Kinetics of substitution of aqua ligands from <i>cis</i>-diaqua(<i>cis</i>-1,2 diaminocyclohexane)platinum(<b>II</b>) perchlorate by glutathione in aqueous medium

Abstract

The kinetics of the interaction of glutathione (reduced) (GSH) with [Pt(DACH)(H₂O)₂]²⁺ (DACH = <i>cis-</i>1,2<i>-</i>diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(DACH)(H₂O)₂²⁺], [glutathione] and<i> </i>temperature at a particular <i>p</i>H (4.0). The reaction has been found to proceed via rapid outersphere association complex<i> </i>formation followed by two slow consecutive steps. The first step involves the transformation of the outersphere complex into<i> </i>the innersphere complex containing Pt-S bond and one aqua ligand while the second step involves chelation when the<i> </i>second aqua ligand is displaced. The association equlibrium constant (<i>K_E</i>)<i> </i>and the two rate constants <i>k₁</i>and <i>k₂ </i>have been<i> </i>evaluated. Activation parameters for both the steps have been calculated using Eyring equation(Δ<i>H^#₁</i> <i> </i>= 32.87 ± 1.31 kJ<i> </i>mol^-1, Δ<i>S^#₁</i> <i> </i>= -187.24 ± 4.2 JK^-1 mol^-1, Δ<i>H^#₂ </i> = 30.50 ± 0.09 kJ<i> </i>mol^-1, Δ<i>S^#₂</i> <i> </i>= - 223.05 ± 0.28 JK^-1 mol^{-1 .}The low enthalpy<i> </i>of activation and large negative values of entropy of activation indicate an associative mode of activation for both steps.