Inner-sphere oxidation of ternary iminodiacetatochromium(III) complexes involving DL-valine and L-arginine as secondary ligands. Isokinetic relationship for the oxidation of ternary iminodiacetato-chromium(III) complexes by periodate

Background In this paper, the kinetics of oxidation of [CrIII(HIDA)(Val)(H2O)2]+ and [CrIII(HIDA)(Arg)(H2O)2]+ (HIDA = iminodiacetic acid, Val = DL-valine and Arg = L-arginine) were studied. The choice of ternary complexes was attributed to two considerations. Firstly, in order to study the effect of the secondary ligands DL-valine and L-arginine on the stability of binary complex [CrIII(HIDA)(IDA)(H2O)] towards oxidation. Secondly, transition metal ternary complexes have received particular focus and have been employed in mapping protein surfaces as probes for biological redox centers and in protein capture for both purification and study. Results The results have shown that the reaction is first order with respect to both [IO4-] and the complex concentration, and the rate increases over the pH range 2.62 – 3.68 in both cases. The experimental rate law is consistent with a mechanism in which both the deprotonated forms of the complexes [CrIII(IDA)(Val)(H2O)2] and [CrIII(IDA)(Arg)(H2O)2] are significantly more reactive than the conjugate acids. The value of the intramolecular electron transfer rate constant for the oxidation of [CrIII(HIDA)(Arg)(H2O)2]+, k3 (1.82 × 10-3 s-1), is greater than the value of k1 (1.22 × 10-3 s-1) for the oxidation of [CrIII(HIDA)(Val)(H2O)2]+ at 45.0°C and I = 0.20 mol dm-3. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of IO4- to chromium(III). Conclusion The oxidation of [CrIII(HIDA)(Val)(H2O)2]+ and [CrIII(HIDA)(Arg)(H2O)2]+ by periodate may proceed through an inner-sphere mechanism via two electron transfer giving chromium(VI). The value of the intramolecular electron transfer rate constant for the oxidation of [CrIII(HIDA)(Arg)(H2O)2]+, k3, is greater than the value of k1 for the oxidation of [CrIII(HIDA)(Val)(H2O)2]+. A common mechanism for the oxidation of ternary iminodiacetatochromium(III) complexes by periodate is proposed, and this is supported by an excellent isokinetic relationship between ΔH* and ΔS* values for these reactions.


Background
Periodate oxidations have been reported to play an important role in biochemical studies [1,2]. They are used in the spectrophotometric determination of glucose and fructose in invert sugar syrups [1]. Alpha-amino acids in proteins can be determined by measuring the ammonia produced through oxidation with periodate in carbonate medium [2]. Periodate oxidation exerts a number of biological effects including the enhancement of lymphocyte activation and increased frequency of effecter to target cell binding [3]. Also, periodate has been used in the modification of human serum transferrin by conjugation to an oligosaccharide [4].
The biological oxidation of chromium from the trivalent to hexavalent states is an important environmental process because of the high mobility and toxicity of chromium(VI) [5]. Recently, Cr(III) oxidation to Cr(V) and/or Cr(VI) in biological systems came into consideration as a possible reason of anti-diabetic activities of some Cr(III) complexes, as well as of long-term toxicities of such complexes [6]. The specific interactions of Cr(III) ions with cellular insulin receptors [7] are caused by intra-or extracellular oxidations of Cr(III) to Cr(V) and/or Cr(VI) compounds, which act as protein tyrosine phosphatase (PTP) inhibitors. The oxidation of some Cr(III) complexes by H 2 O 2 in neutral or weakly basic aqueous media (pH 7.0-9.0), lead to polynuclear species formed on hydrolysis of Cr(III) complexes in neutral aqueous solutions [8]. The relative reactivities of various Cr(III) complexes towards H 2 O 2 may correlate with their reported activities as insulin activators [9]. The use of large doses [Cr(pic) 3 ] supplements may lead to improvements in glucose metabolism for type 2 diabetics; there is a growing concern over the possible genotoxicity of these compounds [10]. The current perspective discusses chemical transformations of Cr(III) nutritional supplements in biological media, with implications for both beneficial and toxic actions of Cr(III) complexes, which are likely to arise from the same biochemical mechanisms, dependent on concentrations of the highly reactive Cr (IV/V/VI) species, formed in the reactions of Cr(III) with biological oxidants [10].
Inner-sphere oxidation of ternary nitrilotriacetatocobalt(II) complexes involving succinate, malonate, tartarate and maleate as a secondary ligands by periodate has been investigated [21,22]. In all cases, initial cobalt(III) products were formed, and these changed slowly to the final cobalt(III) products. It is proposed that the reaction follows an inner-sphere mechanism, which suggested relatively faster rates of ring closure compared to the oxidation step.
Ternary complexes of oxygen-donor ligands and heteroaromatic N-bases such as iminodiacetic (IDA) acid and nitrilotriactic acid (NTA) with some transition metals have attracted much interest as they can display exceptionally high stability and may be biologically relevant [26,27]. The use of transition metal complexes of iminodiacetic 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 chromatography [28].

Materials and methods
All chemicals used in this study were of reagent grade (Analar, BDH, Sigma). Buffer solutions were prepared from NaCl and HCl of known concentration. NaNO 3 was used to adjust ionic strength in the different buffered solutions. Doubly distilled H 2 O was used in all kinetic runs. A stock solution of NaIO 4 (Aldrich) was prepared by accurate weighing and wrapped in aluminum foil to avoid photochemical decomposition [29].  18.42 respectively. To confirm the formula of the complexes, IR spectra and TGA data were carried out. In the IR spectra, bands in the (3565 -3382) cm -1 region, were attributed to ν (OH) of the water mole-cules. The OHof the carboxylic group disappeared and a new (νCOO -) appeared in the region (1467 -1426) 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-(νCOO-Co) band in the region (1586 -1649) cm -1 .  2 ] + by IO 4were followed spectrophotometrically for a definite period of time using the JASCO UV-530 spectrophotometer. All reactants were thermally equilibrated for ca 15 min. in an automatic circulation thermostat, thoroughly mixed and quickly transferred to an absorption cell. The oxidation rates were measured by monitoring the absorbance of Cr VI at 355 nm, on a Milton-Roy 601 spectrophotometer, where the absorption of the oxidation products is maximal at the reaction pH. The pH of the reaction mixture was measured using a Chertsey 7065 pH-meter. The temperature of the reacting solution was controlled, using an automatic circulation thermostat. The thermostat was provided with a special pumping system for circulating water at regulated temperature in the cell holder. The average stabilizing accuracy as measured in the thermostat liquid was ± 0.1°C Pseudo-first-order conditions were maintained in all runs by the presence of a large excess (> 10-fold) of IO 4 -. The ionic strength was kept constant by the addition of NaNO 3 solution. The pH of the reaction mixture was found to be constant during the reaction run.  Figures 1 and 2, respectively, was taken as the criterion for the presence of two absorbing species in equilibrium.

Stoichiometry
A known excess of Cr III complex was added to IO 4 solution, and the absorbance of Cr VI produced was measured at 355 nm after 24 hours from the onset of the reaction. The quantity of Cr III consumed was calculated using the molar absorptivity of Cr VI at the employed pH.

Test for free radical
In order to verify the presence of the free radicals in the reaction, the following test was performed. A reaction mixture containing acrylonitrile was kept for 24 hours in an inert atmosphere. On diluting the reaction mixture with methanol, since no precipitate was formed this suggests no possibility of free radical intervention in the reaction.  2Cr III + 3I VII → 2Cr VI + 3I V The ratio of I VII initially present to Cr VI produced was 1.50 ± 0.05. The stoichiometry is consistent with the observation that IO 3does not oxidize the Cr III -complex over the pH range where the kinetics were investigated.

Kinetics of oxidation of [Cr III (HIDA)(Val)(H 2 O) 2 ] +
Plots of ln (A ∞ -A t ) versus time were linear up to 85% from the beginning of reaction where A t and A ∞ are absorbance at time t and infinity, respectively ( Figure 3). Pseudo-firstorder rate constants, k obs , obtained from the slopes of Figure Figure 4), and the kinetics of the reaction are described by Equation (2): Values of constant (a) and (b) were obtained from the slope and intercept in Figure 4. Plots of 1/k obs versus 1/ [IO 4 -] at different pH values (2.62-3.68) ( Figure 5), show that the reaction rate increased as the pH increased over the range studied ( Table 2). The reaction rate is independent of the ionic strength when varied between 0.20 -0.50 mol dm -3 .  (1). The concentration of periodate initially present to chromium(VI) produced was found to be 3.0:2.0.

Kinetics of oxidation of [Cr III (HIDA)(Arg)(H 2 O) 2 ] +
First-order plots of ln (A ∞ -A t ) versus time were found to be linear for greater than 85% from the beginning of the reaction. Observed rate constants, k obs , obtained from the slopes of these plots, are collected in Table 3. The magnitude of the observed pseudo-first-order rate constant, k obs , was found to be independent of the chromium(III)-complex concentration as shown in Table 3. This indicates the pseudo-first-order dependence on complex concentration.
At constant [H + ] and ionic strength, 1/k obs varies linearly with 1/[IO 4 -] at different temperatures ( Figure 6), and the kinetics of the reaction are described by Equation (4).
At constant temperature 1/k obs varies linearly with 1/[IO 4 -] at different pHs (2.38 -3.34) (Figure 7), showing that the rate of reaction increases with increasing pH ( Table 4). The reaction rate is independent of the ionic strength when varied between 0.20 -0.50 mol dm -3 .

Discussion
The system which consists of a metal ion and more than one type of ligand is defined as a ternary complex such as  The assignment of an inner-sphere mechanism for this reaction seems to be supported by the fact that [IO 4 ]is capable of acting as a ligand as demonstrated by its coordination by copper(III) [31] and nickel(IV) [32], in which the coordinated H 2 O is substituted by I VII [20].
In acid medium the chromium(III)-complex is in equilibrium:   From Equation (16), it follows that the slope of the plots can be represented by Equation (18): It is clear from Equation (18)  In the case of [Cr III (HIDA)(Arg)(H 2 O) 2 ] + , the protonated and deprotonated forms of the chromium(III)-complex are involved in the rate-determining step. In acidic aqueous medium the chromium(III)-iminodiacetic acidarginine complex may be included in the equilibrium shown in Equation (19)     Thermodynamic activation parameters, ΔH* and ΔS* associated with constant (a) in Equation (2), were obtained from a least-squares fit to the transition state theory equation as 15.9 ± 1.2 kJ mol -1 and -277 ± 5 JK -1 mol -1 respectively.  2 ] + by periodate may proceed through an inner-sphere mechanism via one or two electron transfer giving chromium(IV) or chromium(V) respectively in the rate determining step leading to chromium(VI). The fact that acrylonitrile was not polymerized seems to support a two electron transfer process.
Kinetics of oxidation of Cr(III) to Cr(VI) in acidic medium (pH = 2-4) using periodate are biologically important [35]. Since carbohydrates reduce Cr(VI) to a significant extent only in strongly acidic media, it is likely that the first Cr(V) complexes formed in biological systems upon the addition of Cr(VI) are those of strong reductants, such as GSH or ascorbate [5].  [20].
The high negative entropies of activation for this reaction may be largely the result of the charge concentration on complex formation, which causes substantial mutual ordering of the solvated water molecules [36]. The intramolecular electron transfer steps are endothermic as indicated by the positive ΔH* values. The contributions of ΔH* and ΔS* to the rate constant seem to compensate each other. This fact suggests that the factors controlling ΔH* must be closely related to those controlling ΔS*.
Therefore, the solvation state of the encounter complex would be important in determining ΔH* [36]. The relatively low value of ΔH* for [Cr III (HIDA)(Arg)(H 2 O) 2 ] + is due to its composite value; including formation which may be exothermic and intramolecular electron transfer which may be endothermic.  Table 5. A plot of ΔH* versus ΔS* for these complexes is shown in Figure 8. The small change in the rates of the oxidation of iminodiacetatochromium(III) complexes (Table 5) are shown to arise from parallel changes ΔH* and ΔS*. Similar linear plots were found for a large number of redox reactions [38,39] and for each reaction series a common rate-determining step is proposed. An excellent linear relationship is seen; this isokinetic relationship lends support to a common mechanism for the oxidation of chromium(III) complexes, reported here, by periodate. This consists of periodate ion coordination to the chromium(III) complexes in step preceding the ratedetermining intramolecular electron transfer within the precursor complex.

Conclusion
The oxidation of [Cr III (HIDA)(Val)(H 2 O) 2 ] + and [Cr III (HIDA)(Arg)(H 2 O) 2 ] + by periodate may proceed through an inner-sphere mechanism via two electron transfer giving chromium(VI). The value of the intramolecular electron transfer rate constant for the oxidation of [Cr III (HIDA)(Arg)(H 2 O) 2 ] + , k 3 , is greater than the value of k 1 for the oxidation of [Cr III (HIDA)(Val)(H 2 O) 2 ] + . A common mechanism for the oxidation of a ternary iminodiacetatochromium(III) complexes by periodate is proposed, and this is supported by an excellent isokinetic relationship between ΔH* and ΔS* values for these reactions.