Kinetics and Mechanistic Approach to the Permanganate Oxidation of L-Glutamine in Alkaline Medium

Research Article

Austin Chem Eng. 2016; 3(3): 1033.

Kinetics and Mechanistic Approach to the Permanganate Oxidation of L-Glutamine in Alkaline Medium

Fawzy A1,2* and Altass HM1

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

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

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

Received: May 12, 2016; Accepted: May 31, 2016; Published: June 02, 2016


The kinetics of oxidation of L-glutamine (Gln) by permanganate ion has been investigated in alkaline medium at a constant ionic strength of 0.2 mol dm-3 and at 25oC using spectrophotometric technique. A first order kinetics with respect to [permanganate] and less than unit order dependences on [Gln] and [OH-] were revealed. No pronounced effect on the reaction rate by increasing either ionic strength or solvent polarity of the medium was recorded. Intervention of free radicals was observed in the reaction. The reaction mechanism describing the kinetic results was suggested. The final oxidation products of L-glutamine were identified as formyl propanamide, ammonia and carbon dioxide. The rate-law expression for the oxidation reaction was deduced and the reaction constants have been evaluated. The activation parameters associated with the rate-limiting step of the reaction, along with the thermodynamic quantities of the equilibrium constants have been calculated and discussed.

Keywords: L-Glutamine; Permanganate; Oxidation; Kinetics; Mechanism


Amino acids are biologically important organic compounds composed of both amine and carboxylate functional groups, along with a side-chain specific to each amino acid. Due to the biological importance of amino acids, the kinetics and mechanistic studies of their oxidation by a variety of oxidants have received considerable attention [1-20]. L-Glutamine is a a-amino acid that is used in the biosynthesis of proteins. It is non-essential and conditionally essential in humans who are synthesized by the enzyme glutamine synthetase from glutamate and ammonia. Glutamine plays a role in a variety of biochemical functions [21-23]. In human blood, glutamine is the most abundant free amino acid. The demand for glutamine increases with physical and mental stress. Production of this important amino acid, which takes place in the body, often slows down with age and does not generate sufficient amounts. Glutamine plays a decisive role in keeping a balanced acid-base ratio. The most relevant glutamineproducing tissue is the muscle mass. If not enough glutamine is available, the body takes the necessary protein from muscle mass and converts it to glutamine and energy. This leads to muscle proteins being lost, muscle strands becoming thinner and the skin becoming generally saggy. Although the liver is capable of relevant glutamine synthesis, its role in glutamine metabolism is more regulatory than producing, since the liver takes up large amounts of glutamine derived from the gut. Glutamine can be converted to glucose in the kidneys, without effecting glucagon or insulin levels. There are also indications that glutamine can reduce the demand for sugar and alcohol [22,23].

Potassium permanganate is extensively used as an oxidizing agent for numerous organic molecules in various media [24-34]. The oxidation reaction mechanisms by permanganate are governed by pH of the medium [33]. Among six oxidation states of Mn(II) to Mn(VII), permanganate, Mn(VII) is found to be the most powerful oxidation state in both acid or alkaline media. By using permanganate as oxidizing agent, it is understandable that, the Mn(VII) in permanganate is reduced to a variety of oxidation states in acidic, alkaline and neutral media.

There are no reports on the kinetics and mechanism of oxidation of L-glutamine by permanganate ion. This motivates us to investigate the title reaction. The objectives of the present study aimed to shed more light and establish the most favorable conditions affecting oxidation of such amino acid and to elucidate a plausible oxidation reaction mechanism.



All reagents were from Merck or Sigma. A stock solution of L-glutamine was prepared afresh by dissolving the appropriate amount of the sample (E. Merck) in the required volume of bidistilled water. Solution of potassium permanganate was prepared and standardized as reported earlier [35]. Other chemicals were of analytical grade and their solutions were prepared by dissolving requisite amounts of the samples in bidistilled water. Sodium hydroxide solution was used to provide the required alkalinity. Sodium per chlorate and t-butyl alcohol were used to study the effects of ionic strength and dielectric constant of reaction medium, respectively.

Kinetic measurements

The kinetic measurements were followed under pseudo-first order conditions where L-glutamine substrate (Gln) was exist in a large excess over that of permanganate. Initiation of the reaction was done by mixing the formerly thermostatted solutions of permanganate and substrate that also contained the required amounts of NaOH and NaClO4. The course of the reaction was followed up to not less than two half-lives by monitoring the absorbance of permanganate as a function of time at its absorption maximum (λ = 525nm), whereas the other constituents of the reaction mixture did not absorb considerably at the determined wavelength. The absorption measurements were done in a temperature-controlled Shimadzu UV-VIS-NIR-3600 double-beam spectrophotometer. The reactions temperature was controlled to ± 0.1oC.

First order plots of ln(absorbance) versus time were recorded to be straight lines up to at least two-half lives of the reaction completion and the observed first order rate constants (kobs) were calculated as the gradients of such plots. Ordinary values of at least two independent determinations of the rate constants were taken for the analysis. The rate constants were reproducible to 2-3%. The orders of the reaction with admiration to the reactants were calculated from the slopes of the log kobs versus log(concentration) plots by varying the concentrations of both substrate and alkali, in turn, while keeping other conditions constant.


Stoichiometry and product analysis

Reaction mixtures containing different initial concentrations of the reactants with an excess of permanganate ion concentration at [OH-] = 0.05mol dm-3 and at 0.2mol dm-3 ionic strength, were equilibrated for about 24h at room temperature. The unconsumed permanganate was estimated periodically until it reached a constant value, i.e. completion of the reaction. Estimation of unconsumed [MnO4-] revealed that approximately 2.0mol of permanganate consumed 1.0 mol of L-glutamine. This result confirms to the following stoichiometric equation,

H2N (CO) CH2 –CH2 – CH (NH2) COOH + 2MnO4 - + 2OH- =

H2N (CO) CH2 –CH2 – CHO + 2MnO4 2- + NH3 + CO2 + H2O

The above stoichiometric equation is consistent with the results of product analysis. The products were identified as the corresponding aldehyde (formyl propanamide) by spot test [36], intermediate manganate(VI) by its visible spectrum, ammonia by Nessler’s reagent [37] and carbon dioxide by lime water. The product, formyl propanamide was also estimated quantitatively as its 2,4-DNP derivative [37].

Spectral changes

Spectral changes throughout the oxidation of L-glutamine by alkaline permanganate are represented in Figure 1. The main characteristic feature manifested in the figure is the gradual decay of permanganate band at its absorption maximum (λ = 525nm) as a result of its reduction of permanganate by the amino acid.

Effect of permanganate concentration

Permanganate oxidant was diverse in the concentration range of 1.0 x10-4 to 8.0 x 10-4 mol dm-3 while the other reactant concentrations, pH and temperature were kept constant. It has been found that, plots of ln(absorbance) versus time were linear up to about two-half lives of the reaction achievement. Furthermore, the increase in the oxidant concentration did not change the oxidation rate as listed in Table 1. These results confirm the first order dependence with respect to the oxidant.