Kinetic and Mechanistic Study of the Interaction of L-cysteine and N-acetylcysteine with cis-[Pt(en)(H2O)2](ClO4)2 in Aqueous Medium

Abstract

The kinetics of the reactions of L-cysteine and N-acetylcysteine with cis-[Pt(en)(OH 2 ) 2 ] 2+ have been studied spectrophotometrically as a function of [Pt(en) (OH 2 ) 2 ] 2+ T , [L-cysteine] T /[N-acetylcysteine] T and temperature at pH (4.0) where the Pt(II) complex exists predominantly as a diaqua species and L-cysteine and N-acetylcysteine as zwitterions. The substitution reaction shows two consecutive processes; the first step is the ligand assisted anation and the second one is the chelation step. The anation rate constants for N-acetylcysteine were found to be greater than that for L-cysteine. H # 1 (61.6 ± 1.8 kJ mol −1 ) H # 2 (47.5 ± 2.0 kJ mol −1 ), S # 1 (−54.4 ± 5.3 J.K −1 mol −1 ) and S # 2 (−164.8 ± 6.5 J.K −1 mol −1 ) for L-cysteine and H # 1 (23.2 ± 0.8 kJ mol −1 ), H # 2 (61.5 ± 0.6 kJ mol −1 ) and S # 1 (−178.6 ± 2.6 J.K −1 mol −1 ), S # 2 (−115.6 ± 2.0 J.K −1 mol −1 ) for N-acetylcysteine indicate an associative mode of activation for both the ligand substitution processes in the two consecutive steps.