Rate and Equilibrium Data for Substitution Reactions of [Pd(dien)Cl]⁺ with L-Cysteine and Glutathione in Aqueous Solution

Summary.

The kinetics of the complex formation reactions between monofunctional palladium(II) complex, [Pd(dien)Cl]⁺, where dien is diethylene triamine or 1,5-diamino-3-azapentane, with L-cysteine and glutathione were studied in an aqueous 0.10 M perchloric acid medium by using variable stopped-flow spectrophotometry. Second-order rate constants, <$>{k_2}^{298}<$>, were (3.89±0.02) 10² M⁻¹s⁻¹ for L-cysteine and (1.44±0.01) 10³ M⁻¹s⁻¹ for glutathione. The negative entropies of activation support a strong contribution from bond formation in the transition state of the process. The hydrolysis of Pd^II complex gave the monohydroxo species, [Pd(dien)(OH)]⁺ and the dimer with a single hydroxo-bridge species, [Pd₂(dien)₂OH]³⁺. L-Cysteine and glutathione ligands form complexes of 1:1 stoichiometry and a dimer with a single ligand bridge. The formation constants of the complexes were determined, and their concentration distribution as a function of pH was evaluated.