1. Introduction:

Oxidation of amino acids has received considerable attention so far. L-Arginine (Arg), an essential amino acid, is needed to remove toxic ammonia from the body and also plays an important role in cell division, immune function and in the release of hormones. There have been only few investigations on the oxidation of arginine using oxidants like Chloramine-T^1,2, Bromamine-T³, hexacyanoferrate (III)⁴, alkaline per- manganate⁵, diperiodatonickelate (IV) (DPN)⁶, N-bromo succinimide⁷, Mn(III)^8,9, quinquevalent vanadium^10,11, metalcatalysed alkaline permanganate¹², N-chloro- nicotinamide¹³ and N-chlorosaccharin¹⁴.

Kinetic studies are important sources of mechanistic information on the reactions, as demonstrated by the results referring to unsaturated acids both in aqueous and non-aqueous media. Thus in order to explore the mechanism of oxidation by KMnO₄in alkaline medium and to check the selectivity of L-Arginine towards MnO₄^- we have carried out kinetic investigation on Ru(III) catalysed oxidation of L- Arginine by KMnO₄in alkaline medium. Since the reaction between arginine and permanganate was found to be very fast at 35º C and in 0.05mol dm^-3[alkali] hence it is carried out at three different temperature between 20 to 30 ºC.

Ruthenium (III) is known to be an effective catalyst in several redox reactions, particularly in alkaline medium. The mechanism can be quite complicated in presence of catalyst due to the formation of different intermediate complexes, free radicals and different oxidation state of ruthenium.

The kinetics of fast reactions between ruthinate (VII), RuO₄^- and permanganate (VI),MnO₄²^- has been studied ¹⁵.The reaction is presumed to proceed via an outer sphere mechanism. The rapid exchange between MnO₄^2- and MnO₄^- has been studied in detail by variety of tichniques¹⁶. Herein we describe the result of the title reaction in order to determine the active species of oxidant, reductant and catalyst in the given media and to arrive at a plausible mechanism.

MATERIALS AND METHODS:

The standard solution of L-Arginine (E-Merck) was prepared afresh by using double distilled water. A 0.01 mol dm^-3 solution of KMnO₄ (B.D.H) was prepared by dissolving the requisite amount of the salt in doubly distilled water. KMnO₄ solution was standardized with the help of standard oxalic acid solution in acidic medium. The other chemicals used were sodium hydroxide and NaClO₄. All chemicals used were of AR grade. [NaClO₄] was determined using cation exchanger.

The reaction was initiated by mixing previously thermostated solutions of permanganate, L-Arginine with sodium hydroxide and sodium perchlorate to maintain required alkalinity and ionic strength respectively. The temperature was uniformly maintained at suitable temperature t (where t=20, 25, 30).The progress of the reaction was followed by measuring the absorbance of permanganate (Fig.1) at 525 nm using a 1cm qurtz cell in a CECIL-7200 UV-Visible spectrophotometer.

Repetitive spectral scan of Ru(III) catalysed reaction of L-Arginine with alkaline KMnO₄.(1)-[KMnO₄]₌2 x 10^-4 mol.dm^-3,[L-Arginine]_T=2 x 10^-3 mol.dm^-3,[OH^-] = 5 x 10^-2 mol.dm^-3, [Ru (III)]T=1 x 10^-7 mol.dm^-3, I=0.5mol.dm^-3 ,temp=30^oC at (1) 1 minute,(2) 5 minutes,(3) 10 minutes,(4) 15 minutes,(5) 20 minutes,(6) 25 minutes.(7)30 minutes.

Stochiometry and Reaction product:

Known amounts of L-Arginine was allowed to react completely with a known excess of permanganate at 20°C in 0.05mol dm^-3NaOH at an ionic strength of 0.5 mol dm^-3. The remaining permanganate was then analyzed spectrophotometrically. As per these results the stoichiometry was found to correspond to the equations, as represented below.

L-Arginine + Permanganate +OH^-

→ Aldehyde + manganate +Ammonia +CO₂

R-CH(NH₂)-COO^-+2MnO₄^-+2OH^-

→ R-CHO +2MnO₄⁼ +NH₃+CO₂

Where R= (NH=C-NH -CH₂CH₂CH₂)

NH₂

The reaction product were identified as aldehydes1⁷ by boiling point, spot tests and manganate by its visible spectrum and ammonia was identified by Nessler’s reagent. The product aldehyde was quantitatively estimated to about 80%, which is evidenced by its 2,4-DNP derivative¹⁸.The nature of the aldehyde was confirmed by its IRspectrum¹⁹, 3462.56(s) cm^-1 and 1617(w) cm^-1 band may be due to H₂O in trace amount in KBr . Carbonyl stretching at 1716.34 cm^-1 indicates the presence of –CHO group in the product. (fig: 2).

RESULTS AND DISCUSSION:

The effect of variation of [L-Arginine], permanganate, ionic strength is tabulated (Table-1).

The effect of alkali on the rate of the reaction was studied at constant concentrations of L-Arginine and permanganate at a constant ionic strength of 0.5mol dm^-3 at 20^oC. The rate constants obtained were found to increase with the increase in [alkali] (Table-2).

The ruthenium (III) concentration was varied in 2x 10^-7 to 6 x 10^-7 mol.dm^-3 range. The rate constants increased with increase in ruthenium (III) concentration when the concentration of other reactants were constant. This indicates the unit order dependence of rate in [Ruthenium(III)]_T . (Table-3).

The effect of temperature on the rate of the reaction was studied by carrying out the reaction at three different temperatures 20, 25, 30ºC respectively. The plot of log (k /T) verses 1/T was found to be linear indicating that the reaction obeys Eyring equation. ΔH^# and ΔS^# were computed from Eyring equation and datas are collected in Table: 4.

Table:1 Effect of variation of [MnO₄^-], [L-Arginine], and [OH^-] on Ruthenium(III) catalysed oxidation ofL-Arginine by KMnO₄ in aqueous alkaline medium at 25^oC , I=0.5 mol.dm^-3and [Ru(III)]_T =1 x 10^-7mol.dm^-3

10⁴[MnO₄^-] (mol.dm^-3)	10³[L-Arginine] (mol.dm^-3)	[OH^-] (mol.dm^-3)	10⁴k_{obs /}s^-1
2	1	0.05	4.25
2	2	0.05	5.73
2	3	0.05	6.48
2	4	0.05	7.68
2	5	0.05	8.95
3	1	0.05	6.45
4	1	0.05	6.39
5	1	0.05	6.27
6	1	0.05	6.33
7	1	0.05	6.41
2	1	0.03	3.81
2	1	0.07	5.24
2	1	0.09	6.38
2	1	0.10	7.44

Table:2 Effect of variation of [OH^-] at different concentration of [L-Arginine], at 20,25 and 30^oC,[ KMnO₄] =1 x10^-4mol. dm^-3, Ru(III)= 1 x10^-7mol.dm^-3and I=0.5 mol.dm^-3.

[OH^- ] mol.dm^-3	10³[L-Arginine] (mol.dm^-3)	10⁴k_obs/ s ^-1) 20^oC 25^oC 30^oC
0.03	2	2.29 3.38 4.32
0.03	3	4.01 5.68 7.77
0.03	4	5.08 9.57 10.83
0.05	2	2.92 4.79 5.13
0.05	3	5.12 6.65 10.30
0.05	4	7.38 12.89 13.87
0.07	2	3.45 5.29 7.72
0.07	3	6.58 10.17 12.35
0.07	4	9.87 15.99 18.33
0.09	2	4.18 6.48 8.22
0.09	3	7.88 11.95 15.87
0.09	4	12.29 17.27 19.98

Table:3 Effect of variation of [Ru(III)] at 30^oC at fixed [L-Arginine], [ KMnO₄^-] , [OH]^-(I=0.5 mol.dm^-3)

10⁴[MnO₄^-]

(mol.dm^-3)

10³[L-Arginine] (mol.dm^-3)

[OH^-]

(mol.dm^-3)

10⁷[Ru(III)] (mol.dm^-3)

10⁴k_obs

(s^-1)

0.05

3.49

4.56

5.64

6.11

7.29

Table: 4 Value of k,K₂ and activation parameters at various temperature.

Amino Acid

Temp(^oC)

k x 10³

(dm³.mol^-1s^-1)

K₂ x10^-4

(dm³.mol^-1)

ΔH^≠

(kJ/mol)

ΔS^≠

(JK^-1/mol)

Arginine

5.41

7.75

11.4

1.15

3.16

6.94

52.37

-109.46

Arginine is an essential amino acid and has three pKa²⁰values. One of them corresponding to the carboxylic group (pK₁ = 2.17), and the other two for amino (pK₂ = 9.04) and guanidinium groups (pK₃ = 12.48).

Under the present experimental conditions, at a [OH^-] of 0.05mol dm^-3 L-Arginine exists in the form of anionic species, Arg- to the extent of 98.5% and as neutral species, Arg_z to the extent of 1.5%.

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 are active 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 result suggests the formation of a complex between L-Arginine and the hydroxylated ruthenium species. Such complex formation between substrate and catalyst has also been observed in earlier work. Evidence is provided by the fractional order found on [amino acid].The formation of the complex was also proved kinetically by the nonzero intercept of the plot of [Ru(III)] / k_obs verses 1/ [L-Arginine].

The existence of two isobestic points in the UV-Vis spectrum of permanganate in alkaline medium indicates the presence of two equilibrium steps before the slow step of the mechanism. Since scheme -1 is in accordance with the generally well accepted principle of non-complementary oxidations taking place in sequence of one electron steps, the reaction between the substrate and oxidant would provide a radical intermediate .This type of radical intermediate has also been observed in earlier work on the alkaline permanganate oxidation of amino acids .In agreement with the experimental results obtained, a mechanism as in scheme-1 may be delineated. The probable structure of complex C is:

Based on the mechanism as described in Scheme-1, rate law for the reaction can be written as :

(1)

According to equation (2), the plots of [Ru(III)] /k_obsversus 1/ [L-Arginine] (r >0.9988) and [Ru(III)]/k_obs verses 1/[OH-] (r>0.9913, s<0.046) is linear. The slopes and intercepts of the plots lead to the values of, K₂ and k which were collected in table 4. Using the temperature variation of k (rate constant), activation parameters were determined by using Eyring equation (please see Table:4).

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Received on 26.07.2015 Modified on 10.08.2015

Research J. Science and Tech. 7(4):Oct. – Dec. 2015; Page 230-233

DOI: 10.5958/2349-2988.2015.00033.9