Investigation on the toxic interaction of toluidine blue with calf thymus DNA

The gene toxic interactions of toluidine blue (TB) with calf thymus DNA (ct-DNA) were examined in vitro with UV-visible absorption spectroscopy, fluorescence spectroscopy, fluorescence polarization and circular dichroism techniques. The experimental results showed that TB interacted with ct-DNA by two binding modes. At low TB concentrations, TB intercalated into the DNA base pairs. At higher TB concentrations, TB was attached to the negatively charged phosphate groups. For the intercalation binding mode and electrostatic binding mode, the binding constants were 1.76 × 10⁶ L mol^-1 and 6.18 × 10⁵ L mol^-1 and the number of binding sites was 0.48 and 0.79, respectively. Circular dichroism results showed that the two binding modes had different effects on the ct-DNA conformation. At high dye concentrations, Z-form DNA appears, while at low TB concentrations, a B to A form transition is observed.