Mechanistic aspects of ligand substitution on [(H 2 O)(tap) 2 RuORu(tap) 2 (H 2 O)] 2 +  {tap=2-(m-tolylazo)pyridine} ion by three glycine-containing dipeptides in aqueous medium at physiological pH

Abstract

The interaction of the title complex with selected amino acids such as glycine (L 1 H), L-valine (L 2 H), and L-leucine (L 3 H) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], and temperature. The reaction has been monitored at 600 nm, where the spectral difference between the reactant and product is maximum. At pH 7.4, the reaction has been found to proceed via two distinct consecutive steps, i.e., it shows a nonlinear dependence on the concentration of ligands: The first process is [ligand] dependent, but the second step is [ligand] independent. The rate constants for the processes are k 1 ∼10 −3 s −1 and k 2 ∼10 −4 s −1 . The activation parameters were calculated from Eyring plots. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also