Kinetics and Mechanistic Studies of Oxidation of a Ternary Nitrilotri-Acetatocobalt(II) Complexes Involving DL-valine and DL-aspartic Acid as a Secondary Ligands by Periodate

Oxidation of ternary complexes, [CoII(NTA)(L)(H2O)X] -n by periodate in aqueous medium has been studied spectrophotometrically over the (25.0 – 45.0) ± 0.1°C range. The reaction show first order kinetics with respect to both [IO4 ] and the complexes, and the rate of the reaction increases over the [H + ] range (1.05 – 28.20) x 10 -5 mol dm -3 in both cases. Preparation and characterization of [Co II (NTA)(Asp)(H2O)2] -3 and [Co II (NTA)Val(H2O)2] 2is performed. Conformation of the formation of the ternary complexes has been done using IR spectrum, TGA, UV-visible spectroscopic and cyclic voltammetry measurements. The thermodynamic activation parameters have been calculated. It is assumed that electron transfer takes place via an inner-sphere mechanism.


Introduction
Amino acid residues are the main constituents of proteins and the study of its sensitivity towards oxidation open up a new area to understand the mechanism involved in the protein and amino acid modification [1].
The use of transition metal complexes of nitrilotriacetic acid have been widely adopted in biology, and are gaining increasing use in biotechnology, particularly in the protein purification technique known as immobilized metal-ion [2].
Ternary complexes of oxygen-donor ligands and heteroaromatic N-bases have attracted much interest as they can display exceptionally high stability and biological important [3,4].
Periodate oxidation exerts a number of biological effects including the enhancement of lymphocyte activation and increased the frequency of effecter to target cell binding [5]. Also, periodate was used in the modification of human serum transferrin by conjugation to oligosaccharide [6].
Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) by means of sodium periodate was reported, that mimics the mechanism of polyphenyleoxidase. The reaction leads to the formation of the antioxidant product 2-S-cysteinylecaffeic adduct which exhibits slightly improved antiradical activity in relation with the parent molecule (caffeic acid) [7]. Also, kinetics studies of periodate oxidations on a series of dextran oligomers, polymers, some dimeric carbohydrates [8] and of chitosans with different chemical composition [9] were investigated to show the dependence of the kinetics on the molecular weight.
Oxidation of the ternary complexes involving nitrilotriacetatocobaltate(II) succinic acid, [ 2 ] 3− (nta = nitrilotriacetate, ox = oxalic acid and ph = phthalic acid) by periodate have been studied kinetically in aqueous solution over 20-40°C and a variety of pH ranges [18], These reactions proceeded via the formation of initial cobalt (III) products, which were converted slowly into final cobalt (III) products fitting an inner-sphere mechanism.
A kinetic study of the oxidation of [Co(H 2 L)(H 2 O) 2 ] 2+ (H 2 L = N,N-bis (salicylaldehyde-1,2-diaminoethane) Schiff base) by periodate in aqueous solution was performed reaction was inhibited as the concentration of Cu(II) increased, and it was independent on Fe(II) concentrations over the ranges studied. An inner-sphere mechanism is proposed for the oxidation pathways of both the protonated and deprotonated Co II complex species [19].
The aim of the present study is to propose the most probable reaction path for the kinetics of oxidation of biologically important ternary nitrilotri-acetatocobalt(II) complexes involving DL-valine and DL-aspartic acid as a secondary ligands. [Co II (NTA)(L)(H 2 O) X ] -n where (NTA = nitrilotriactate acid, L= aspartic acid, Val = DL-valin, x= no of coordinated water molecule, and n = no of negative charge).
Choice of these ternary complexes was attributed to two considerations. Firstly, study the effect of aspartate and DLvalin as a secondary ligand on the stability of [Co II NTA(H 2 O) 2 ]towards oxidation and the reaction pathways. Secondly, study the effect of secondary ligand on the biological activity of NTA and [Co II (NTA (H 2 O) 2 ] -.

Materials and Methods
All reagent grade or analar chemicals were used without further purification. Co(NO 3 ) 2 (BDH) solutions were standardized volumetrically with EDTA [20]. A stock solution of (IO 4 )was made up by weight and covered with aluminum foil to avoid photochemical decomposition [21]. Solutions of NaOAC, HOAC, and NaNO 3 were prepared by weighing.
HOAC / NaOAC buffers of known [H + ] were used, and the ionic strength was adjusted with NaNO 3 .
Potentiometric measurements were performed with a Metrohm 702 SM titrino. The titroprocessor equipped with a 665 dosimat (Switzerland-Heriau). The titroprocessor and electrode were calibrated with standard buffer solution [22].
Calculations were performed using computer program MINIQUAD-75 loaded on an IBM-550 computer. The solution containing 5.0 ml 0.01 mol dm -3 complex, 5.0 ml 0.20mol dm -3 NaNO 3 , 5 ml 0.04 mol dm -3 HNO 3 and 25 ml deionized water, was titrated with 0.01 mol dm - IR spectra, thermal gravimetric analysis (TGA), UV Visible spectra and cyclic voltammetric data were carried out to confirm the formula of the complexes.
IR spectra show bands in the (3516 -3363) cm -1 region, were attributed to ν (OH) of the water molecules. The OH of the carboxylic group disappeared and a new (ν COO -) appeared in the region (1464 -1432) cm -1 indicating that the carboxylic group of the ligands participates in the coordination with the metal ions through deprotonation.
All the spectra of the complexes studied showed asym-(ν COO-Co) band in the region (1582 -1658) cm -1, the bands in the range (2928 -2967) cm -1 in the spectra, are due to ν (NH) as shown in Figures 1 and 2.     Confirmation of the formation of binary and ternary complexes in solution has been carried out by studying the electrochemical behavior ( Figures 5-8). Cyclic Voltametric measurements were operated using potentiostat / Galvaostate winking PGS 95 with single-compartment Voltametric cell equipped with a platinum working electrode (area = 0.5 cm 2 ). A platinum wire was used as counter electrode, and a SCE as reference electrode. A sample volume of 25 cm 2 containing the free metal ion 3 x 10 -3 mol dm -3 , ternary complex 3 x 10 -3 mol dm -3 . All solutions are investigated in water at 30°C, the solutions are purged with nitrogen for 120s, and the potential is scanned at the scan rate 25 MVs -1 from (+ 1.5 to -1.0) V.    Metal cobalt ion is difficulty oxidized at more positive potential, +732 mV vs. SCE, Figure (3), but the oxidation process of the binary system of Co(II)-nitrilotriacetate becomes slightly easy where the oxidation potential was shifted to more negative value, -375 mV vs. SCE, while in the case of ternary complex involving (aspartic with nitrilotriacetato-cobalt(II), the complex become more stable than binary complex, + 478 mV vs. SCE), but in case of ternary complex involving (valin with nitrilotriacetatocobalt(II), the complex become less stable than Co II (NTA)Asp(H 2 O) 2 ] 3-, because it oxidizes firstly, at +179 mV vs. SCE.

Kinetic Procedures
The U.V.    Milton-Roy 601 spectrophotometer equipped with a temperature cell holder and connected to a thermo-circulating water bath, was used to measure the oxidation rates by monitoring the absorbance of initial Co III complexes absorbance at 572 and 558 nm, respectively. All reactants, except IO 4 -, were equilibrated at required temperature for 15-20 min. The required amount of separately thermostated IO 4 stock solution was rapidly mixed, and then the recording of absorbance was commenced.
The [H + ] of the reaction mixture was measured using a Chertsey, Surrey, 7065 pH-meter.
Pseudo-first order conditions were maintained in all runs by the presence of a large excess of IO 4 -, (> 10-fold). The ionic strength was kept constant by addition of NaNO 3 solution. The [H + ] of the reaction mixture was found to be always constant during the reaction run.
The error limits for results are calculated using Microcal ™ Origin ® (Version 6.0).    Plots of ln (A α -A t ) versus time were linear up to 85% of reaction where A t and A α are absorbance at time t, and infinity, respectively. Pseudo-first order rate constants, k obs , obtained from the slopes of these plots, Table (2 & 3). These data show that, k obs , was unaffected when the concentration of the Co II -complex was varied at constant (IO 4 )concentration indicating first order dependence on the complex concentration. Plots of k obs against [IO 4 -] were found to be linear, (Figures  13 and 14). The dependence of k obs on [IO 4 -], Table (2 & 3)., is described by

Kinetics
The kinetics of the reaction was studied over [H + ] range (1.05 -28.20) x 10 -5 mol dm -3 at different temperatures  (Figures 15 and 16). This behavior can be described by equation 3.
Values of k 2 and k 3 were calculated at different temperatures, and were listed in Table (6 & 7).

Discussion
The lability of cobalt(II) reactants and the inertness of cobalt(III) products could be utilized as a diagnostic tool for ascertaining inner-sphere electron transfer [10,[13][14][15]. Oxidation of some cobalt(II) complexes by periodate, where an initial cobalt (III) product was observed, was interpreted in terms of this mechanism.
An inner-sphere mechanism is proposed for the bath way of oxidation of both two Co II complexes Co II (NTA)Asp(H 2 O) 2 ] 3and [Co II (NTA)Val(H 2 O) 2 ] 2-,. This proposal seems to be supported for the following reasons: Formation of the initial cobalt (III) products, which were slowly converted to the final cobalt products [13,15,24]. Periodate ion is capable of acting as a ligand, as evidenced from its coordination to copper(III) [25] and nickel [26].
K 1 and K 2 were potentiometrically determined as 1.38 x 10 -6 and 5.01 x10 -7 , respectively. It was observed from values of K 1 and K 2 at the employed pH range that these equilibriums were not prevailing and the protonated Co IIcomplex species was the predominant.
In aqueous solutions, periodate species are found in the equilibria [27]. -. The hydrogen ion dependence of the reaction rate of both complexes is in agreement with the involvement of both deprotonated and protonated forms of periodate in the rate determining steps, proceeded by a rapid deprotonation equilibrium and in which both forms are reactive.
The concomitant reaction rate increase with the increase of ionic strength may be attributed to that the reaction rate, in the rate determining step, occurred between two species of the same sign.
In view of the above considerations, kinetics of oxidation of [Co II (NTA)L(H 2 O) x ] -n by periodate proceeds by one firstorder pathway in each reactant. The mechanism could be described by the following equations (10)(11)(12)(13)(14).  (13) From the above mechanism, the rate of the reaction can be described by Equation (14) Comparison Equations (16) and (2), gives k 1 = k 10 K 6 + (k 11 K 7 K 3 / [H + ]) Values of k 2 and k 3 were obtained by comparison of Equations (17) and (3), as follows: k 2 = k 10 K 6 and k 3 = (k 11 K 7 K 3 ) The initial cobalt(III) product of oxidation of [Co II (NTA)L(H 2 O) x ) -n ] by IO 4 may be converted to final products according to the equation (18) According to crystal field theory, Co +2 has d 7 distributed as (t 2g ) 6 and (e g ) 1 . Increase of crystal field energy ∆ o between t 2g and e g orbital, by strong field ligands, labilize the free electron in e g . Values of pK a for valine and aspartic acid are 2.3 and 1.88 respectively, i.e. strength of conjugate bases are in the sequence Asp > Val. Therefore ∆ o for aspartate ligand is more than both valinate. Thus, loss of e g electron from the central Co +2 ion is easier in case of [Co II (NTA)Val(H 2 O) 2 ] 2than Co II (NTA)Asp(H 2 O) 2 ] 3-. This is consistent with the order of stability of these complexes.
The high negative entropies of activation for this reaction were climbed to be largely the result of the charge concentration on encounter complex formation, which causes substantial mutual ordering of the solvated water molecules [28]. The intramolecular electron transfer steps are endothermic as indicated by the value of ∆H * . The contributions of ∆H* and ∆S* to the rate constant seem to compensate each other. This fact suggests that the factors controlling ∆H* must be closely related to those controlling ∆S*. Therefore, the solvation state of the encounter complex would be important in determining ∆H. Thus, the relatively small enthalpy of activation, ∆H*, can be explained in terms of the formation of more solvated complex [29].
Oxidation of Co II complexes shows the formation of an initial Co III product. This may be due to the oxidation process being inner sphere. This is consistent with inner sphere oxidation which is generally proposed for IO 4 -reactions. Through transformation of initial to final Co III product an I VI in the initial product is probably substituted by a water molecule with a slow rate due to inertness of Co III and Co II -OIO 3 bond is being stronger than Co-H 2 O bond. (2)-The using of Mn ++ as a catalyst in the reaction mixture is responsible for the oxidation of the complexes.

Conclusion
( ] as evidenced from values of electron transfer rate constant for the two complexes. (5)-The high negative entropies of activation for which reaction were climbed to be largely the result of the charge concentration on the formed encounter complex, which causes substantial mutual ordering of the solvent molecules of the solvated complex. The intramolecular electron transfer steps are endothermic as indicated by the value of ∆H*.
The contributions of ∆H* and ∆S* to the rate constant seem to compensate each other. This fact suggests that the factor controlling ∆H* must be closely related to those controlling ∆S*. Therefore, the solubility state of the encounter complex would be important in determining ∆H*. Thus, the relatively small enthalpy of activation, ∆H*, can be explained in terms of the formation of more solved complex.