Asian Journal of Biochemical and Pharmaceutical Research

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1 Research Article Asian Journal of Biochemical and Pharmaceutical Research Reactivity of Valine towards Vanadium (V) in Presence of Ru + Catalyst in Perchloric Acid Medium Maheshwar Prasad Sah Department of Chemistry, P. B. S. College, Banka, India Received: 5 January 011; Revised: 15 January 011; Accepted: 5 January 011 Abstract: A simple titrimetric method has been used for the oxidation of valine. The method is based on the reduction of vanadium (V) to vanadium (IV) and oxidation of valine. The main oxidation product of valine has been identified as -methylpropanal, ammonia and carbon dioxide. In this method, known concentrations of valine was treated with a measured excess of vanadium(v) in acid medium in presence of [RuCl6] - catalyst and the unreacted vanadium (V) was determined by titration with standard Fe + solution using barium diphenylamine sulphonate as redox indicator. In this reaction two moles of vanadium (V) consume one mole of valine. The effect of ionic strength, [SO4 - ], [HSO4 - ], [H + ] and temperature were carried out and these effects are also in support of the mechanism proposed. The activation parameters have been determined. Keyword: Ruthenium (III), catalysis, valine, vanadium (V), perchloric acid. INTRODUCTION: Vanadium is the most abundant transition metallic element, occurs in seven oxidation states [1] in several compounds. Vanadium is usually recovered from vanadinite [Pb5(VO4)Cl] and carnotite [K(UO)VO4 HO]. It occurs in certain petroleum, widely those from Venezuela. Vanadium is one of the most important metal for modern technology and used extensively in alloy steels and cast iron, space technology, atomic energy industry, pharmaceutical industrial processes. Trace amount of vanadium is a nutritional requirement [1] for cell growth, but toxic at higher concentrations. The toxicity of vanadium increases as the solubility in the water and oxidation state increases, vanadium in +5 oxidation state being the most toxic and highly mobile [-5]. Generally, vanadium enters in fresh water by leaching from vanadium containing rocks and present in the form of oxovanadium (IV) ions or vanadyl ions (VO + ) and vanadate ions (VO4 - ). Vanadium can exist in human body in equilibrium states between V 5+ and V 4+ and it can also bind strongly with blood protein by adsorption or complexing. In oxidation of organic and inorganic compounds, several workers reported [6-10] that vanadium (V) has been used as a potential oxidant, because in aqueous acid media the redox potential [1,11] of the couple V 5+ /V 4+ is 1.0 V. Reported [1-16] that amino acids have been used as reductants for the reduction of various oxidising agents. The study of oxidation of amino acids towards oxidant is interest, due to the biological significance and formation of different products [1,1]. Valine is an essential amino acid and it cannot be manufactured in the body. Meats, dairy products, mushrooms, peanuts, soy proteins, grains etc. are the main sources of valine. Valine is a branched chain amino acid that works with leucine and isoleucine to promote normal growth, regulate blood sugar, repair tissues, used as an energy source by muscle tissue 180

2 and maintains nitrogen balance in human body. Valine may be helpful for the treatment of liver and gallbladder disease. The aim of the present investigation is to determine the most toxic vanadium (V), kinetics, mechanism and rate law based on reaction with valine titrimetrically in presence of ruthenium (III) catalyst with respect to each reactant of the reactions. EXPERIMENTAL: Material: Ammonium metavanadate, perchloric acid, sodium perchlorate, sodium sulphate, sodium bisulphate and other reagent were used of analytical reagent grade. Vanadium (V) solution was prepared in double distilled water by dissolving appropriate amount of ammonium metavanadate and perchloric acid and standardised by ferrous ammonium sulphate solution using barium diphenylamine sulphonate as redox indicator [17]. Stock solution of valine was prepared by dissolving it in double distilled water. Stock solutions of sodium perchlorate, sodium sulphate and sodium bisulphate were prepared in double distilled water. Ruthenium (III) chloride solution was prepared in HCl of known strength. Sodium perchlorate and perchloric acid were used to maintain the required ionic strength and acidity respectively. Methods: Appropriate quantities of the solutions of vanadium (V), sodium perchlorate, ruthenium (III), sodium sulphate, sodium bisulphate, perchloric acid were placed in separate conical flasks and kept in a thermostated water bath. After half an hour when the solutions acquired the temperature of the bath of 40 0 C, then the calculated amount of each of the solution was added together into a conical flask, followed by the addition of required volume of double distilled water. The reaction mixture was then placed in a thermostated water bath maintained at 40 0 C (±0.6 0 C). The reaction was initiated by addition of required amount of valine solution placed separately in the water bath at 40 0 C. As soon as the half of the valine solution passed out from the pipette the stop watch was started to record the time. The kinetic studies were carried out by quenching the aliquot of the reaction mixture in measured excess of ferrous ammonium sulphate and back titrating the unreacted Fe + solution against standard vanadium (V) solution using barium diphenylamine sulphonate as redox indicator [8]. Polymerization test: No gel formation was observed on addition of acrylonitrile to the partially oxidised reaction mixture followed by addition of large excess of methanol. This suggest the absence of free radical intermediates in the reaction mixture. Stoichiometry and product analysis: For product analysis, the reaction was determined by mixing a known concentrations of valine, ruthenium (III), acid with a known excess of vanadium (V). The amount of unreacted vanadium (V) was estimated by titration with standard Fe + solution using barium diphenylamine sulphonate as redox indicator. The stoichiometry thus determined was found to be two moles of vanadium (V) to one mole of valine. Ammonia, -methylpropanal and carbon dioxide were identified as products in the stoichiometric studies. Carbon dioxide, ammonia [18] as ammonium ion and -methylpropanal [19] were identified. The overall stoichiometric equation could be written as: HN-CHR-CO+V() + +[RuCl6] - [HVO] + +RCHO+NH4 + +[RuCl6] - +CO+HO + (1) Where, R = (CH)CH 181

3 RESULTS AND DISCUSSION: Order of reaction with respect to[v 5+ ]: To find out the order of reaction with respect to [V 5+ ], the reactions were studied at various concentrations of vanadium(v) in the range from mol dm - to mol dm - but at constant [valine], [H + ], [ruthenium(iii)], ionic strength and temperature of mol dm -, mol dm -, mol dm -, mol dm - and 40 0 C respectively. The average pseudo first order rate constant (kobs) was found to be (.14 ± 0.4) 10-5 s -1 indicates that the reaction is first order with respect to vanadium (V). -d[v 5+ ] / dt = kobs [V 5+ ] -dln [V 5+ ] / dt = kobs () Order of reaction with respect to [valine]: To find out the order of reaction with respect to [valine], the reactions were studied at different concentrations of valine in the range from mol dm - to mol dm - but at constant [V 5+ ], [H + ], [ruthenium(iii)], ionic strength and temperature of mol dm -, mol dm -, mol dm -, 0.55 mol dm - and 40 0 C respectively. The average pseudo first order rate constant (kobs) was found to be (15. ± 0.5) 10-5 s -1 indicates that the reaction is first order with respect to valine. The plot of log kobs vs log [valine] is linear with a slope equal to indicating the reaction is first order with respect to [valine] (Figure 1). kobs = ks [Valine] () The values of ks thus obtained are (8.59 ± 0.45) 10 - mol -1 dm s -1, (19.67 ± 1.1) 10 - mol -1 dm s -1, (.9 ±.5) 10 - mol -1 dm s -1 and (45.58 ± 1.67) 10 - mol -1 dm s -1 at 5 0 C, 40 0 C, 45 0 C and 50 0 C respectively. Effect of [H + ] on reaction rate: The rate of reaction was investigated at different hydrogen ion concentrations in the range from mol dm - to mol dm - but at constant [valine], [V 5+ ], [ruthenium(iii)], ionic strength and temperature of mol dm -, mol dm -, mol dm -,.55 mol dm - and 40 0 C respectively. It was observed that the reaction rate increases with increase in hydrogen ion concentration (Figure 4). The plot of log kobs vs -HO are linear with slope equal to 0.7 in the acid range mol dm - to mol dm - (Figure ). From the plot of log kobs vs log [H + ] (Figure 4) the order with respect to perchloric acid concentration was determined and found to be less than unity (0.501). Vanadium(V) exists [1] only in the form of VO + in strongly acidic medium, whereas in ph range to 6 vanadium(v) exists as HV10O8 4-, HV10O8 5-, V10O8 6-, HV10O8 - and H4V10O8 -. In higher acid concentration, VO + ion is converted into V() + as given below VO + + HO + V() + (4) The order with respect to [H + ] was found to be less than unity i.e , the protonated species V() + is assumed to be active species. Effect of [SO 4 - ] and [HSO 4- ] on the rate of reaction: At constant ionic strength, the rate of reaction decreases on increasing the [SO4 - ] (Figure 5) and [HSO4 - ] (Figure 6). The inhibitory action of [SO4 - ] and [HSO4 - ] on the rate of reaction is due to the fact that the active species of vanadium (V) is removed to the inactive species according to the following equilibria. SO H SO H V V HSO HS O H O V V (5) 18

4 HSO V SO HSO V H SO O H O V V Effect of ionic strength: To establish the nature of intermediate species in the rate determining step, the effect of concentration of sodium perchlorate was studied. It was found that the rate constant is independent of ionic strength of the medium, indicating that at least one of the reacting species in the rate determining step was molecular in nature [0]. Effect of [Ru III ]: The rate of reaction was studied at different concentrations of [Ru III ] in the range from mol dm - to mol dm - but at constant [V 5+ ], [valine], [H + ], ionic strength and temperature of mol dm -, mol dm -, mol dm -, mol dm - and 40 0 C respectively. It was observed that the rate of reaction increases with increase in [Ru III ], indicating catalytic effect of [Ru III ] on rate of reaction. The plot of log kobs vs log [Ru III ] are linear with slope equal to 0.96 also indicate that the rate of reaction increases with increase in [Ru III ] (Figure 7).The values of catalytic ratios thus obtained are 1.17, 1.45, 1.76 and.14 indicating the positive catalytic effect of [Ru III ] on the rate of reaction. The catalytic constant kc was determined at various concentrations of [Ru III ] as mol -1 dm s -1, mol -1 dm s -1, mol -1 dm s -1 and mol -1 dm s -1 are fairly constant confirming the catalytic action of [Ru III ]. From the experimental results, it has been observed that the dependence of rate on [Ru III ], [valine] and [HO] can be expressed as: (6) k obs = k 1 k k [Valine][Ru + ] k 1 k +k k [Ru + ] 1 k = 1 k + 1 k obs k 1 k k [Valine][Ru + ] k 1 [Valine] (7) (8) Where k = k [HO] and k = k + k Effect of temperature on the rate of reaction: The reaction was studied at four different temperatures, 08K, 1K, 18K and K at constant [V 5+ ], [valine], [H + ], [Ru III ] and ionic strength of mol dm -, mol dm -, mol dm -, mol dm - and 0.55 mol dm - and 40 0 C respectively. The rate of reaction increases with increasing temperature. The activation parameters were calculated from the pseudo first order rate constant. The slope of the plot of log kobs versus 1/T i.e. Arrhenius plot has been used to calculate the energy of activation (Ea). From the Eyring s plot, log (kobs/t) versus 1/T (Figure 8), the enthalpy of activation ( H # ) was calculated and from which entropy of activation ( S # ) and Gibbs energy of activation ( G # ) were calculated. The values of Ea, H #, G # and S # are calculated as ( ± 0.068) kj mol -1, ( ± 0.) kj mol -1, ( ± 0.7) kj mol -1 and (-88.7±0.9) J K -1 mol -1 respectively. The negative value of S # and positive value of G # suggest the formation of more ordered activated complex and transition state is highly solvated as compared to the reactive species [0]. The high value of energy of activation (Ea) suggests that the reaction is slow at rate determining step. (9) 18

5 (10) HOVNH CH C O RuCl R Complex 6 k HO HOVNH Slow Complex O CHCORuCl R 6 H O (11) HOVNH Complex O CH C O RuCl R fast 6 H N CH VOCO R H ORuCl 6 (1) fast H 4 NCH HO RCHO NH R (1) V O V fast HVO H O Valine has been oxidised by vanadium (V) via unstable intermediate complexes followed by fission of C-C bond to form reaction product. The most reactive species of vanadium (V), V() + combines with NH of valine to form Complex1. [RuCl6] - combines with Complex1 to form Complex. Complex forms Complex by the liberation of HO +. This is the slow and rate determining step. According to J. F. Bunnett [1] hypothesis, the plot of log (kobs+ho) versus log aho is generally linear and the slope determines a parameter ω. The slope obtained from these plots equal to indicates that the water molecule acts as a proton transfer agent at rate determining step (Figure 9). Highly unstable Complex decomposes to form aldehyde, ammonia, carbon dioxide and VO(). VO() combines with V() + to form HVO +. There was no evidence in favour of the formation of proposed Complexes, but cannot be invalidated the proposed mechanism due to the very small steady state concentrations of the proposed complexes. On the basis of above mechanism the rate law could be written as: (14) d[v 5+ ] dt = k 1 k k [H O][Valine][Ru + ][V 5 + ] k 1 k +k 1 k [H O]+k k [H O][Ru + ] (15) 184

6 The above rate law (15) explains the first order dependence of rate on each in [V 5+ ] and [Valine] and fractional order dependence on [Ru + ]. The equation (15) could be written as dln[v 5 + ] dt = k obs = k 1 k k [H O][Valine][Ru + ] k 1 k +k 1 k [H O]+k k [H O][Ru + ] (16) k obs = k 1 k k [Valine][Ru + ] k 1 k +k 1 k +k k [Ru + ] (17) Where, k = k [HO] k obs = k 1 k k [Valine ][Ru + ] k 1 (k +k )+k k [Ru + ] k obs = k 1 k k [Valine ][Ru + ] k 1 k +k k [Ru + ] (18) Where, k = k + k k obs = p [Valine][Ru + ] q + r [Ru + ] (19) Where, p =k 1 k k, q = k 1 k and r = k k By taking reciprocal of equation (19) 1 k obs = 1 k obs = q + r [Ru + ] p [Valine][Ru + ] q p [Valine][Ru + ] + r p [Valine] (0) (1) Where, p, q and r are constants and expressed in terms of different rate constants. The slope, intercept on 1/kobs axis and extrapolated intercept on 1/[Ru + ] axis, give the values of q/p[valine], r/p[valine] and -1/q respectively. At constant [V 5+ ], a plot of 1/kobs versus 1/[Valine] was linear with small intercept (45.069) on 1/kobs axis, offering support for complex formation (Figure ). CONCLUSION: The negative value of S # and positive value of G # suggest the formation of more ordered activated complex and transition state is highly solvated as compared to the reactive species. The high value of energy of activation (Ea) suggests that the reaction is slow at rate determining step. The slope obtained from J. F. Bunnett plots equal to indicates that the water molecule acts as a proton transfer agent at rate determining step. At constant [V 5+ ], a plot of 1/kobs versus 1/[Valine] was linear with small intercept (45.069) on 1/kobs axis, offering support for complex formation. ACKNOWLEDGEMENT: The author is thankful to Dr. L. Thakur, Retired Professor and Head, P. G. Department of Chemistry, T. M. Bhagalpur University, Bhagalpur for good suggestions. 185

7 1 / kobs 5 + log kobs log [Valine] Fig. 1: Plot of log kobs vs log [Valine] at at 7 K. [V 5+ ]=.5 x 10 - mol dm -, [RuCl6] - = 5.0 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm - and I = 0.55 mol dm / [Valine] Fig. : Plot of 1 / kobs vs 1 / [Valine] at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [RuCl6] - = 5.0 x 10 - mol dm - [HClO4] = 5.0 x 10-1 mol dm - and I = 0.55 mol dm -. Fig. : Plot of log kobs vs Ho at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [RuCl6] - = 5.0 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm - and I =.55 mol dm - 186

8 Fig. 4: Plot of log kobs vs log [H + ] at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [RuCl6] - = 5.0 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm - and I =.55 mol dm - Fig. 5: Plot of log kobs vs log [SO4 - ] at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm -, [RuCl6] - = 5.0 x 10 - mol dm -, and I = mol dm - Fig. 6: Plot of log kobs vs log [HSO4 - ] at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm -, [RuCl6] - = 5.0 x 10 - mol dm -, and I = mol dm - 187

9 Fig. 7: Plot of log kobs vs log [Ru + ] at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm -, and I = mol dm - Fig. 8: Plot of log (kobs/t) vs 1/T. [V 5+ ] =.5 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm -, [Ru + ] = 5.0 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm - and I = 0.55 mol dm - Fig. 9: Plot of log (kobs +H0) vs log aho at 1 K. [V 5+ ] =.5 x 10 - mol dm -, [Valine] =.5 x 10 - mol dm -, [HClO4] = 5.0 x 10-1 mol dm -, [Ru + ] = 5.0 x 10 - mol dm - and I =.55 mol dm - 188

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