# Kinetics and Mechanism of Oxidation of Leucine and Alanine by Ag(III) Complex in  Alkaline Medium

Authors: Changying Song; Lei Chen; Jinhuan Shan
Journal: Research Letters in Inorganic Chemistry
 (2008)

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## Abstract

Kinetics and mechanism of oxidation of leucine and alanine by Ag(III) complex were studied spectrophotometrically in alkaline medium at constant ion strength. The reaction was in first order with respect to Ag(III) complex and amino acids (leucine, alanine). The second-order rate constant,k−, decreased with the increasing in [OH−] and [IO4−]. A plausible mechanism was proposed from the kinetics study, and the rate equations derived from mechanism can explain all experimental phenomena. The activation parameters were calculated at 298.2 K.


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## Body

## 1. Introduction

Transition metals in a higher oxidation state can be
stabilized by chelation with suitable polydentate ligands, such as diperiodatargentate(III) [1–4],
[ethylenebis(biguanide)silver(III)] [5, 6]. These complexes were stable in a
suitable aqueous solution. Ag(III) complexes were stabilized in alkaline medium
by periodate or tellurate ions [7, 8]. These complexes such as Ag(III), Cu(III),
and Ni(IV) complex can be used as an oxidation reagents in organic chemistry
and analytical chemistry [9–11]. As a kind of oxidation reagents, those
complexes have been used widely in kinetic study [12].The kinetics and mechanism of oxidation of some amino acids
have been studied by some researchers. However, the mechanism is different in
the different reaction systems [13–23]. In order to
perfect the mechanism of oxidation of amino acids, the kinetics and mechanism
of oxidation of alanine and leucine by [Ag(HIO6)2]5− have been studied in
this paper.


## 2. Experimental

All the
reagents used were of A.R. grade. All solutions were prepared with doubly
distilled water. Solution of[Ag(HIO6)2]5− (DPA) was prepared and
standardized by the method reported earlier [7, 8]. Solution of DPA was always
freshly prepared before used.


### 2.1. Apparatus and Kinetics Measurements and Product Analysis

All kinetics measurements were carried out as previously [12].
The way for product analysis was similar to previous study [24]. After
completion of the reaction, the products of oxidation were identified as
ammonia and the corresponding ketone by their characteristic spot test [25].


## 2.1. Apparatus and Kinetics Measurements and Product Analysis

All kinetics measurements were carried out as previously [12].
The way for product analysis was similar to previous study [24]. After
completion of the reaction, the products of oxidation were identified as
ammonia and the corresponding ketone by their characteristic spot test [25].


## 3. Result and Discussion

### 3.1. Kinetics

The pseudo-first-order rate constantskobs were calculated by the method as previously [12].kobs values increased with the increase in [AA],  the plots of kobs versus [AA] are straight
line which pass the grid origin at different temperatures (Table 1), the order
with respect to [AA] was unity. Thus, an overall second-order rate law described
by (1) is established here, where k′ represents observed second-order rate constant: (1)r=kobs[Ag(III)]=k′[Ag(III)][AA].k′ values decreased with the increase in [IO4−]. The
plot of 1/k′ versus [IO4−] was a straight line (Figures 1 and 2). The values of k′ decreased with the increase in [OH−] and the plots of 1/k′ versus f(OH)[OH−] were
line (Figures 3 and 4).Table 1
kobs varied with different
[AA] at different temperatures.
[AA]/(mol/L)0.010.020.030.040.05T/(K)103kobs/(s−1) for oxidation of leucine298.22.7405.3607.78910.1713.69303.24.0667.97311.2915.9219.64308.26.77512.0418.1824.6231.11313.28.25815.2522.5032.4038.30103kobs/(s−1) for oxidation of alanine298.25.87210.3715.8722.1429.05303.28.67114.3823.2431.9740.89308.212.4722.5233.4644.4759.78313.217.7132.7647.1865.9893.53[Ag(III)]=2.965×10−5mol/L, [OH−]=0.06mol/L, [IO4−]=5.00×10−4mol/L, μ=0.15mol/L.Figure 1
The second-order rate constant,k′, as a function of the concentration of IO4− at different temperatures for oxidation of leucine.Figure 2
The second-order rate constant,k′, as a function of the concentration of IO4− at different temperatures for oxidation of alanine.Figure 3
The second-order rate constant,k′, as a function of the concentration of OH− at different temperatures for oxidation of leucine.Figure 4
The second-order rate constant,k′, as a function of the concentration of OH− at different temperatures for oxidation of alanine.


### 3.2. Reaction Mechanism

In aqueous periodate solution, (2)–(4) were detected and the corresponding equilibrium constants at 273.2 K were determined by Aveston [26] as follows: (2)2IO4−+2OH−⇌H2I2O104−,logβ1=15.05,(3)IO4−+OH−+H2O⇌H3IO62−,logβ2=6.21,(4)IO4−+2OH−⇌H2IO63−,logβ3=8.67.The distribution
of all species of periodate in alkaline solution can be calculated from (2)–(4). In the [OH−] range
used in this work, the dimer and IO4− species of periodate can be neglected. The main species of periodate are H2IO63− and H3IO62−.
Equation (5) can be obtained from (2), (3), (4) as follows: (5)[H2IO63−]=β3[OH−]21+β2[OH−]+β3[OH−]2[IO4−]ex=f([OH−])[IO4−]ex, where [IO4−]ex represents
the concentration of original overall periodate ion.From the discussion above, the
reaction mechanism was proposed as below:(6)[Ag(HIO6)2]5−+3H2O⇌K1[Ag(H2O)2(HIO6)]−+H2IO63−+OH−,(7)[Ag(H2O)2(HIO6)]−+RCH(NH2)COO−→K[Ag(H2O)2(HIO6)(RCH(NH2)COO−)]2−, R means (CH3)2CHCH2—for
leucine and H—for alanine (8)[Ag(H2O)2(HIO6)(RCH(NH2)COO−)]2−→fastAg(I)+RCOCOO−+H2IO63−+H2O+NH3,[Ag(III)]tot=[Ag(OH−)2(H2IO6)2]e5−+[Ag(OH−)2(HIO6)]e3−.
Here “e” and “tot” mean
the equilibrium concentration of the all kinds of Ag(III) complexes and the
total concentration of all kinds of Ag(III) complexes, respectively. The
reaction (7) is the rate-determining step, k means the rate-determining step constant, the rate equation can be expressed as
below: (9)−d[Ag(III)]Tdt=kobs⋅[Ag(III)]tot=k′[Ag(H2O)2(HIO6)−]⋅[RCH(NH2)COO−]=kK1[R][Ag(III)]totK1+[H2IO63−][OH−],(10)kobs=kK1[R]K1+[H2IO63−][OH−]. Then substitute (5) for [H2IO63−] in (10) is as follows: (11)kobs=kK1[R]K1+f([OH−])[IO4−]ex[OH−],(12)k′=kK1K1+f([OH−])[IO4−]ex[OH−]. From (12), the
plot of 1/k′ versus f(OH)⋅[OH−] is a straight line, and 1/k′ versus [IO4−]ex is straight line too, which can explain all the experimental phenomena, and the
activation parameters according to k are
calculated [27] (see Table 2).Table 2
Thermodynamic
activation parameters for the oxidation of leucine and alanine by Ag(III)
complex in alkaline medium.
Thermodynamic activation parametersReaction for oxidation of leucineTemperature (K)10k/(M−1⋅s−1)298.22.75Ea/(KJ⋅mol−1)59.34±4.37303.24.10△H#/(KJ⋅mol−1)56.86±4.37308.26.54△S#/(J⋅K−1⋅mol−1)−64.83±14.3313.28.40103K1/(mol⋅L−1)27.09±0.42Reaction for oxidation of alanineTemperature (K)10k/(M−1⋅s−1)298.25.93Ea/(KJ⋅mol−1)60.95±3.15303.28.47△H#/(KJ⋅mol−1)58.47±3.15308.212.3△S#/(J⋅K−1⋅mol−1)53.40±10.31313.219.4103K1/(mol⋅L−1)26.44±0.49


## 3.1. Kinetics

The pseudo-first-order rate constantskobs were calculated by the method as previously [12].kobs values increased with the increase in [AA],  the plots of kobs versus [AA] are straight
line which pass the grid origin at different temperatures (Table 1), the order
with respect to [AA] was unity. Thus, an overall second-order rate law described
by (1) is established here, where k′ represents observed second-order rate constant: (1)r=kobs[Ag(III)]=k′[Ag(III)][AA].k′ values decreased with the increase in [IO4−]. The
plot of 1/k′ versus [IO4−] was a straight line (Figures 1 and 2). The values of k′ decreased with the increase in [OH−] and the plots of 1/k′ versus f(OH)[OH−] were
line (Figures 3 and 4).Table 1
kobs varied with different
[AA] at different temperatures.
[AA]/(mol/L)0.010.020.030.040.05T/(K)103kobs/(s−1) for oxidation of leucine298.22.7405.3607.78910.1713.69303.24.0667.97311.2915.9219.64308.26.77512.0418.1824.6231.11313.28.25815.2522.5032.4038.30103kobs/(s−1) for oxidation of alanine298.25.87210.3715.8722.1429.05303.28.67114.3823.2431.9740.89308.212.4722.5233.4644.4759.78313.217.7132.7647.1865.9893.53[Ag(III)]=2.965×10−5mol/L, [OH−]=0.06mol/L, [IO4−]=5.00×10−4mol/L, μ=0.15mol/L.Figure 1
The second-order rate constant,k′, as a function of the concentration of IO4− at different temperatures for oxidation of leucine.Figure 2
The second-order rate constant,k′, as a function of the concentration of IO4− at different temperatures for oxidation of alanine.Figure 3
The second-order rate constant,k′, as a function of the concentration of OH− at different temperatures for oxidation of leucine.Figure 4
The second-order rate constant,k′, as a function of the concentration of OH− at different temperatures for oxidation of alanine.


## 3.2. Reaction Mechanism

In aqueous periodate solution, (2)–(4) were detected and the corresponding equilibrium constants at 273.2 K were determined by Aveston [26] as follows: (2)2IO4−+2OH−⇌H2I2O104−,logβ1=15.05,(3)IO4−+OH−+H2O⇌H3IO62−,logβ2=6.21,(4)IO4−+2OH−⇌H2IO63−,logβ3=8.67.The distribution
of all species of periodate in alkaline solution can be calculated from (2)–(4). In the [OH−] range
used in this work, the dimer and IO4− species of periodate can be neglected. The main species of periodate are H2IO63− and H3IO62−.
Equation (5) can be obtained from (2), (3), (4) as follows: (5)[H2IO63−]=β3[OH−]21+β2[OH−]+β3[OH−]2[IO4−]ex=f([OH−])[IO4−]ex, where [IO4−]ex represents
the concentration of original overall periodate ion.From the discussion above, the
reaction mechanism was proposed as below:(6)[Ag(HIO6)2]5−+3H2O⇌K1[Ag(H2O)2(HIO6)]−+H2IO63−+OH−,(7)[Ag(H2O)2(HIO6)]−+RCH(NH2)COO−→K[Ag(H2O)2(HIO6)(RCH(NH2)COO−)]2−, R means (CH3)2CHCH2—for
leucine and H—for alanine (8)[Ag(H2O)2(HIO6)(RCH(NH2)COO−)]2−→fastAg(I)+RCOCOO−+H2IO63−+H2O+NH3,[Ag(III)]tot=[Ag(OH−)2(H2IO6)2]e5−+[Ag(OH−)2(HIO6)]e3−.
Here “e” and “tot” mean
the equilibrium concentration of the all kinds of Ag(III) complexes and the
total concentration of all kinds of Ag(III) complexes, respectively. The
reaction (7) is the rate-determining step, k means the rate-determining step constant, the rate equation can be expressed as
below: (9)−d[Ag(III)]Tdt=kobs⋅[Ag(III)]tot=k′[Ag(H2O)2(HIO6)−]⋅[RCH(NH2)COO−]=kK1[R][Ag(III)]totK1+[H2IO63−][OH−],(10)kobs=kK1[R]K1+[H2IO63−][OH−]. Then substitute (5) for [H2IO63−] in (10) is as follows: (11)kobs=kK1[R]K1+f([OH−])[IO4−]ex[OH−],(12)k′=kK1K1+f([OH−])[IO4−]ex[OH−]. From (12), the
plot of 1/k′ versus f(OH)⋅[OH−] is a straight line, and 1/k′ versus [IO4−]ex is straight line too, which can explain all the experimental phenomena, and the
activation parameters according to k are
calculated [27] (see Table 2).Table 2
Thermodynamic
activation parameters for the oxidation of leucine and alanine by Ag(III)
complex in alkaline medium.
Thermodynamic activation parametersReaction for oxidation of leucineTemperature (K)10k/(M−1⋅s−1)298.22.75Ea/(KJ⋅mol−1)59.34±4.37303.24.10△H#/(KJ⋅mol−1)56.86±4.37308.26.54△S#/(J⋅K−1⋅mol−1)−64.83±14.3313.28.40103K1/(mol⋅L−1)27.09±0.42Reaction for oxidation of alanineTemperature (K)10k/(M−1⋅s−1)298.25.93Ea/(KJ⋅mol−1)60.95±3.15303.28.47△H#/(KJ⋅mol−1)58.47±3.15308.212.3△S#/(J⋅K−1⋅mol−1)53.40±10.31313.219.4103K1/(mol⋅L−1)26.44±0.49


## 4. Conclusion

Through the kinetics study, we supposed a simple reaction
mechanism, which can explain the reaction very well. Activation parameters and
rate determining step constant were calculated from the rate law derived from
the proposed reaction mechanism. The negative value of△S# suggested that intermediate complex is more in
order than the reactants. The higher rate constant for the rate determining
step indicates that the oxidation presumably occurs via innersphere mechanism.
The pre-equilibrium constants, K1, are calculated at 298.2 K, which
are smaller values. This also shows that rate law from equilibrium hypothesis
is suitable for the
reaction mechanism.



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*Source: 786857-2008-10-14.xml*