Gold(III) Complexes Containing N-Heterocyclic Carbene Ligands: Thiol 'Switch-on' Fluorescent Probes and Anti-Cancer Agents

摘要

Cellular thiol levels are linked to many diseases, including cancer and AIDS. Soft metal ions and their complexes are usually sought after in the design of metal-based thiol probes and new anti-cancer drug leads that specifically target proteins or enzymes containing SH or SeH moieties. Au~I is one of the soft metal ions with a high binding affinity towards thiol, and can be generated by the reduction of Au~(III). Indeed, a majority of Au~(III) complexes are known to be readily reduced to Au~I or even Au~0 by cellular thiol-containing compounds. As Au~(III) is usually 4-coordinated, whereas Au~I is usually 2-coordinated, the reduction of Au~(III) to Au~I is accompanied by the release of the coordinated ligand(s). Thus, we conceived that Au~(III) complexes containing strongly fluorescent organic ligand(s) could be potentially useful biological probes. The intraligand emission would be quenched owing to the presence of the low energy Au~~(III) 5d_(x~2_y~2) orbital. Upon reduction of Au~(III) to Au~I by thiols, the fluorescent ligand(s) will be released, and hence emission would be switched on.