] (r ≥ 0.995, S ≤ 0.014), 1.k versus 1.[L-TRP] (r ≥ 0.999, S ≤ 0.012)
obs
2-
and 1.kobs versus [H IO6 ] (r ≥ 0.999, S ≤ 0.011) should be linear and are found
3
to be so (Fig. 7). The slopes and intercepts of such plots lead to the values
-
2
-3
-4
-3
of K , K , K and k as (9.1±0.4)!10 mol dm , (3.0±0.1)!10 mol dm ,
(
4
5
6
2
3
-1
-2 -1
25.50±0.6)!10 dm mol and (2.68±0.01)!10 s respectively. The value
of K is in good agreement with earlier literature [18]. These constants were used
4
to calculate the rate constants and compared with the experimental values and
found to be in reasonable agreement with each other (Table 1), which fortifies
the Scheme 2. The equilibrium constant K is far greater than K . This may be
4
5
attributed to the greater tendency of DPC to undergo hydrolysis compared to the
dissociation of hydrolyzed species in alkaline medium. The effect of ionic
strength and dielectric constant of the medium on the rate explains qualitatively
the reaction between two negatively charged ions, as seen in Scheme 2.
The thermodynamic quantities for the first, second and third equilibrium
2
-
steps of Scheme 2 can be evaluated as follows. The [H IO ], [L-TRP] and
3
6
-
[
OH ] (as in Table 1) were varied at four different temperatures. The plots of 1.
kobs versus [OH ], 1.k versus 1.[L-TRP] and 1.kobs versus [H IO6 ] should
-
2-
obs
3
be linear (Fig. 7). From the slopes and intercepts, the values of K4, K and K6
5
were calculated at different temperatures and these values are given in the Ta-
ble 4. The vant Hoff's plots were made for variation of K4, K and K with
5
6
temperature (log K versus 1.T (r ≥ 0.985, S ≤ 0.004), log K versus 1.T (r ≥
4
5
0
.973, S ≤ 0.007) and (log K versus 1.T (r ≥ 0.992, S ≤ 0.008) and the values
6
of enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of reaction
ΔG, were calculated for the first, second and third equilibrium steps. These val-
ues are given in Table 5.
A comparison of the thermodynamic quantities of first step of Scheme 2 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 rate
determining step is fairly fast and involves low activation energy [26].
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Kinetic and Mechanistic Investigations …
313
Fig. 7. Verification of rate law (9) for the of oxidation of L-tryptophan by DPC at T = 298 K.
#
#
The moderate values of ΔH and ΔS were both favorable for electron trans-
#
fer processes. The negative value of ΔS suggests that the intermediate complex
is more ordered than the reactants [27] The observed modest enthalpy of activa-
tion and as well as a higher rate constant of the slow step indicate that the
oxidation presumably occurs via inner-sphere mechanism. This conclusion is
supported by earlier observation [13,28].
The activation parameters for the oxidation of some acids by DPC are sum-
marized in Table 6.
According to Exner [29], if the rates of several reactions in a series have
been measured at two temperatures and log k (at T ) is linearly related to log k
1
2
2
(
at T ), i.e., log k = a + b log k , he proposes that ꢀ can be evaluated from the
1 2 1
equation.
We have calculated the isokinetic temperature to be 311 K by plotting log
k at 298 K versus log k at 303 K (r ≥ 0.997, S ≤ 0.006) as in Fig. 8. The value
2
1
of ꢀ (311 K) is higher than experimental temperature (298 K). This indicates
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14
N. P. Shetti and S. T. Nandibewoor
Table 6. Activation parameters for oxidation of some amino acids (for isokinetic temperature).
Amino acids
k
1
3
!102
k
2
3
!102
ΔH#
dm mol s ) (dm mol s ) (J.K.mol)
at T = 298 K at T = 303 K
ΔS#
Reference
-1 -1
-1 -1
(
(kJ.mol)
L-Aspartic acid
Vanillin
1.3
3.7
2.1
2.7
1.1
1.6
4.2
41.5
13.1
72.0
29.0
48.0
-142
-227
-32.0
-181
-113
31
32
L-Lysine
4.1
33
34
L-Proline
3.16
1.98
L-Tryptophan
Present work
2 1
Fig. 8. Plot of log k at 303 K versus log k at 298 K for isokinetic temperature (Table 6). (1)
L-Tryptophan ; (2) L-Aspartic acid ; (3) L-Lysine ; (4) L-Proline ; (5) Vanillin.
that the rate is governed by the enthalpy of activation. [30] The linearity and the
slope of the plot obtained may confirm that the kinetics these reactions follows
a similar mechanism, as previously suggested.
5. Conclusion
Among various species of DPC in alkaline medium, monoperiodatocuptrate(III)
-
(
MPC) [Cu(OH) (H IO )] is considered as active species for the title reaction.
2 3 6
The results indicate that, the role of pH in the reaction medium is crucial. Rate
constant of slow step and other equilibrium constants involved in the mechanism
are evaluated and activation parameters with respect to slow step of reaction
were computed. The overall mechanistic sequence described here is consistent
with product studies, mechanistic and kinetic studies.
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Kinetic and Mechanistic Investigations …
315
Appendix
According to Scheme 2,
(9)
(
(
10)
where T and f refer to total and free concentrations.
11)
(
(
12)
13)
Similarly,
(14)
In view of low concentrations of DPC used, the second, third and fourth
terms in the above equation are neglected. Therefore,
(15)
Similarly,
(16)
Substituting Eqs. (12), (15) and (16) in (9) and omitting the subscripts T and
f we get,
(17)
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