Electronic properties of gold(I) compounds - Relevance to chemical reactions

The chemical reactivity of Au(I) compounds is related to their HOMO-LUMO energy separation. For coordinately unsaturated complexes of Au(I), the excited state (HOMO-LUMO separation) is only about 2 eV above the ground state. Many Au(I) complexes such as [Au(TPPTS)3]8- and [Au(TPA)3]+ show visible luminescence from this state even in water solution. Quenching of the phosphorescence relates to the rate of reaction of the substrate with the gold complex. Dioxygen and radicals such as NO rapidly react with the Au(I) to quench the excited state, as do halocarbons capable of electron and/or atom transfer. Although Corey and Khan suggested that a plausible mechanism for gold drug behavior in treatment for rheumatoid arthritis, RA, might be quenching of singlet oxygen, our work demonstrates that NO and O2- (which leads to singlet oxygen) each react rapidly to quench emission from Au(I). Drug action may relate to the prevention of build-up in cells by Au(I) of reactive, cytotoxic peroxynitrite, O2NO-, formed quickly from O2- and NO in cells.