The UV-VIS spectroscopy and fluorescence spectroscopy were applied to investigate the mechanism of the interaction between methylene blue (MB) and herring sperm DNA by using acridine orange as a spectral probe in Tris-HCl buffer (pH 7.0). The chief action mode was comfirmed to be an intercalation binding at a lower MB concentration but the mixed action modes at a higher concentration. Combining ratio of the binary complex is n(MB)∶n(DNA)＝10∶1, the binding constant of MB with DNA is ＝2.46×10⁵ L•mol^－1 and the apparent molor absorptivity is ε＝5.70×10⁶ L•mol^－1•cm^－1. Meanwhile, the interaction of MB with DNA was effected by the changes of pH and temperature. The results deduce that the interaction between MB and DNA is an enthalpy driven reaction.

Key words: methylene blue, acridine orange, herring sperm DNA, action mode