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 glycine-containing dipeptides(L-L′H) such as glycyl-glycine(L-L1H), glycyl-L-alanine (L-L2H) and glycyl-L-leucine(L-L3H) 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 a maximum. At pH 7.4, the interaction with studied dipeptides shows two parallel steps. i.e., it shows a non-linear dependence on the concentration of dipeptides; both processes are ligand-dependent. The rate constants for the processes are: k1 ~10 − 3 s − 1 and k 2 ~10 − 5 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 calculated. The product of the reaction has been characterized by IR and ESI-mass spectroscopic analysis. The kinetics of the interaction between [(H2O)(tap)2RuORu(tap)2(H2O)]2+ {tap=2-(m-tolylazo)pyridine} ion and glycine-containing dipeptides such as glycyl-glycine, glycyl-L-alanine and glycyl-L-leucine has been studied spectrophotometrically in aqueous medium as a function of nucleophile concentration, temperature and pH at constant ionic strength. The rate and activation parameters, ESI-mass spectra and IR data were used to deduce a plausible mechanism.