Ag+ and Cysteine Quantitation Based on G-Quadruplex-Hemin DNAzymes Disruption by Ag+

Some G-quadruplex-hemin complexes are DNAzyme peroxidases that efficiently catalyze H2O2-mediated reactions, such as the oxidation of ABTS (2,2'-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid) by H2O2. Since Ag+ chelates guanine bases at the binding sites are involved in G-quadruplex formation, the presence of Ag+ may disrupt these structures and inhibit the peroxidase activity of G-quadruplex-hemin DNAzymes. On the basis of this principle, a highly sensitive and selective Ag+-detection method was developed. The method allows simple detection of aqueous Ag+ with a detection limit of 64 nM and a linear range of 50-3000 nM. Cysteine (Cys) is a strong Ag+-binder and competes with quadruplex-forming G-rich oligonucleotides for Ag+-binding, promoting the reformation of G-quadruplexes and increasing their peroxidase activity. Therefore, the Ag+-sensing system was also developed as a Cys-sensing system. This turn-on process allowed the detection of Cys at concentrations as low as 50 nM using a simple colorimetric technique. The Cys-sensing system could also be used for the detection of reduced glutathione (GSH). Neither the Ag+-sensing nor the Cys-sensing systems required labeled oligonucleotides. In addition, both gave large changes in absorbance signal that could be observed by the naked eye. Thus, a simple visual method for Ag+- or Cys-detection was developed.