Kinetics and Mechanism of Silver(I)-Catalyzed Oxidations of α-Aminobutyric Acid by Platinum(IV) in Perchloric and Sulfuric Acid Solutions

Research Article

Austin Chem Eng. 2016; 3(2): 1029.

Kinetics and Mechanism of Silver(I)-Catalyzed Oxidations of α-Aminobutyric Acid by Platinum(IV) in Perchloric and Sulfuric Acid Solutions

Fawzy A1,2*, Zaafarany IA1, Khairou KS1, Yarkandi N1 and Bawazeer T1

1Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, 21955 Makkah, Saudi Arabia

2Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt

*Corresponding author: Fawzy A, Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, 21955 Makkah, Saudi Arabia, Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt

Received: April 09, 2016; Accepted: May 03, 2016; Published: May 05, 2016


The kinetics of oxidations of α-aminobutyric acid (ABA) by platinum(IV) has been investigated spectrophotometrically in perchloric and sulfuric acids solutions in the presence of silver(I) catalyst at a constant ionic strength of 1.0 mol dm-3 and at 25°C. The reactions were very slow to be measured in the absence of the catalyst. The reactions in both acids showed a first order dependence on [PtIV] and less than unit order dependences with respect to [ABA], [H+] and [AgI]. Increasing ionic strength and dielectric constant was found to decrease the oxidation rates. Under comparable experimental conditions, the oxidation rate of α-aminobutyric acid in perchloric acid solutions was approximately found to be twice that recorded in sulfuric acid solutions. A plausible oxidations mechanism has been proposed and the rate law expression has been derived. Both spectral and kinetic evidences revealed formation of 1:1 intermediate complexes between ABA and AgI in both acids before the rate-controlling step. Then the formed complexes react with the oxidant (PtIV) by an inner-sphere mechanism to give rise to the oxidation products of ABA which were identified as the corresponding aldehyde (propionaldehyde), ammonium ion and carbon dioxide. The activation parameters of the second order rate constants were evaluated and discussed.

Keywords: α-Aminobutyric acid; Oxidation; Silver(I) catalyst; Platinum(IV); Kinetics; Mechanism


Amino acids play a significant role in the metabolism and the specific metabolic role of them includes the biosynthesis of polypeptides and proteins, as well as the synthesis of nucleotides. Oxidation of amino acids is a significant field of organic chemistry because of its bearing on the mechanism of amino acid metabolism. Kinetics of oxidation of amino acids by various oxidants in different media has been studied earlier [1-18], and they often undergo oxidative decarboxylation and deamination. α-Aminobutyric acid (ABA), is a non-proteinogenic alpha amino acid which is biosynthesised by transaminating oxobutyrate, a metabolite in isoleucine biosynthesis.

The chemistry of biologically active platinum(IV) complexes has increased interest in the last decades due to their remarkable anticancer properties [19-22]. Kinetic studies on the oxidation of inorganic and organic substrates using one of the platinum(IV) complexes such as hexachloroplatinate(IV) complex, [PtCl6]2-, are scarce and limited to a few cases [10-16,23-26], in which [PtCl6]2- may behave as one or two electron oxidant, depending upon the substrate and experimental conditions. The knowledge of the reactivity of platinum(IV) compounds towards their reduction by potential bioreductant such as α-aminobutyric acid may be important for understanding the mechanism of where antitumor activity as well as for designing new compounds with the least side effect.

Transition metal ions have been widely employed as homogenous catalysts for oxidation of organic and inorganic substrates [7-17] by different reaction pathways such as formation of complexes with the reactants, oxidation of a substrate, or the formation of free radicals [27]. Kinetic investigations on the oxidation of amino acids catalyzed by different metal ions are an important field of chemistry due to the role played by metals in biological systems.

Literature survey reveals that there are no reports about the kinetics of oxidation of α-aminobutyric acid by any oxidant. In view of the above mentioned arguments, we have carried out a detailed study on the kinetics and mechanism of oxidation of this important amino acid by biologically active platinum(IV) in different acid media, namely perchloric and sulfuric acids solutions in the presence of silver(I) catalyst. This work aims to study the selectivity of the studied amino acid towards platinum(IV) in acid media, to check the catalytic efficiency of AgI catalyst, and to elucidate a plausible reaction mechanism.



Reagent grade chemicals and doubly distilled water were used throughout the work. A stock solution of α-aminobutyric acid was prepared afresh by dissolving the amino acid sample (E. Merck) in bidistilled water. Chloroplatinic acid solution (Johnson Matthey) was used without further purification. Required solution of the oxidant was freshly prepared before each experiment by proper dilution of its original solution which is standardized spectrophotometrically [28]. The solution was stored in a bottle away from light and re- standardized periodically. Sodium pechlorate and sodium sulfate were used to vary the ionic strength in both perchloric and sulfuric acids solutions, respectively, and acetic acid to vary the dielectric constant of the reactions media.

Kinetic measurements

All kinetic investigations were conducted under pseudo-firstorder conditions where α-aminobutyric acid was present in a large excess over that of platinum(IV). The ionic strength, I, of the reactions mixtures was adjusted to 1.0 mol dm–3. The reactions temperature (25°C) was controlled within ±0.1°C unless stated otherwise. The reactions were initiated by rapid addition of known amounts of the pre-equilibrated PtIV to the reactions mixtures containing the required amounts of the investigated amino acid, perchloric or sulfuric acid, silver(I) nitrate, sodium perchlorate or sodium sulfate and water, thermostated at the same temperature. The solutions were then mixed and transferred to a cell with a path length of 1 cm. The courses of the reactions were followed spectrophotometrically by monitoring the decrease in the absorbance of PtIV at λ = 261nm, its absorption maximum, as a function of time using Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer with a cell compartment kept at constant temperature. The applicability of Beer’s law was verified at 261 nm under the reaction conditions. The molar extinction coefficient, e, was determined, e = (1.32 ± 0.04) x 104 dm3 mol–1 cm–1, and was found to be in a good agreement with that reported previously [28]. In addition, there is no interference from other reagents at this wavelength. It was observed that the oxidation reactions were very slow to be measured in the absence of silver(I) catalyst. The pseudo-first order rate constant values of the catalyzed reactions (kC) were obtained from the linear portion of in (absorbance) versus time plots, which were the average of at least two independent kinetics runs and were reproducible to within ±2- 3%. Double logarithmic plots were used to determine the order with respect to each reactant. The concentration of the particular species being examined was varied while the concentrations of the other species were held fixed.


Time-resolved spectra

Time-resolved spectra during the silver(I) catalyzed-oxidations of α-aminobutyric acid by platinum(IV) in perchloric and sulfuric acids solutions are shown in Figure 1 (a) and (b), respectively. The scanned spectra indicate gradual disappearance of the PtIV absorption band with time as a result of its reduction. A hyposchromic shift in the PtIV band of about 6 nm from 261 to 255nm as well as two is osbestic points located at 245 and 280nm are apparent in the spectra.

Stoichiometry and product identification

Different reactions mixtures with different sets of reactants containing various amounts of PtIV and α-aminobutyric acid in both acids solutions at fixed acidity, ionic strength, and temperature were allowed to react for about 24 h. After completion of the reactions, the unreacted [PtIV] was assayed spectrophotometrically. Results indicated that one mole of the amino acid consumed one mole of PtIV in the predominant reactions, as represented in the following stoichiometric equation 1: