Spectroscopic studies of the interaction between methylene blue and G-quadruplex

G-rich single-stranded DNA (5'-TTAGGG-3') adopted a G-quadruplex structure in buffer containing potassium ions. The spectroscopic feature and the interaction between methylene blue and G-quadruplex have been investigated by circular dichroism, and nuclear magnetic resonance spectroscopy. The UV-Vis absorption and fluorescence spectral results show that the fluorescence behavior of MB by single-stranded DNA fits Stern-Volmer static quenching equation very well and they formed 1 : 1 complexes; dimeric G-quadruplexes were bound to MB with 1 : 1 or 2 : 1, and their equilibrium constants were 1.047x10(5) and 8.79x10(4) L/mol, respectively. Based on the above results and H-1-NMR spectral data, one may conclude that MB stacked either the terminal tetrads to form 1 : 1 complexes or between two terminal tetrads of G-quadruplexes to form 1 : 2 sandwich complexes with G-qudruplexes.