Kinetic Study of the Interaction of Three Glycine-Containing Dipeptides with Hydroxopentaaquarhodium(III) Ion in Aqueous Medium

The kinetics of the interaction of three glycine-containing dipeptides, namely, glycyl-L-valine (L 1 -L′H), glycyl-glycine (L 2 -L′H) and glycyl-L-glutamine (L 3 -L′H), with [Rh(H 2 O) 5 OH] 2+ has been studied spectrophotometrically in aqueous medium as a function of the Rh(H 2 O) 5 OH 2+ and dipeptide concentrations, pH and temperature, at constant ionic strength. At pH = 4.3, the substrate complex exists predominantly as the hydroxopentaaqua species and dipeptides as zwitterions. The reaction has been found to proceed via two parallel paths: both processes are ligand dependent. The rate constants for the processes are of the order: k 1 ∼10 −3 s −1 and k 2 ∼10 −5 s −1 . The activation parameters for both steps were evaluated from Eyring plots. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for both of the interaction processes. The low and values and large negative values of and support the associative mode of activation for both processes. The product of the reaction has been characterized using IR and ESI-mass spectroscopic analysis.