INTRODUCTION:
Amino acids act not only as the building blocks in protein syntheses but they also play a significant role in metabolism and have been oxidized by a variety of oxidizing agents1. The study of the oxidation of amino acids is of interest because of their biological significance and selectivity towards the oxidant to yield the different products 2–4.
Potassium permanganate is used as an oxidizing agent in synthesis as well as in analytical chemistry and also as a disinfectant. Oxidation by permanganate ion is applied extensively in organic synthesis. Among the six oxidation states of manganese (+2 to+7), permanganate, Mn (VIII)is the most potent oxidation state in acidic as well as in alkaline medium. The mechanism by which the multivalent oxidant oxidizes a substrate depends not only on the substrate but also on the medium5 used for study. In strongly alkaline medium the stable reduction product 6,7 of the permanganate ion is manganate ion , MnO4-2. No mechanistic information is available to distinguish between a direct one electron reduction to Mn (VI) and a mechanism, in which hypo permanganate is formed in a two electron reduction followed by rapid oxidation of hypo magnate ion 8.
Received on 20.08.2012 Modified on 29.08.2012
Accepted on 28.09.2012 © AJRC All right reserved
Asian J. Research Chem. 5(9): September, 2012; Page 1182-1185
The kinetic investigation of the oxidation of biologically important amino acid by variety of oxidant has been carried out under different experimental conditions 9. In many cases undergo oxidative decarboxylation. But other studies with amino acids report the oxidation product as the corresponding aldehydes 10. Thus the study of amino acids becomes important because of their biological significance and selectivity towards the oxidants. Although a variety of organic substrates11and inorganic substrates 6, 7 are oxidized by permanganate in aques alkaline medium, these are only few reports on oxidation of amino acids by aqueous alkaline permanganate. L-Isoleucine is an active site residue enzyme and helps in maintaining the correct conformation of enzymes by keeping them in their proper ionic states. Thus oxidation of this may help in understanding enzyme kinetics. In order to explore the mechanism of oxidation by permanganate ion in a strongly alkaline medium and to check the reactivity of amino acids towards permanganate, L-Isoleucine has been selected as a substrate.
The kinetics of fast reactions between ruthinate(VII),RuO4- and permanganate(VI), MnO42- has been studied12.The reaction is presumed to proceed via an outer sphere mechanism. The rapid exchange between MnO42- and MnO4- has been studied in detail by variety of techniques13. Here in we describe the result of the title reaction in order to determine the active species of oxidant, reductant and catalyst in such media and to arrive at a plausible mechanism.
EXPERIMENTAL:
Stock solution of L-Isoleucine (S.R.L. Chemicals) was prepared by dissolving the appropriate amount of sample in double distilled water. The solution of KMnO4 (B.D.H) was prepared and standardized with standard solution of H2C2O414. Ruthenium (III) chloride (S.D. fine chemicals and other reagents of AnalaR grade were prepared by dissolving requisite amounts of samples in doubled distilled water and standardized. NaOH and NaClO4 were used to provide the required alkalinity and to maintain ionic strength respectively.
Kinetic Measurements:
All kinetic measurements were performed under pseudo first order conditions where [L-Isoleucine]T used is at least 10- fold excess over [Permanganate]T at a constant ionic strength of 0.5 mol. dm-3. The course of reaction was followed by monitoring the decrease in absorbance of MnO4- on a 1 cm quartz cell in a CECIL-7200 UV-Vis spectrophotometer at its absorption maximum (525nm) .The spectral changes during the reaction are shown in Fig.1.
The first order rate constants, kobs were evaluated from the slope of ln(At-A∞) Vs t plots, where At and A∞ are absorbance of the reaction mixture at time t and at equilibrium respectively. The first order plots in all most all cases were liner up to 90% of the reaction and k obs were reproducible within ±3 %.The correlation coefficient of plots used to determine k obs were found to be 0.99 in most of the cases.
All the effects of variation of concentration of alkali, MnO4-, catalyst, temperature and concentrations are collected in table 1 and 2.
Stochiometry and characterization of Reaction product:
The reaction mixture containing an excess of permanganate over L-Isoleucine and 0.05 mol.dm-3 sodium hydroxide at a constant ionic strength of 0.5mol dm-3 was allowed to react for 2 hrs at 35±1oC under inertatmosphere. After completion of the reaction the remaining MnO4- was analyzed spectrophotometrecally. Results showed that two moles of MnO4- were consumed by one mole of L-Isoleucine. So it is concluded that the stochiometry of the reaction under kinetic study is
R-CH (NH2) COOH + 2MnO4- + 2OH- →R-CHO +2 MnO42- + NH3+CO2 + H2O ………………………..….(1)
Where R = C2H5 -CH
(CH3)
The reaction product were identified as aldehydes 15 by boiling point, spot tests and ammonia 16 by Nesslers reagent and manganate by its visible spectrum .The product aldehyde was quantitatively estimated to about 80% , which is evidenced by its 2,4-DNP derivative17. The nature of the aldehyde was confirmed by its IR spectrum18. 2933cm-1 due to the C-H stretching of –CHO, 3422.43(s) cm-1 and 1630(w) cm-1 band may be due to H2O in trace amount in KBr. Carbonyl stretching at 1759.55 cm-1 indicates the presence of –CHO group in the product. (fig: 2). The same type of aldehyde as above was obtained when the product analysis was carried under first order conditions. It was also observed that the aldehyde does not undergo further oxidation under the present kinetic conditions.
DISCUSSION:
Under the prevailing experimental conditions at pH ≥12, the reduction product of Mn (VII) is stable and further reduction of Mn (VI) might be stopped. Diode Arrey rapid scan spectrophotometric (DRRAS) studies have shown that at pH ≥ 12, the product of manganese (VII) was manganese (VI) and no further reduction was observed as reported19,20. However, on prolonged standing green manganese (VI) is reduced to manganese (IV) under our experimental conditions.
It is known that in aqueous solutions, amino acid exists in Zwitterionic 21 form, whereas in aqueous alkaline medium it exists as anionic form.
Under the conditions [OH-]>> [Ru(III)], ruthenium (III) is mostly present as the hydroxylated species, [Ru(H2O)5OH]2+ . Increase in rate with increase in [OH-] indicates the presence of the hydroxylated species of ruthenium (III) as a reactive species which is shown by the following equllibrium in accordance with the earlier work.
[Ru(H2O)6]3+ + OH- ↔ [Ru(H2O)5OH]2+ + H2O
The rate law for the reaction can be written as: