Correlation of Photosensitization and Binding Mode of Methylene Blue and DNA

Binding of methylene blue (MB) and DNA, photosensitization of MB in DNA, and correlation of photosensitization and binding mode are studied at different concentration ratios of DNA and MB. The absorption spectra indicate that the electrostatic binding is the main mode at low γ ratios (γ≤2), while at high γ ratios (γ>2) the intercalative binding is dominated. Studies on dynamics of photosensitization formation of singlet oxygen (¹O₂) for MB in DNA are carried out by using time−resolved technology. There are no obvious changes of the singlet oxygen lifetime and the triplet state MB molecule (MB^3+∗) lifetime at low ratios, they are about 4 μs and 1 µs, respectively. However, we could not obtain the ¹O₂ lifetime and MB^3+∗ lifetime due to the great decrease of ¹O₂ phosphorescence signals at high ratios. These results show that the photosensitization and binding mode of MB in DNA possess high correlation. When MB binds with DNA by electrostatic interaction, type−II photosensitization of MB plays a major role in photodynamic effect, the damage of DNA probably could be ascribed to ¹O₂. However, at high ratios, binding mode between MB and DNA turns to intercalative binding, which greatly weakens the type−II photosensitization process. Charge transfer between MB and DNA possibly becomes the main damage mechanism.