Reactivity of hydroxopentaaquarhodium(III) ion towards glycine-containing dipeptides in aqueous medium

Department of Chemistry, The University of Burdwan, Burdwan-713 104, West Bengal, India E-mail : alakghosh2002@yahoo.co.in Manuscript received online 07 May 2012, revised 10 June 2012, accepted 13 June 2012 The kinetics of the interaction of two glycine-containing dipeptides, namely, glycyl-L-leucine (L1-L'H) and glycyl-L-isoleucine (L2-L'H) with [Rh(H2O)5OH]2+ has been studied spectrophotometrically in aqueous medium as a function of [Rh(H2O)5OH2+], [dipeptide], pH and temperature at constant ionic strength. At pH 4.3, where the substrate complex and dipeptides exists predominantly as hydroxopentaaqua species and zwitter ion respectively. The reaction has been found to proceed via two parallel paths : both processes are ligand dependent. The rate constant for the processes are : k1 ~10-3 s-1 and k2 ~10-5 s-1. The activation parameters for both the steps were evaluated using Eyring's equation. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for both the interaction processes. The low ∆H1# and large negative value of ∆S1#-. as well as AH1# and AS1# indicate an associative mode of activation for both the aqua ligand substitution processes for both the parallel paths. The product of the reaction has been characterized, as IR and ESI-mass spectroscopic analysis