Nanoparticles-Free Fluorescence Anisotropy Amplification Assay for Detection of RNA Nucleotide-Cleaving DNAzyme Activity

Fluorescence anisotropy is a homogeneous, sensitive, ratiometric, and real-time analytical technology. However, it is a great challenge to produce a large fluorescence anisotropy change upon the presence of target small molecules without nanoparticles-dependent amplification. This work reports a nanoparticle-free and multiple G-enhanced fluorescence anisotropy assay for detection of DNAzyme activity. A Pb2+-dependent GR-5 DNAzyme was used as a model. We hybridized the rA-cleavable substrate strand containing a TMR label at the 5′-end with the DNAzyme strand containing an extended three G bases at the 3′-end. By this design, we demonstrate that both fluorescence quenching and the enhanced DNAzyme activity contribute to a Pb2+-induced large fluorescence anisotropy change (|Δr| = 0.168). The limit of detection for Pb2+ is estimated to be about 100 pM with a dynamic range from 200 pM to 100 nM. The interference from the other nine divalent metal ions of 1000-times excess amount is negligible. Moreover, we show an extended assay for evaluation of the interactions of Pb2+ with cysteine and glutathione by the detection of GR5 DNAzyme activity. Collectively, we developed a novel fluorescence anisotropy amplification assay, enabling us to detect DNAzyme activity and associated cofactors and inhibitors and to characterize the Pb2+-chelation capability of free thiols.