Abstract
Background: In this paper, the kinetics of oxidation of [Cr-III(HIDA)(Val)(H2O)(2)](+) and [Cr-III(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 [Cr-III(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 [Cr-III(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)](+), k(3) (1.82 x 10(-3) s(-1)), is greater than the value of k(1) (1.22 x 10(-3) s(-1)) for the oxidation of [Cr-III(HIDA)(Val)(H2O)(2)](+) at 45.0 degrees 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 [Cr-III(HIDA)(Val)(H2O)(2)](+) and [Cr-III(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 [Cr-III(HIDA)(Arg)(H2O)(2)](+), k(3), is greater than the value of k(1) for the oxidation of [Cr-III(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 Delta H* and Delta S* values for these reactions.