Ultrasensitive fluorometric glutathione assay based on a conformational switch of a G-quadruplex mediated by silver(I)

The authors describe a silver(I) mediated fluorescent assay for glutathione (GSH). An allosteric oligonucleotide strand containing a G-rich sequence is used to produce a G-quadruplex, and N-methylmesoporphyrin IX (NMM) is chosen as the fluorescent probe. In the absence of Ag(I), the DNA strand is partially intramolecularly hybridized to form a hairpin structure wherein the G-rich sequence is partially caged. On addition of Ag(I), the hairpin is disrupted by forming C-Ag(I)-C base pairs. As a result, the G-rich sequence is released and folds into a G-quadruplex structure, which is able to bind NMM to generate strong fluorescence at 612 nm. However, in the presence of GSH, due to the strong binding ability between GSH and Ag(I), the C-Ag(I)-C structure is not formed. Hence, the DNA probe reverts back to its original structure and fluorescence is not increased. Based on these findings, a method was worked out that has a detection limit as low as 3.5 nM. Due to the inherent selectivity of the interaction between GSH and Ag(I), the method is highly selective over common potential interfering species. It was successfully applied to the fluorometric determination of GSH in cell extracts.