The interaction of paraquat with calf thymus DNA (ctDNA) has been performed using methylene blue (MB) as a molecular probe by absorption spectra, fluorescence spectra, fluorescence polarization and circular dichroism spectra. MB primarily bound to double helical of ctDNA via the intercalative mode. Scatchard curves indicated the non-competitive inhibition of MB binding to ctDNA in the presence of paraquat. The results of fluorescence polarization and circular dichroism spectra all showed that the intercalative binding between paraquat and ctDNA was excluded. The results of the effect of sodium chloride on the binding of paraquat with ctDNA showed that sodium ions could remarkably constrain the above interaction. It was suggested that positively charged paraquat ions could interact with the negatively charged phosphate moieties of ctDNA through electrostatic attraction and stacked on the surface of double-stranded ctDNA, which might cause ctDNA shrinked and reduce the binding affinity of MB to ctDNA in the surrounding sites. Maybe this is because sodium ions may neutralize the negatively charged phosphate backbone of ctDNA, and then weaken the electrostatic attraction between paraquat and ctDNA. The binding constant of paraquat to ctDNA was 1.80×10⁴ L•mol^－1 obtained by Scatchard equation.

Key words: paraquat, calf thymus DNA, methylene blue, fluorescence polarization, circular dichroism spectra, interaction