# Mechanistic Aspects of Osmium(VIII) Catalyzed  Oxidation of L-Tryptophan by Diperiodatocuprate(III)  in Aqueous Alkaline Medium: A Kinetic Model

Authors: Nagaraj P. Shetti; Ragunatharaddi R. Hosamani; Sharanappa T. Nandibewoor
Journal: Research Letters in Inorganic Chemistry
 (2008)

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## Abstract

In presence of osmium(VIII), the reaction between L-tryptophan and diperiodatocuprate(III) DPC in alkaline medium exhibits 1:4 stochiometry                     (L-tryptophan:DPC). The reaction shows first-order dependence on [DPC] and [osmium(VIII)], less than unit order in both [L-tryptophan] and [alkali], and negative fractional order in [periodate]. The active species of catalyst and oxidant have been identified. The main products were identified by spectral studies and spot test. The probable mechanism was proposed and discussed.


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## Body

## 1. Introduction

In the recent past [1], some relatively stable copper(III) complexes have been prepared, namely, the periodate, guanidine, and
tellurate complexes. The Cu3+/Cu2+ reduction potential is  –1.18 V in
alkaline solution [2]. The copper(III) periodate
complex (DPC) exhibits different multiple equilibria involving different
copper(III) species in aqueous alkaline medium. It is interesting to know which
of the copper(III) species is the active oxidant.L-tryptophan (L-trp) is an essential aminoacid and it is
needed to maintain optimum health. Osmium(VIII) acts as an efficient catalyst in
many redox reactions [3, 4] involving
different complexities due to the formation of different intermediate
complexes, free radicals, and multiple oxidation states of osmium.The uncatalyzed reaction of oxidation of L-tryptophan by
DPC has been studied [5]. We have observed that the microamounts of
osmium(VIII) catalyze the oxidation of L-trp by DPC in alkaline medium. In
order to understand the active species of oxidant and catalyst and to propose
the appropriate mechanism, the title reaction is investigated in detail, in
view of various mechanistic possibilities.


## 2. Experimental

All chemicals used
were of reagent grade and
millipore water was used throughout the
work. A solution of L-trp (s.d. fine) was prepared by dissolving an appropriate amount of recrystallized sample in millipore
water. A stock solution of osmium(VIII)
was prepared and standardized by
the method reported earlier [6]. The copper(III) periodate complex was
prepared [7] and standardized by standard procedure [8].


### 2.1. Kinetic Measurements

All kinetics measurements were carried out as in
earlier work [6].


## 2.1. Kinetic Measurements

All kinetics measurements were carried out as in
earlier work [6].


## 3. Results and Discussion

The
results indicated 1:4 stoichiometry as given in Scheme1.Scheme 1
1:4 stochiometry of osmium(VIII) catalyzed
oxidation of L-trp by DPC reaction.The main
product, indole-3-acetic acid, was separated by TLC, using the mixture of
methyl acetate, isopropanol, and 25% ammonium hydroxide in the ratio of
45:35:20. IR, NMR spectra and its melting points were compared with the literature and were in good agreement. The LC-MS analysis of
isolated product indicated the presence of indole-3-acetic acid as molecular
ion peak, m/z 175.In
the presence of catalyst, the reaction is understood to occur via parallel
paths with contributions from the uncatalyzed and catalyzed paths. The total
rate constant (kT) is equal to the sum of the rate constants of the
catalyzed (kC) and uncatalyzed (kU) reactions. Hence, kC=kT−kU. The reaction orders have been determined from
the slopes of log kc versus log
(concentration) plots by varying the concentration of L-trp, Os(VIII), OH−, and IO4−,
in turn, while keeping the other concentrations constant. The order in both
[DPC]  and [Os(VIII)]    was found to be unity. The order in [L-trp]  and [OH−]  was
found to be less than unity, and
in [periodate]  to be negative and less than unity. It is well known that
[9] Os(VIII) exists as (OsO4(OH)2]2+ in
aqueous alkaline medium. It was found that the
increase in ionic strength increased the rate of reaction and decrease in
dielectric constant of the medium increased the rate of reaction. Initially added products did not have any significant effect on the
rate of reaction. Test for free radicals indicated the participation of free
radical in the reaction [6]. These experimentally determined orders and results
could be well accommodated in Scheme 2.Scheme 2
The osmium(VIII) catalyzed oxidation of
L-trp by DPC.Based on the experimental results, monoperiodatocuprate
MPC was considered to be the active species of DPC complex. The fractional
order with respect to L-trp concentration indicates the formation of a complex
between L-trp and osmium(VIII) species. Spectroscopic evidence for the complex
formation between catalyst and substrate was obtained from UV-vis spectra of
Os(VIII), L-trp, and a mixture of both. A bathochromic shift of about 6 nm from
255 nm to 261 nm in the spectra of Os(VIII) was observed. The Michaelis-Menten
plot also proved the complex formation between catalyst and reductant. Such a
complex between a substrate and a catalyst has been observed in other studies
[6].Scheme2 leads
to the following rate law: (1)Rate[DPC]=kC=kT−kU=kK1K2K3[L-trp][OH−][Os(VIII)]ℛ, where ℛ denotes [H3IO63−]+K1[OH−][H3IO63−]+K1K2[OH−]+K1K2K3[OH−][L-trp] which explains all the observed kinetic orders of
different species. The rate law (1) can be rearranged into the following form
which is suitable for verification: (2)[Os(VIII)]kc=[H3IO63−]kK1K2K3[L-trp][OH−]+[H3IO63−]kK2K3[L-trp]+1kK3[L-trp]+1k. According
to (2), others being constant, the plots of [Os(VIII)]/kC versus 1/[L-trp],
[Os(VIII)]/kC versus 1/[OH−],
and [Os(VIII)]/kC versus [H3IO62−]
were linear as in Figure 1. From the intercepts and slopes of such plots, the
reaction constants K1, K2,
K3, and k were calculated as (15.6±0.4) dm3 mol−1,
(3.3±0.10)  ×  10−4 mol dm−3, (0.71±0.02)  ×  104 dm3 mol−1, (3.2±0.04)  ×  103 dm3 mol−1s−1,
respectively. The values of K1 and K2 obtained were in good agreement with previously
reported values [10]. These constants were used to calculate the rate constants
over different experimental conditions; when compared with the
experimental kC values, they were found to be in reasonable
agreement with each other, which fortifies Scheme 2.Verification of rate law (1) of Os(VIII) catalyzed oxidation of L-tryptophan by DPC at 298 K (conditions as in Table 1). (a) [Os(VIII)]/kc versus 1/[L-trp]; (b) [Os(VIII)]/kc versus 1/[OH−]; (c) [Os(VIII)]/kc versus [H2IO63−].
(a)(b)(c)SimilarlyK1, K2,
K3, and k were calculated at four different temperatures (288, 293, 298, and 303 K) and used to compute the activation parameters and
thermodynamic quantities. The values of Ea, ΔH#, ΔS#, and ΔG# and logA
were obtained and found to be 42.0  ±  2 kJ mol−1,
44.0  ±  2 kJ mol−1–30.0±1.5 J K−1 mol−1, 53.0  ±  3 kJ mol−1, and 11.0±0.5, respectively. (Ea, ΔS#,
ΔH#, and logA were 51.7  ±  3 kJ mol−1, −114  ±  6 J K−1 mol−1,
48.2  ±  2 kJ mol−1, and 10.5, resp., for the uncatalyzed reaction
[5].) The catalyst Os(VIII) alters the reaction path by lowering the energy of
activation (i.e., it provides an alternative pathway with lower activation
parameters for the reaction).The
thermodynamic quantities,ΔH
(kJ mol−1), ΔS  (J K−1 mol−1), and ΔG
(kJ mol−1) using K1 were calculated to
be –47, 182, and −6.4,
respectively. Similarly the values using K2 were calculated to
be 97.7, 262.8, and 18.6, respectively and the values using K3 to be  –144.0, −412.0,
and −22.0, respectively.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 Scheme1. The moderate
ΔH# and ΔS# values are favorable for electron transfer
reaction. The negative value of ΔS# suggests that the intermediate
complex is more ordered than the reactants [11]. The observed modest enthalpy
of activation and a higher-rate constant for the slow step indicate that the
oxidation presumably occurs via an innersphere mechanism. This conclusion is
supported by earlier observations [12].


## 4. Conclusion

Among various species of Cu(III) in alkaline medium,
monoperiodatocuprate(III) is considered to be the active species for the title
reaction. The active species of osmium(VIII) is understood to be as [OsO4(OH)2]2−.
The activation parameters evaluated for the catalyzed and uncatalyzed reactions explain the
catalytic effect on the reaction. The Os(VIII) catalyst alters the reaction
path by lowering the energy of activation.Table 1
Effects of [DPC], [L-trp], [OH−], [IO4−],  and [Os(VIII)]  on the osmium(VIII) catalyzed oxidation of L-trp by DPC in
alkaline medium at 298 K, I  =  0.20 mol dm−3.
105[DPC] (mol dm−3)104[L-trp] (mol dm−3)[OH−] (mol dm−3)104[IO4−] (mol dm−3)106[Os(VIII)] (mol dm−3)102kT (s−1)103kU (s−1)102kC (s−1)FoundCalculated1.05.00.15.08.01.905.841.321.343.05.00.15.08.01.915.811.341.345.05.00.15.08.01.895.701.321.347.05.00.15.08.01.965.751.391.34105.00.15.08.01.895.911.301.345.01.00.15.08.00.611.340.470.445.03.00.15.08.01.453.801.100.995.05.00.15.08.01.895.701.321.345.07.00.15.08.02.286.831.561.545.0100.15.08.02.698.821.811.755.05.00.025.08.01.688.230.870.855.05.00.065.08.01.726.591.051.135.05.00.15.08.01.895.701.321.345.05.00.155.08.01.965.191.451.395.05.00.25.08.02.014.451.561.475.05.00.11.08.02.9211.441.771.835.05.00.13.08.02.458.061.641.535.05.00.15.08.01.895.701.321.345.05.00.17.08.01.664.831.181.165.05.00.110.08.01.373.720.990.985.05.00.15.03.01.185.700.610.605.05.00.15.05.01.575.701.000.965.05.00.15.08.01.895.701.321.345.05.00.15.010.02.945.702.372.295.05.00.15.0154.015.703.343.38



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*Source: 216058-2009-01-11.xml*