Kinetics and Mechanism of Oxidation of L-Leucine by Alkaline Diperiodatonickelate(IV) - A Free Radical Intervention, Deamination, and Decarboxylation

Article abstract of DOI:10.1007/s00706-003-0608-x

The kinetics of oxidation of L-leucine by alkaline diperiodatonickelate(IV) was studied spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [DPN] and apparently less than unit order, each in [L-leucine] and [alkali] under the experimental conditions. However, the order in [L-leucine] and [alkali] changes from first order to zero order as the concentrations change from lower to higher values. Addition of periodate has no effect on the reaction rate. A mechanism involving the deprotonated diperiodatonickelate(IV) (DPN) as the reactive oxidant is proposed. The reaction constants involved in the mechanism were evaluated. There is a good agreement between the observed and calculated rate constants under varying experimental conditions. The isokinetic temperature was determined and the activation parameters with respect to the slow step of the reaction scheme were evaluated and discussed.

Full text of DOI:10.1007/s00706-003-0608-x

Monatshefte f €u r Chemie 134, 1341–1352 (2003)
DOI 10.1007/s00706-003-0608-x
Kinetics and Mechanism of Oxidation
of L-Leucine by Alkaline
Diperiodatonickelate(IV) – A Free
Radical Intervention, Deamination,
and Decarboxylation
Mahesh R. T., Pandurang D. Pol, and Sharanappa T. Nandibewoorꢀ
Post-Graduate Department of Studies in Chemistry, Karnatak University,
Dharwad-580 003, India
Received September 19, 2002; accepted (revised) January 1, 2003
Published online September 11, 2003 # Springer-Verlag 2003
Summary. The kinetics of oxidation of L-leucine by alkaline diperiodatonickelate(IV) was studied
spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [DPN]
and apparently less than unit order, each in [L-leucine] and [alkali] under the experimental conditions.
However, the order in [L-leucine] and [alkali] changes from first order to zero order as the concentra-
tions change from lower to higher values. Addition of periodate has no effect on the reaction rate. A
mechanism involving the deprotonated diperiodatonickelate(IV) (DPN) as the reactive oxidant is
proposed. The reaction constants involved in the mechanism were evaluated. There is a good agree-
ment between the observed and calculated rate constants under varying experimental conditions. The
isokinetic temperature was determined and the activation parameters with respect to the slow step of
the reaction scheme were evaluated and discussed.
Keywords. L-Leucine; Alkaline diperiodatonickelate(IV); Oxidation; Kinetics.
Introduction
The use of diperiodatonickelate(IV) (DPN) as an oxidant in alkaline medium is
new and restricted to a few cases [1–6] due to the fact of its limited solubility and
stability in aqueous medium. The reduction of nickel(IV) complexes has received
considerable attention in order to understand the nature of intermediate oxidation
states such as nickel(III). Indeed, stable nickel(III) complexes are known [7–9].
Moreover, when nickel(IV) periodate is the oxidant, it needs to be known which of
the species is the active form, since multiple equilibria between different nickel(IV)
ꢀ Corresponding author. E-mail: karuni@bgl.vsnl.net.in

1
342
R. T. Mahesh et al.
species are involved. Amino Acids act not only as the building blocks in protein
synthesis but they also play a significant role in metabolism. Amino Acids can
undergo many types of reaction depending on whether a particular amino acid
contains non-polar groups or polar substituents. L-Leucine is an essential amino
acid. It forms active sites of enzymes and helps in maintaining their proper con-
formation by keeping them in proper ionic states. So, oxidation of L-leucine may
help in understanding some aspects of enzyme kinetics. The oxidation of amino
acids is of interest as the oxidation products differ for different oxidants [10–12].
Thus, the study of amino acids becomes important because of their biological
significance and selectivity towards the oxidant.
In earlier reports [4–7] on DPN oxidation, periodate had a retarding effect and
ꢁ
the order in [OH ] was found to be less than unity in almost all the reactions and
monoperiodatonickelate(IV) (MPN) is considered to be the active species. How-
ever, in the present study entirely different kinetic observations have been made
and deprotonated diperiodatonickelate(IV) (DPN) is found to be the active form of
the oxidant. The literature survey reveals that there are no reports on mechanistic
studies of L-leucine oxidation by DPN. Thus, L-leucine has been selected as a
substrate in order to explore the mechanism of oxidation by DPN in alkaline
medium and to check the reactivity of amino acids towards DPN. The title reaction
is studied to investigate the redox chemistry of the nickel(IV) in such media and to
arrive at a plausible mechanism.
Results and Discussion
Reaction Order
The orders with respect to [L-leucine], [periodate] and [alkali] were found by
log kobs versus log [concentration] plots; these orders were obtained by varying
the concentration of reductant, periodate, and alkali in turn while keeping the
respective others constant.
The concentration of diperiodatonickelate(IV) was varied in the range,
ꢁ
5
ꢁ4
ꢁ3
ꢁ
1
.0 ꢂ 10 to 1.0 ꢂ 10 mol dm at fixed [L-leucine], [OH ], and ionic strength.
The non-variation in the pseudo-first order rate constants at various concentrations
of DPN indicates the order in [DPN] to be unity (Table 1). This was also confirmed
by the linearity of plots of log [DPN] versus time up to 85% completion of
ꢁ
4
the reaction. The substrate L-leucine was varied in the range of 4.0 ꢂ 10 to
ꢁ
3
ꢁ3
ꢃ
4
.0 ꢂ 10 mol dm at 25 C keeping all other reactants concentrations and con-
ditions constant (Table 1). The k values increased with the concentration of L-
obs
leucine and indicated an apparent order in [L-leucine] less than unity (Table 1).
However, at lower concentration of L-leucine the reaction was first order in
[
L-leucine] and at higher concentration of L-leucine the reaction was independent
of [L-leucine]. The order in [L-leucine] changes from first to zero order as the
L-leucine] decreases. The effect of [alkali] on the rate of reaction was studied
at constant concentrations of L-leucine and DPN at an ionic strength of
[
ꢁ
3
0.6 mol dm . The rate constants increased with [alkali] indicating a less than unit
order of this reactant (Table 1). Similar as in the L-leucine case the order changes
from first to zero order as [alkali] decreases. The effect of [IO ] was observed by
4
ꢁ

Kinetics and Mechanism of Oxidation of L-Leucine
1343
ꢁ
ꢁ
Table 1. Effect of variation of [DPN], [L-leucine], [IO4 ], and [OH ] on oxidation of L-leucine by
ꢃ
ꢁ3
DPN at 25 C, I ¼ 1.0 mol dm
4
3
ꢁ
4
ꢁ
2
ꢁ1
[
DPN] ꢂ 10 =
[L-leucine] ꢂ 10 =
[IO4 ] ꢂ 10 =
[OH ]=
kobs ꢂ 10 =s
ꢁ
3
ꢁ3
ꢁ3
ꢁ3
mol dm
mol dm
mol dm
mol dm
exptl.
calcd.
1
2
4
6
1
.0
.0
.0
.0
0
0.8
0.8
0.8
0.8
0.8
2.0
2.0
2.0
2.0
2.0
0.2
0.2
0.2
0.2
0.2
2.4
2.4
2.5
2.5
2.4
2.5
2.5
2.5
2.5
2.5
6
6
6
6
6
.0
.0
.0
.0
.0
0.4
0.6
0.8
2.0
4.0
2.0
2.0
2.0
2.0
2.0
0.2
0.2
0.2
0.2
0.2
1.6
2.0
2.5
3.4
4.0
1.7
2.1
2.5
3.5
4.0
6
6
6
6
6
.0
.0
.0
.0
.0
0.8
0.8
0.8
0.8
0.8
2.0
4.0
0.2
0.2
0.2
0.2
0.2
2.30
2.35
2.40
2.45
2.50
2.5
2.5
2.5
2.5
2.5
8.0
15.0
20.0
6
6
6
6
6
.0
.0
.0
.0
.0
0.8
0.8
0.8
0.8
0.8
2.0
2.0
2.0
2.0
2.0
0.1
0.2
0.5
0.8
1.0
1.4
2.0
2.5
2.7
3.0
1.6
2.1
2.5
2.7
2.9
error ꢆ4%
varying the concentration from 0.2 ꢂ 10ꢁ to 2.0 ꢂ 10 mol dm keeping all
other concentrations of reactants constant. It was found that the added periodate
has no effect on the rate of reaction.
4
ꢁ4
ꢁ3
Effect of Ionic Strength and Solvent Polarity
The effect of ionic strength was studied by varying the potassium nitrate concen-
tration in the reaction medium. The ionic strength of the reaction medium was
ꢁ
3
varied from 0.2 to 1.0 mol dm at constant concentrations of DPN, L-leucine, and
alkali. It was found that the rate of reaction increases with increasing ionic strength
1
=2
of the medium. The plot of log kobs versus I
Fig. 1) (r ꢄ 0.969, s ꢅ 0.0548).
The relative permittivity (D) effect was studied by varying the t-butyl alcohol:
was linear with positive slope
(
water ratio in the reaction mixture with all other conditions being constant.
Attempts to measure the relative permittivities were not successful. However, they
were computed from the values of the pure liquids [13]. There was no reaction of
the solvent with the oxidant under the experimental conditions used. The rate
constants, k , decreased with the decrease in the dielectric constant of the me-
obs
dium. The plot of log kobs versus 1=D was linear (r ꢄ 0.976, s ꢅ 0.055) (Fig. 1)
with a negative slope.

1
344
R. T. Mahesh et al.
1=2
Fig. 1. Plot of log kobs versus I and log kobs versus 1=D
Effect of Initially Added Products
The initially added products, such as Ni(II) in the form of NiSO did not have any
4
significant effect on the rate of the reaction.
Test for Free Radicals
To test the intervention of free radicals, the reaction mixture was mixed with
acrylonitrile and kept for 24 hours under nitrogen atmosphere. On dilution with
methanol, a precipitate of polymer was formed, indicating the intervention of free
radicals in the reaction.
The water soluble Ni(IV) periodate complex is reported [5, 6, 14–16] to be
6
ꢁ
Ni(HIO ) (OH) ] , although periodate is involved in multiple equilibria depend-
6 2 2
[
ing on the pH employed. Under the conditions of high pH as maintained in this
2
ꢁ
study, periodate is likely to exist [4, 5] as H IO6 . Hence, the species of Ni(IV) in
3
3ꢁ
alkali can be expected to be [Ni(H IO )(H IO )(OH) ] (DPN). This conclusion
3
6
2
6
2
is also supported by earlier work [2, 3].
It is known that in aqueous solution, amino acid exists as Zwitterionic [17]
whereas in aqueous alkaline medium they exists as the anionic form according to
the following equilibria (Scheme 1).
The reaction between DPN and L-leucine in alkaline medium has 2:1 stoichi-
ometry of oxidant to reductant with first order dependence in [DPN] and the appar-
Scheme 1

Kinetics and Mechanism of Oxidation of L-Leucine
1345
ꢁ
ent order of less than unity in [L-leucine] and [OH ]. In most of the reports [4–7]
ꢁ
on DPN oxidation, periodate had a retarding effect and the order in [OH ] was
found to be less than unity and monoperiodatonickelate(IV) (MPN), is considered
to be the active species. However, in the present kinetic study, different kinetic
observations have been obtained, i.e., periodate has no effect whatsoever on the
rate of reaction. Accordingly, the deprotonated DPN is considered to be the active
species of the oxidant. The fractional order in [L-leucine] presumably results from
a complex formation between the oxidant and substrate prior to the formation of
the products. Spectroscopic evidence for the complex formation between oxidant
and substrate was obtained from Uv-Vis spectra of the L-leucine, DPN, and mix-
ture of both. A bathochromic shift of about 7 nm from 459 to 463 nm in the spectra
of L-leucine was observed and hyperchromicity was observed at 463 nm. The Uv-
Vis spectral changes occurring in the oxidation of L-leucine by DPN show the
absence of isosbestic points, which supports the complex formation [18]. Indeed,
it is to be noted that a plot of 1=kobs versus 1=[L-leucine] (r ꢄ 0.986, s ꢅ 0.0058)
shows a straight line with non-zero intercept (Fig. 2). Such complex formation
between substrate and oxidant has been observed earlier [19]. Based on the
observed kinetic results a mechanism involving complex formation between oxi-
dant and substrate has been proposed which is shown in Scheme 2.
The thermodynamic quantities for the first equilibrium step in Scheme 2 can
be evaluated as follows: The hydroxyl ion and the L-leucine concentration as in
Table 1 were varied at four different temperatures and the K value was determined
1
3
ꢁ1
at each temperature. The values of K (dm mol ) were obtained as 7.5, 8.1, 8.9,
1
ꢃ
and 9.3 at 25, 30, 35, and 40 C, respectively. A Van’t Hoff’s plot was made for
the variation of K with temperature (r ꢄ 0.9992, s ꢅ 0.0122) and the values of
1
ꢃ
ꢃ
the enthalpy of the reaction, ꢀH , the entropy of the reaction, ꢀS , and the
ꢃ
ꢁ1
free energy of the reaction, ꢀG , were calculated as 11.4 ꢆ 0.6 kJ mol ,
ꢁ
1
ꢁ1
ꢁ1
1
7.7 ꢆ 1.0 J K mol , and ꢁ54.5 ꢆ 2.5 kJ mol , respectively. A comparison of
the latter values with those obtained for the slow step of the reaction shows that
ꢁ
Fig. 2. Verification of rate law described by Eq. (2); plot of 1=kobs versus 1=[OH ] and 1=k versus
obs
1=[L-leucine] (conditions as given in Table 1)

1
346
R. T. Mahesh et al.
Scheme 2
Fig. 3. Probable structure of complex C
these values mainly refer to the rate limiting step, supporting the fact that the
reactions before the rate determining step are fairly rapid and involve only little
activation energy [20].
Scheme 2 leads to the rate law given by Eq. (1).
ꢁ
d½NiðIVÞꢇ
dt
kK K ½NiðIVÞꢇ½OH ꢇ½L-leucineꢇ
1
2
Rate ¼ ꢁ
¼
ð1Þ
ꢁ
ꢁ
1 þ K ½OH ꢇ þ K K ½L-leucineꢇ½OH ꢇ
1
1
2

Kinetics and Mechanism of Oxidation of L-Leucine
1347
2
ꢁ
ꢁ
The terms ½1 þ K K ½½NiðOHÞ ðH IO Þ ꢇ ꢇ½OH ꢇꢇ and ½1 þ K ½½NiðOHÞ
1
2
3
6
2ꢁ
1
2
2
2
2ꢁ
ðH IO Þ ꢇ ꢇ þ K K ½½NiðOHÞ ðH IO Þ ꢇ ꢇ [L-leucine]] should also be in the
3
6
1
2
3
6
2
2
2
denominator of Eq. (1), but in view of the low concentration of [Ni(IV)] used they
approximate to unity.
Therefore, Eq. (2) is obtained.
ꢁ
Rate
kK K ½OH ꢇ½L-leucineꢇ
1
ꢁ
2
kobs
¼
¼
ð2Þ
ꢁ
½
Equation (2) can be rearranged to Eq. (3), which is suitable for verification
NiðIVÞꢇ 1 þ K ½OH ꢇ þ K K ½L-leucineꢇ½OH ꢇ
1
1
2
1
1
1
1
¼
þ
þ k
ð3Þ
ꢁ
kobs kK K ½L-leucineꢇ½OH ꢇ kK ½L-leucineꢇ
1
2
2
According to Eq. (3), other conditions being constant, the plots of 1=k
ꢁ
obs
versus 1=[OH ] (r ꢄ 0.986, s ꢅ 0.0058) and 1=[L-leucine] (r ꢄ 0.972,
s ꢅ 0.0081) should be linear as shown in Fig. 2. From the slopes and intercepts,
3
ꢁ1
the values of K ; K ; and k could be derived as 7.5 ꢆ 0.1 dm mol ,
1
3
2
2
ꢁ1
ꢁ2 ꢁ1
2
3.8 ꢆ 0.4 ꢂ 10 dm mol , and 4.7 ꢆ 0.2 ꢂ 10 s , respectively. The value of
K is in agreement with earlier values [21]. Using these constants, the rate con-
1
stants were calculated over different experimental conditions and there is a rea-
sonable agreement between the calculated and experimental values (Table 1)
which fortifies the proposed mechanism. The increase in the rate with increasing
ionic strength qualitatively explains the reaction between two negatively charged
species of reactants (Scheme 2). The effect of solvent on the reaction rate was
already described previously [22]. The plot of log kobs versus 1=D gives a straight
line with a negative slope for a reaction between negative ions and a neutral
molecule, whereas a positive slope results for a positive ion and a neutral mole-
cule. In our present study, a plot of log kobs versus 1=D (r ꢄ 0.984, s ꢅ 0.0053)
is linear with a negative slope (Fig. 1) which is in accordance with the involve-
ment of negative ions as given in Scheme 2.
The activation parameters for the oxidation of some amino acids by DPN are
summarized in Table 2. 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 and=or entropy of activation. Changes in the rate
Table 2. Activation parameters for some amino acids (for isokinetic temperature)
ꢁ
2
3
ꢁ2
3
6
ꢁ1
6
6
Amino acid k ꢂ 10 dm
k ꢂ 10 dm
DS =JK
mol
DH =kJ
mol
DG =kJ
mol
Refs.
ꢁ
1
ꢁ1
ꢁ1 ꢁ1
mol
s
mol
s
at
at 298 K
303 K
L-arginine
L-aspartic
Acid
1.0
2.0
1.4
2.8
ꢁ36 ꢆ 4
45.0 ꢆ 2
51.0 ꢆ 3
56 ꢆ 4
55 ꢆ 3
[30]
[31]
ꢁ11.5 ꢆ 1
L-valine
L-leucine
1.8
4.7
2.2
5.6
ꢁ45.3 ꢆ 2.4 26.2 ꢆ 1.3 40.0 ꢆ 2
[32]
ꢁ49.9 ꢆ 2.5 18.0 ꢆ 1
33.3 ꢆ 1.8 present
work

1
348
R. T. Mahesh et al.
Fig. 4. Plot of log k at 298 K versus log k at 303 K (conditions as given in Table 2); 1. L-leucine;
1
2
2
. L-aspartic acid; 3. L-valine; 4. L-arginine
6
6¼
are caused by changes in both DH and DS , but the latter quantities are highly
6
6¼
correlated. A plot of DH versus DS is linear according to Eq. (4).
6
6¼
DH ¼ ꢀDS þ constant
is called the isokinetic temperature. It has been asserted that apparently linear
ð4Þ
ꢀ
6
6¼
correlations of DH with DS are sometimes misleading and the evaluation of ꢀ by
means of the above equation lacks statistical validity [23]. Exner [24] 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 log k (at T ) is
2
2
linearly related to log k (at T ), i.e. log k ¼ a þ b log k , he proposes that ꢀ can be
1
1
2
1
evaluated from Eq. (5).
ꢀ
¼ T T ðb ꢁ 1Þ=T b ꢁ T
ð5Þ
1
2
2
1
We have calculated the isokinetic temperature as 342.2 K by plotting log k at
1
2
98 K versus log k at 303 K (Fig. 4) (r ꢄ 0.979, s ꢅ 0.0051). The value of ꢀ
2
(
342.2 K) is higher than the experimental temperature (303 K). This indicates that
the rate is governed by the enthalpy of activation [25]. The linearity and the slope
of the plot obtained may confirm that the kinetics of these reactions follow a
similar mechanism, as previously suggested.
Effect of Temperature
The kinetics were studied at four different temperatures under varying L-leucine
concentrations. The rate constants k of the slow step were obtained from the
intercept of 1=kobs versus 1=[L-leucine] (r ꢄ 0.998, s ꢅ 0.0054) plot at four dif-
ferent temperatures and were used to calculate the activation parameters. The
2
ꢁ1
values of k ꢂ 10 were 4.76 ꢆ 0.10, 5.60 ꢆ 0.10, 6.45 ꢆ 0.10, and 7.40 ꢆ 0.15 s
ꢃ
at 25, 30, 35, and 40 C, respectively. The energy of activation corresponding to
these constants was evaluated from the Arrhenius plot of log k versus 1=T, and
other activation parameters were obtained and are listed in Table 3.
The mechanism is also supported by moderate values of activation parameters
6¼
(
Table 2). A high negative value of DS suggests that the intermediate complex is

Kinetics and Mechanism of Oxidation of L-Leucine
1349
Table 3. Thermodynamic activation parameters for the L-leucine oxidation by alkaline di-
periodatonickelate(IV) complex with respect to slow step of Scheme 2
Activation parameters
Values
Ea (kJ mol 1
ꢁ
)
ꢁ1
20.6 ꢆ 0.3
18.1 ꢆ 0.2
33.3 ꢆ 1.1
ꢁ50 ꢆ 2
6¼
DH (kJ mol
)
)
ꢁ1
6¼
ꢁ1
DG (kJ mol
6¼
ꢁ1
DS (J K mol
)
more ordered than the reactants. The modest value of enthalpy of activation and the
higher rate constant of the slow step of the mechanism indicated that the oxidation
presumably occurs by an inner-sphere mechanism. This conclusion was supported
by earlier studies [26].
Conclusions
Among various species of Ni(IV) in alkaline medium, in earlier reports the mono-
periodatonickelate(IV) was the active species, whereas deprotonated diperiodateo-
nickelate(IV) is considered to be the active species for the title reaction. Rate
constants of the slow step and other equilibrium constants involved in the mecha-
nism are evaluated and activation parameters with respect to the slow step of the
reaction were computed.
Experimental
Materials
All chemicals used were of reagent grade. Doubly distilled water was used throughout the work. The
solution of L-leucine was prepared by dissolving an appropriate amount of a recrystallised sample in
distilled water. The solid complex nickel(IV) periodate was prepared by a known method [27]. The
purity of the complex was checked by its UV=Vis spectrum which shows a broad absorption band
at 410 nm [14]. Aqueous solutions of DPN were obtained by dissolving the solid complex in
1
.0 mol dm 3 KOH solution. The Ni(IV) in alkaline solution was estimated gravimetrically [28] after
ꢁ
reducing it to Ni(II) and precipitating Ni(II) as its dimethyl glyoxime complex. Potassium hydrox-
ide and potassium nitrate were employed to maintain the required alkalinity and ionic strength
respectively.
Since periodate is present in excess in the DPN complex, the possibility of L-leucine oxidation by
periodate in aqueous alkaline medium has been tested. The results indicated that the reaction between
ꢁ
IO4 and L-leucine is negligibly slow compared to the rate of reaction between DPN and L-leucine
under the experimental conditions.
Kinetic Measurements
Since the reaction was too fast to be monitored by the usual methods, kinetic measurements were
performed on a Hitachi 150-20 spectrophotometer connected to a rapid kinetic accessory (HI-TECH
SFA-12). Kinetic runs were followed under pseudo-first order conditions with the L-leucine concen-
ꢃ
tration in excess over that of the oxidant at 25 ꢆ 0.1 C, unless otherwise stated. The reaction was

1
350
R. T. Mahesh et al.
initiated by mixing thermally equilibrated solutions of DPN and L-leucine, which also contained a
ꢁ
definite quantity of KOH, KNO , and IO4 . Here the total concentration of hydroxide ion was
3
calculated considering the KOH in DPN as well as the KOH additionally added. Similarly, the total
metaperiodate concentration was calculated by considering the amount present in the DPN solution
and additionally added one. The course of reaction was followed by measuring the absorbance of
unreacted DPN in the reaction mixture in a 1 cm quartz cell located in the thermostated compartment
of a Hitachi 150-20 spectrophotometer, at its maximum absorption wavelength of 410nm as a function
of time. Earlier it was verified that there is negligible interference from other species present in the
reaction mixture at this wavelength. The obedience of Beer’s law by DPN at 410 nm was verified
3
ꢁ1
ꢁ1
earlier and the molar absorbance coefficient, ", was found to be 7500 ꢆ 375 dm mol cm at this
wavelength. The reaction was followed up to more than 80% completion. The first-order rate constants,
kobs, were calculated from the slopes of log [DPN] versus time plots. The rate constants were repro-
ducible to within ꢆ5%.
In view of the modest concentration of alkali used in the reaction medium, attention was also
directed to the effect of the reaction vessel surface on the kinetics. Use of polythene=acrylic wares and
quartz or polyacrylate cells gave the same results, indicating that the surface does not have any
significant effect on reaction rates.
Kinetic runs were also carried out in an N atmosphere in order to understand the effect of
2
dissolved oxygen on the rate of reaction. No significant difference in the results was obtained
under a N atmosphere and in the presence of air. In view of the ubiquitous contamination of
2
carbonate in the basic medium, the effect of carbonate was also studied. Added carbonate had no
effect on the reaction rates, however, fresh solutions were nevertheless used for carrying out each
kinetic study.
Regression analysis of experimental data to obtain regression coefficients r and s, the stan-
dard deviations of points from the regression line, was performed with a Pentium-III personnel
computer.
Stoichiometry and Product Analysis
Different reaction mixtures with different sets of concentrations of reactants where [Ni(IV)] was in
excess over [L-leucine] at constant ionic strength and alkali concentration were kept for about 6 h at
ꢃ
2
5 ꢆ 0.1 C in a nitrogen atmosphere in a closed vessel. The remaining [DPN] was assayed spectro-
photometrically by measuring the absorbance at 410 nm. The results indicated that two moles of DPN
consumed one mole of L-leucine as shown in Scheme 3.
The main oxidation products were identified as pentanoic acid by spot test [29], Ni(II) was
confirmed by its dimethyl glyoxime complex, and ammonia by Nessler’s reagent. CO was qualita-
2
tively detected by bubbling N gas through the acidified reaction mixture and passing the liberated gas
2
through a tube containing lime water. The nature of the carboxylic acid was confirmed by the IR
spectrum which showed a carbonyl (C¼O) stretch at 1708 cmꢁ and a signal of 3042 cm due to O–H
1
ꢁ1
stretching of the acid.
Scheme 3

Kinetics and Mechanism of Oxidation of L-Leucine
1351
Appendix
According to Scheme 2 the reaction rate is given by Eq. (I).
2
ꢁ
f
Rate ¼ k½Cꢇ ¼ kK1K2½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ ½OH ꢇ ½L-leucineꢇ
ðIÞ
2
2
f
f
The total [Ni(IV)], [Ni(IV)] , is given as in Eq. (II) where the subscripts t and f stand for total and free,
t
respectively.
2
ꢁ
3ꢁ
½
NiðIVÞꢇ ¼ ½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ þ ½½NiðOHÞ ðH2IO6ÞðH3IO6Þꢇ ꢇ þ ½Cꢇ
t
2
2
f
2
2
ꢁ
2ꢁ
f
ꢁ
¼
¼
½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ þ K1½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ ½OH ꢇ
2
2
f
2
2
ꢁ
2ꢁ
2
þ K1K2½OH ꢇ½L-leucineꢇ½½NiðOHÞ ðH3IO6Þ ꢇ ꢇf
2
2
ꢁ
f
ꢁ
ꢁ
½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ ½1 þ K1½OH ꢇ þ K1K2½OH ꢇ½L-leucineꢇꢇ
2
2
½
NiðIVÞꢇ
½NiðOHÞ ðH3IO6Þ ꢇ ꢁꢇf ¼
2
t
ðIIÞ
½
2
2
ꢁ
ꢁ
1
þ K1½OH ꢇ þ K1K2½OH ꢇ½L-leucineꢇ
ꢁ
Similarly, [OH ] and [L-leucine] are obtained as described by Eqs. (III, IV).
f
f
ꢁ
½
OH ꢇ
ꢁ
t
½
OH ꢇ ¼
ðIIIÞ
ðIVÞ
f
þ K1½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ þ K1½½NiðOHÞ ðH3IO6Þ ꢇ2ꢁꢇ½L-leucineꢇ
2ꢁ
1
2
2
2
2
½
L-leucineꢇ
t
½
L-leucineꢇ ¼
f
þ K1K2½OH ꢇ½½NiðOHÞ ðH3IO6Þ ꢇ2ꢁꢇ
ꢁ
1
2
2
After substituting the values of (II), (III), and (IV) in Eq. (I), we get Eq. (V).
ꢁ
kK1K2½NiðIVÞꢇ ½L-leucineꢇ ½OH ꢇ
t
t
t
Rate ¼
ðVÞ
ꢁ
ꢁ
2ꢁ
ꢁ
t
½
½
1 þ K1½OH ꢇ þ K1K2½OH ꢇ ½L-leucineꢇ ꢇ½1 þ K1K2½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ½OH ꢇ ꢇ
1 þ K1½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ þ K1K2½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ ½L-leucineꢇ ꢇ
t
t
t
2
2ꢁ
2
2ꢁ
2
2
t
2
2
t
t
2
ꢁ
ꢁ
2ꢁ
2
The
terms
½1 þ K1K2½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ½OH ꢇꢇ,
½1 þ K1½½NiðOHÞ ðH3IO6Þ ꢇ ꢇ þ
2
2
2
2ꢁ
K1K2½½NiðOHÞ ðH3IO Þ ꢇ ꢇ½L-leucineꢇꢇ in the denominator of Eq. (V) approximate to unity in view
2
6
2
of they low concentration of [Ni(IV)] used. Therefore, Eq. (V) becomes Eq. (VI) (omitting the sub-
script t and f).
ꢁ
Rate
kK1K2½L-leucineꢇ½OH ꢇ
kobs ¼
¼
ðVIÞ
ꢁ
ꢁ
½
NiðIVÞꢇ 1 þ K1½OH ꢇ þ K1K2½OH ꢇ½L-leucineꢇ
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