A kinetic and mechanistic study on the oxidation of arginine and lysine by hexacyanoferrate (III) catalysed by iridium (III) in aqueous alkaline medium

The kinetics of Ir (III) catalysed oxidation of some amino acids like arginine and lysine by hexacyanoferrate (abbreviated as HCF) (III) ions in aqueous alkaline medium at constant ionic strength 0.5 mol dm -3 and temperature 35oC has been studied spectrophotometrically. The reactions exhibit 2:1 stoichiometry and follow first order kinetics in [HCF (III)] and [alkali]. The dependence of the rate on substrate concentration has been found to be of Michaelis-Menten type. The ionic strength of the reaction mixture shows positive salt effect on the reaction rate. To calculate thermodynamic parameters, the reactions have been studied at four different temperatures between 35 to 50oC. A complex mechanism involving the complex formation between catalyst and the substrate has been


Introduction
Oxidation reactions are of fundamental importance in nature, and are key transformations in organic synthesis 1 . Oxidation of a-amino acids is of great importance both from chemical point of view and its bearing on the mechanism of amino acids metabolism 2 . Amino acids have been oxidised by a variety of reagents under different experimental condition 3 -10 . It was shown by the previous workers that the oxidation of a-amino acids by hexacyanoferrate(m) proceeds very slowly in absence of any catalyst while it follo~s a complex kinetics in presence of a catalyst 11 -t3. The oxidation rate was improved by the use of some metal ions Os, Ru and Ag 14 -17 . These reactions follows a complex kinetics in presence of cata-lyst18-21. Thus to understand about the catalysis of Ir 111 in oxidation of amino acids by HCF(m) and to explore the mechanism of these oxidations in aqueous alkaline medium, two amino acids, arginine and lysine, have been selected as substrate for oxidation.

Results and discussion
Kinetic experiments were performed at different concentrations of one reactant keeping the concentration of other constant.
The concentration of substrate (lysine and arginine, abbreviated as S) was varied in the range of I x I0-3 -10 x w-3 mol dm-3 at 35 °C keeping all other reactants concentration constant (Table 1). The data presented in Table I shows first order dependence on lower concentration of substrate which tends to be zero order at its higher concentration. To calculate the thermodynamic parameters, the effect of temperature on the rate of the oxidation was stud- following mechanistic path has been suggested for the oxidation of arginine and lysine by hexacyanoferrate(lll) in Table 2. Effect of ionic strength on reaction rate aqueous alkaline medium in presence of Ir 111 as catalyst. [ )l X 10 1 Lysine Arginine (mol dm-3) RCHNH 2 Coo-+ IrCI 6 It is reported that Ir 1 and Ir 11 are the stable species of iridium, but in alkaline medium [IrCI 6 ] 3 -is the only reacting species of iridium 25 · 26 . Srivastava eta/. have reported that the oxidation of amino acids involves the cleavage of N-H and C-H bonds in the rate determining step27.
Based on the above facts it is assumed in the present study that substrate anion forms a loosely bonded complex C 1 with iridium trichloride. The carbonyl oxygen of acid is most likely involved in the formation of complex C 1 . In the next step the complex C 1 then combines with HCF(IIl) through electron abstraction to form complex (C2). The complex (C 2 ) then slowly-disproportionates into Ir 1 + and HCF(II) along with final product. Ir' + is reoxidized to Ir3+ by two moles of HCF(III) via one electron transfer process. The metal ion complexes with organic substrate make the electron transfer easier28. The formation of the complex was proved kinetically by Michaelis-Menton plot i.e. a non-zero intercept of the plot of llrate vs 1/[S] (Fig. 2). The complex formation between oxidant and substrate was also reported in Iitera-ture29.
The reaction rate (r) is measured in terms of rate of disappearance of HCF(IIl). According to step (III) the rate of disappearance of HCF(III) would be  The rate Jaw (11) clearly accounts for the first order kinetics with respect to HCF(Ill), organic substrate, hydroxide ion and catalyst at their lower concentrations. In order to verify this law (10) at higher concentration of above said reactants it could be re-written as All chemicals and reagents used were of AR grade. distilled water. Absorbance was recorded on Systronic UV-Vis spectrophotometer. Amax for the reaction mixture was 420 nm at which the absorbance was noted only All solutions and reaction mixture were prepared in double h, a required amount of amino acid solution was added to the mixture and stirred to start the reaction mixture. Aliquots were withdrawn from the reaction mixture after repeated intervals of 5 min and the absorbance was recorded. Initial rates (dA!dt)i were evaluated after 5 min from the start of the reaction by plane mirror method and pseudo-first order rate constant k 1 were calculated by Guggenheim's method.
The stoichiometry of the reaction was studied by estimating the amount of HCF(II) ions produced after definite interval of time with standard solution of ceric(Iv) sulphate using ferroin as redox indicator. Estimation of the residual oxidant showed that one mole of substrate consumed two mole of hexacyanoferrate(III), corresponding to the stoichiometry, for the kinetic determinations, solutions of substrate and oxidant, in NaOH (ionic strength adjusted by the addition of the requistic amount of KCl), were mixed and kept at atmospheric conditions for 24 h. The main reaction products were identified as keto acid and ammonia. Ammonia was identified by Nessler's reagent 3 · 21 and keto acid by the following methods. The reaction mixture was extracted with diethyl ether and then concentrated. The concentrated extract was subject to TLC which shows the presence of single white product, keto acid 22 • 23 . The concentrated extract was evaporated at room temperature. A solid residue was left. It was analysed by melting point determination, spot test analysis and IR spectroscopy. The melting point of the two keto acids-E-guanidino-a-oxo valerie acid and 6-amino-a-oxo caproic acid separated are 214 °C and 211 °C respectively (literature value 216 °C and 212 °C respectively 3 ). TheIR bands at frequency 1644 cm-1 (carbonyl group) and 1632 cm-1 (acid group) for arginine and 1635 cm-1 (carbonyl group) and 1614 cm-1 (acid group) for lysine shows the presence of keto acid group in the final product extracted 24 .