All DNA oligonucleotides were ordered from Sangon Biotech. Oligonucleotides with a length greater than 10 nt were purified by high affinity purification, while those with a length less than 10 nt and modified oligonucleotides were purified by high performance liquid chromatography. Upon receipt, the lyophilized oligonucleotides were suspended in 1× TE buffer (10 mM Tris•HCl, pH 8.0, 1 mM EDTA), and stored at 4 ℃. The concentration of each oligonucleotide was quantitated by measuring the absorbance at 260 nm using Agilent Cary 60 spectrophotometer. All DNA duplexes in thermodynamic fluorescence experiments were prepared by annealing top and bottom strands in a 1:1 ratio. All DNA substrate complexes in kinetic fluorescence experiments were prepared by annealing a 1.2:1.2:1.1:1 ratio of fluorophore labeled, quencher labeled, top and bottom strands. The buffer for all experiments and annealed complexes was 1× TE with 12.5 mM Mg 2+ , pH 8.0. All complexes were annealed in an ABI 2720 thermal cycler by heating to 95 ℃ for 5 min and then gradually cooling to 20 ℃ at a constant rate over a period of 2 h.