Mechanistic Aspects of Os(VIII)/Ru(III) Catalysed Oxidation of L-phenylalanine by Ag(III) Periodate Complex in Aqueous A
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Mechanistic Aspects of Os(VIII)/Ru(III) Catalysed Oxidation of L-phenylalanine by Ag(III) Periodate Complex in Aqueous Alkaline Medium Shekappa D. Lamani Æ Jyothi C. Abbar Æ Sharanappa T. Nandibewoor
Received: 12 July 2009 / Accepted: 18 August 2009 / Published online: 10 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The kinetics of oxidation of ruthenium(III) (Ru(III)) and osmium(VIII) (Os(VIII)) catalysed oxidation of L-phenylalanine (L-Pal) by diperiodatoargentate(III) (DPA) in aqueous alkaline medium at 27 °C and a constant ionic strength of 0.25 mol dm-3 was studied spectrophotometrically. The involvement of free radicals was observed in the reactions. The reaction between DPA and L-Pal in alkaline medium exhibits stoichiometry as [L-Pal]:[DPA] = 1:1. The reaction is of first order in [Os(VIII)], [Ru(III)] and [DPA] and has negative fractional order in [IO4-]. It has less than unit order in [L-Pal] and [OH-]. However, the order in [L-Pal] and [OH-] changes from first order to zero order as their concentrations increase. The main oxidation products were identified by spot test and spectral studies. The probable mechanisms were proposed and discussed. The catalytic constant (Kc) was also calculated for Os(VIII) and Ru(III) catalysis at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed and thermodynamic quantities were also calculated. It has been observed that the catalytic efficiency for the present reaction is in the order of Os(VIII) [ Ru(III). The active species of catalyst and oxidant have been identified. Keywords Kinetics Oxidation L-phenylalanine Diperiodatoargantate(III) Osmium(VIII) catalysis Ruthenium(III) catalysis
S. D. Lamani J. C. Abbar S. T. Nandibewoor (&) Department of Chemistry, Karnatak University, Dharwad 580 003, India e-mail: [email protected]
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1 Introduction The oxidation of amino acids is of interest as the oxidation products differ for different oxidants [1, 2]. Thus, the study of amino acids becomes important because of their biological significance and selectivity towards the oxidants. Amino acids have been oxidized by variety of oxidizing agents [3]. L-phenylalanine (L-Pal) is an essential amino acid. It forms active sites of enzyme and helps in maintaining their proper confirmation by keeping them in proper ionic states. So oxidation of L-Pal may help in understanding some aspects of enzyme kinetics. L-phenylalanine supplementation helps in the suppression of the pain and aid weight loss through the suppression of appetite. It is converted in to tyrosine, which is precursor to dopamine levels, have a definite function in sexual desire. Diperiodatoargentate(III) (DPA) is a powerful oxidising agent in alkaline medium with the reduction potential [4] 1.74 V. It is widely used as a volumetric reagent for the determination of various organic and inorganic species [5]. Jayaprakash Rao and other researchers have studied DPA as an oxidizing agent for the kinetics of oxidat
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