New gold carbene complexes as candidate anticancer agents.

Three structurally related gold(I) carbene complexes with bulky hydrophobic ligands i.e. 1-3 were investigated in solution for further consideration as candidate anticancer agents. Cytotoxic assays were subsequently conducted on bone marrow-derived preosteoclast cell line of human origin (FLG 29.1) and human colon cancer cells (HCT-116). A far greater cytotoxic activity was measured for compound 1 against HCT-116 cells compared to 2 and 3; conversely, all compounds were highly and similarly active against FLG 29.1 cells. Results obtained for the reaction of complexes 1 and 2 with RNase A documented the occurrence of a weak interaction with this model protein and the formation of a tiny amount of the corresponding adduct. Moreover, a certain reactivity of the complex 2 was also detected toward GSH. The general implications of the obtained results are discussed.

Studies on the reactions of [AuCl4](-) with different nucleophiles in aqueous solution.

In order to distinguish between the different types of reactions that can occur between Au(III) species and simple nucleophiles, including iodide, bromide, nitrite, thiourea, pyridine and dimethyl sulfoxide, spectrophotometric techniques including stopped-flow and rapid-scan measurements were employed under specific reaction conditions. All experiments were performed in a 0.4 M NaCl aqueous solution to maintain a high chloride concentration and a constant ionic strength. The temperature dependence of the observed rate constants confirmed the associative nature of the ligand substitution reactions. The redox behaviour of the Au(III) species was studied by cyclic voltammetry and confirmed the reversible redox transitions at ca. 0.38 V (SCE, E = 0.1 V s(-1)). Results obtained during the reaction progress were attributed to the formation of Au(0). This oxidation state was observed for the reactions with thiourea, iodide and nitrite, whereas pyridine showed a potential shift only to Au(i) formation, while bromide showed potential shifts typical of ligand substitution reactions. The reaction with dimethyl sulfoxide was studied using (1)H NMR and ab initio (RMP2(full)/LANL2DZp) techniques, which revealed why Au(III) does not react with sulfoxide. The results are discussed in terms of the importance of the stability of the Au(III) species in aqueous solutions of the selected salts and bases. In this way, one could differentiate between a possible three-electron inner-sphere redox process and/or a substitution process during the rapid initial step of the reactions.

Substitution versus redox reactions of gold(III) complexes with L-cysteine, L-methionine and glutathione.

The influence of tridentate, nitrogen donor ligands, on the stability of gold(III) complexes under physiological conditions was investigated. The interaction of [Au(terpy)Cl](2+) (terpy = 2,2':6'2'' terpyridine), [Au(bpma)Cl](2+) (bpma = bis(pyridyl-methyl)amine), [Au(dien)Cl](2+) (dien = diethylenetriamine) and [AuCl4](-) with the biologically relevant thiols, L-cysteine (L-Cys) and glutathione (GSH), and thioether, L-methionine (L-Met), was studied using UV-Vis spectroscopy, cyclic voltammetry, (1)H NMR spectroscopy and ESI-MS. In this study, the rate constants for substitution reactions between monofunctional gold(III) complexes and sulfur donor ligands in aqueous solution were determined at different initial concentrations of reactants, chloride ions, pH and constant ionic strength. The obtained second-order rate constants for the reaction with L-methionine in the absence of added chloride at pH 2.5 and 25 °C follow the sequence (7.5 ± 0.4) × 10(3) > (4.5 ± 0.1) × 10(2) > 88.3 ± 0.8 M(-1) s(-1) for the terpy, bpma and dien complexes, respectively, demonstrating that the substitution step could be detected prior to the reduction step. This behavior was expected due to the influence of a decreasing π-donor ability of the chelate ligands, which slows down the substitution reactions along the series of complexes studied. In order to throw more light on the mechanism of biological activity of gold(iii) compounds, such a systematic study was performed for all the mentioned thiols and thioether.