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Autocatalyzed Oxidation of Amino Acid, L-Citrulline by Diperiodatocuprate(III) Complex in Aqueous Alkaline Medium: a Kinetics and Mechanistic Approach

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Autocatalyzed Oxidation of Amino Acid, L-Citrulline by Diperiodatocuprate(III) Complex in Aqueous Alkaline Medium: a Kinetics and Mechanistic Approach J. Chem. Sci. Ó (2020) 132:17 Indian Academy of Sciences https://doi.org/10.1007/s12039-019-1718-2 Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) REGULAR ARTICLE Autocatalyzed oxidation of amino acid, L-Citrulline by diperiodatocuprate(III) complex in aqueous alkaline medium: a kinetics and mechanistic approach SANTOSH B KONNUR and SHARANAPPA T NANDIBEWOOR* P. G. Department of Studies in Chemistry, Karnatak University, Dharwad, Karnataka 580 003, India E-mail: [email protected] MS received 4 July 2019; revised 3 September 2019; accepted 6 September 2019 Abstract. Autocatalysed oxidation of important amino acid, L-Citrulline(L-Cit) by coordination complex, diperiodatocuprate(III) in aqueous alkaline medium at a constant ionic strength of 0.05 mol dm-3 was studied spectrophotometrically. Autocatalysis was observed by one of the products formed, i.e., Cu (II). 1:4 stoichiometry (L-Cit:DPC) exhibited between the reaction of L-Cit and DPC in aqueous alkaline medium. The path is of first-order in [DPC], less than unit order in [L-Cit] and independent in [OH-]. Periodate has a retarding effect on the rate of reaction. Ionic strength has a negligible effect on the reaction rate. The main reaction products were identified by spot tests and spectroscopic analysis. A composite mechanism involving uncatalysed and an autocatalysed reaction path (via free radical) was proposed. The activation parameters for the slow step of the mechanism and also the thermodynamic quantities for different steps of the mechanism were determined and discussed. Keywords. Kinetics; mechanism; diperiodatocuprate(III); L-citrulline; autocatalysis. 1. Introduction widely used in the examination of several organic compounds.12 The oxidation reactions by DPC studies Autocatalytic processes are crucial of the mechanisms are reported.13 Complexes of copper have occupied a leading to the symmetry breaking of chemical com- major role in oxidation chemistry due to their profu- 1,2 pounds towards homochirality and identified in sion and relevance in biological chemistry.14 In the 3,4 several experimental systems. Catalytic oxidation of copper(II) catalyzed oxidation of various amino acids several substrates by various different oxidants is by peroxy, disulphate involves copper(III) as an 5,6 reported. Oxidation of various biological substrates intermediate.15 DPC is a versatile one-electron oxidant 2? and polyphenol by air oxygen catalysed by Cu and and involves multiple equilibria between different 2? 7,8 Fe complexes exhibit transient oscillations. copper(III) species. Hence, it can be a matter of The study of the metals in their highest oxidation intrigue to many researchers to know which of the state has fascinated many researchers in current years. active species of the copper(III) is involved in the Such transition metals can be stabilized by chelating reaction. with suitable bi or polydentate ligands. Diperioda- Human kidney converts L-Citrulline (L-Cit) into tocuprate(III) (DPC) is an excellent oxidant due to its another important amino acid called L-arginine and restricted solubility and stability in aqueous medium. oxide of nitrogen. In the human body, L-Cit boosts the The reactivity of a few alcohols with diperioda- production of nitric oxide. Oxide of nitrogen (nitric 9 tocuprate(III) (DPC) is reported by Movius. The oxide) helps arteries to relax and work better, which characterization, structural determination and synthe- also regulates the flow of blood throughout the body. It 10,11 sis of DPC have been reported. Copper periodate is used in the treatment of Parkinson’s disease and complexes in their ?3 oxidation state have been certain dementias. The human body needs certain *For correspondence Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-019-1718-2) contains supple- mentary material, which is available to authorized users. 17 Page 2 of 11 J. Chem. Sci. (2020) 132:17 Scheme 1. Mechanism for the oxidation of L-Cit by alkaline DPC. proteins. L-Cit helps to increase the required supply of unity is [alkali]. Rate of reaction is increased with an ingredients. L-Cit is a key intermediate compound in increase in ionic strength. In the present kinetics and the urea cycle. In the body, L-Cit is produced as a mechanistic study of L-Cit by DPC, the order with byproduct of the enzymatic production of nitric oxide respect to DPC, L-Cit is unity and less than unity, from the amino acid arginine, catalyzed by nitric oxide respectively. Rate is independent of [OH-] and ionic synthase.16 strength. The retarding effect of periodate on rate is In many of the cases, amino acids are oxidised observed. One more interesting fact is that the auto- through decarboxylation and deamination.17 The oxi- catalysis of Cu(II) was observed. dation of amino acids is of interest as the oxidation In view of the observed study of autocatalysis by products differ for different oxidants.18,19 Such oxi- Cu(II) in the oxidation of L-Cit by DPC and of interest dation studies may throw some light on the mechanism to study mechanism of oxidation L-Cit by Cu(III) of conversion of amino acid in the biological system. complex in aqueous alkaline medium, L-Cit as a Kinetics and mechanistic study of L-Cit by per- substrate for oxidation has been selected. The title manganate in acidic and basic media have been reaction represents a full mechanistic and kinetic reported.20 In basic media, order with respect to oxidation of L-Cit by DPC in alkali media to establish [permanganate] and [L-Cit] is unity, whereas less than the optimum conditions affecting such oxidation J. Chem. Sci. (2020) 132:17 Page 3 of 11 17 reactions. To understand more about the active species first-order rate constants, ‘kobs’ and they were linear up to of the reactants in these media, and finally to deter- 30% of the completion of the reaction under the range of - mine the plausible oxidation mechanisms on the basis [OH ] used. Above 30% of completion of the reaction, a of the observed kinetic, spectral studies were taken up. non-linearity was due to the autocatalysis, by copper(II) and ± By understanding the mechanism, the reactivity of the rate constants were reproducible about 5%. During the kinetics, a constant (5.0 x 10-5 mol dm-3) concentra- L-Cit towards DPC could be interpreted and tion of KIO4 was used throughout the study unless other- understood. wise stated. Since the excess amount of periodate was present in DPC, the probability of oxidation of L-Cit by periodate in an alkaline medium at 25 °C was tested. 2. Experimental Iodometrically, the progression of reaction was studied. However, it was found that there was no significant reaction Materials and reagents 2.1 under the experimental conditions employed compared to the DPC oxidation of L-Cit. Reagent grade chemicals and double distilled water were The total [OH-] and [IO- ] were calculated by the con- used throughout the experimental work. The required 4 sideration of previously present ions in the DPC solution solution of L-Cit (S D fine-CHEM Ltd., Mumbai, India) and that additionally added. No effect of added carbonate was prepared by dissolving the appropriate amount of was observed on the rate of reaction. The spectral change of sample in double-distilled water. The diperioda- reaction progression has been shown in Figure 1A, in which tocuprate(III) compound was prepared21 and standardized a decrease in [DPC] at 418 nm during the reaction was through the standard procedure.22a The UV-vis spectral observed. The autocatalysed oxidation of Cu(II) is also study revealed that the maximum absorption peak at 418 nm verified the existence of copper(III) complex. Periodate solution was prepared by dissolving the appro- A 0.6 priate amount of periodate sample in hot water and kept for 1 24 h to attain the equilibrium. Its concentration was deter- 0.5 1 23 mined iodometrically at neutral pH maintained using 0.4 5 2 phosphate buffer. The Cu(II) solution was made by dis- 3 0.3 solving the known amount of copper sulphate (BDH) in OD 4 double-distilled water. KOH (BDH) and KNO3 (AR) were 0.2 5 employed to maintain the required alkalinity and ionic strength, respectively. The constant temperature within ± 0.1 ° 0.1 C range was maintained unless otherwise stated. 0 300 400 500 600 Wavelength (nm) 2.2 Kinetics measurements B 0.8 Kinetic measurements were carried out by using a Peltier 0.7 accessory for temperature control attached to Varian CARY 0.6 50 Bio UV–vis Spectrophotometer (Varian, Victoria-3170, 0.5 (1) Australia). For pH measurement, an Elico pH meter model (2) 0.4 (6) (5) (4) (3) LI 120 was used. For product analysis, the liquid chro- OD matography (LC) technique (Acquity UPLC) mass spec- 0.3 trometry (MS) instrument, SynaptG2 and FTIR technique: 0.2 Nicolet-5700 USA, were used. 0.1 Kinetic runs were performed under pseudo-first-order 0 ([L-Cit] [[ [DPC] at 25 ± 0.1 °C) conditions. The 0 50 100 150 200 required amount previously thermostatted solution of L-Cit Time (sec) and DPC were used to initiate the reaction, to which Figure 1. (A) Spectroscopic changes of DPC during the required a concentration of KNO3, KOH and KIO4 were also added to maintain reaction conditions. The progression reaction period in the Cu(II) autocatalysed oxidation of L-Cit by diperiodatocuprate(III) at 25 °C (scanning interval of the reaction was studied spectrophotometrically at - 30 s). [DPC] = 1.0 x10 4, [OH-] = 0.02, [L-Cit] = 1.0 x 418 nm, by recording the decrease in absorbance due to - - 10 3, I = 0.05 mol dm 3.
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