Kinetics and mechanism of interactions of some monofunctional Au(III) complexes with sulphur nucleophiles

Abstract

Kinetics of the substitution reactions between monofunctional Au(III) complexes, [Au(dien)Cl]2+, [Au(bpma)Cl]2+ and [Au(terpy)Cl]2+ (dien = 3-azapentane-1,5-diamine, bpma = di-(2-picolyl) amine, terpy = 2,2′;6′,2″-terpyridine), and biologically relevant sulphur ligands, namely glutathione (GSH), l-methionine (l-Met) and l-cysteine (l-Cys), were studied in 0.1 M HCl (pH = 1.0). The reactions were followed under pseudo-first-order conditions as a function of ligand concentration and temperature using stopped-flow spectrophotometry. The [Au(terpy)Cl]2+ complex proved to be more reactive than the [Au(bpma)Cl]2+ and [Au(dien)Cl]2+ complexes. The reactivities of the nucleophiles follow the same order for all three complexes, viz. l-Met > GSH > l-Cys. Values of the activation parameters of the reactions support an associative substitution mechanism. In order to confirm that these monofunctional Au(III) complexes undergo a single substitution process in strongly acidic medium, the reaction between [Au(terpy)Cl]2+ and l-Met was studied by HPLC. At pH = 1.0, only one reaction product was detected.