New gold pincer-type complexes: synthesis, characterization, DNA binding studies and cytotoxicity

With the aim of assessing whether Au( iii ) compounds with pincer type ligands might be utilized as potential antitumor agents, three new monofunctional Au( iii ) complexes of the general formula [Au( N - N' - N )Cl]Cl 2 , where N - N' - N = 2,6-bis(5- tert -butyl-1 H -pyrazol-3-yl)pyridine (H 2 L t Bu , 1 ), 2,6-bis(5- tert -butyl-1-methyl-1 H -pyrazol-3-yl)pyridine (Me 2 L t Bu , 2 ) or 2,6-bis((4 S ,7 R )-1,7,8,8-tetramethyl-4,5,6,7-tetrahydro-1 H -4,7-methanoindazol-3-yl)pyridine (Me 2 *L, 3 ) were synthesized. All complexes were characterized by elemental analysis, spectroscopic techniques (IR, UV-Vis, 1D and 2D NMR) and mass spectrometry (MALDI TOF MS). The chemical behavior of the complexes under physiological conditions was studied by UV-Vis spectroscopy, which showed that all compounds were remarkably stable and that the gold center remained in the 3+ oxidation state. The kinetics and the mechanism of the reaction of complexes 1-3 with guanine derivatives ( i.e. guanosine (Guo) and guanosine-5′-monophosphate (5′-GMP)) and calf thymus DNA (CT DNA) were studied by stopped-flow spectroscopy. The three complexes displayed moderately different rate constants in their reactions with Guo, 5′-GMP and CT DNA, which can be explained by the steric hindrance and σ-donicity of the methyl substituent on the bis-pyrazolylpyridine fragment in complexes 2 and 3 . The measured enthalpies and entropies of activation (Δ H ≠ > 0, Δ S ≠ < 0) supported an associative mechanism for the substitution process. The interaction of the newly synthesized complexes 1-3 with CT DNA was investigated by UV-Vis and fluorescence spectroscopy, and also by viscosity measurements, which all indicated that complexes 1-3 bound to CT DNA with moderate binding affinity ( K b = 1.6-5.7 × 10 3 M −1 ) and stabilized the duplex of CT DNA. Molecular docking indicated that complexes 1-3 interacted with DNA via intercalation. Complex 1 reduced the cell survival of all the investigated cell lines (A549, A375, and LS-174) with IC 50 values being up to 20 μM. We have shown that 1 induced perturbations of the cell cycle and led to apoptosis in human melanoma A375 cells. Complex 1 also affected the level of reactive oxygen species (ROS) in the same cells. However, pre-treatment of A375 cells with NAC (ROS scavenger) reversed the effect of 1 on their survival. The complex [Au(H 2 L t Bu )Cl]Cl 2 ( 1 ) induced perturbations of the cell cycle and led to apoptosis in human melanoma A375 cells.