Effect of the photocatalytic activity of TiO(2) on plasmid DNA.

We investigated the photodynamic DNA strand-breaking activity of TiO(2). A solution of super-coiled pBR 322 DNA was irradiated with 5 J/cm(2) of UVA in the presence of TiO(2) and the products were analyzed by using gel electrophoresis. The ratio of open-circular DNA to super-coiled circular DNA was calculated from the resulting peak areas as a DNA strand-breaking index (SBI). The SBI of anatase-structure TiO(2) (band gap=3.23 eV) was greater than that of rutile structure (band gap=3.06 eV), and the level of SBI correlated with the photocatalytic activity for degradation of 2-propanol. The inhibitory effects of active oxygen scavengers, including DMSO, glutathione and histidine, on the DNA strand-breaking activity were examined. All of the scavengers except ascorbic acid showed inhibitory effects, as did several polyhydric alcohols including mannitol, a well-known hydroxyl radical scavenger. These results suggest that the photodynamic DNA strand-breaking activity of TiO(2) is due to active oxygen species, especially hydroxyl radicals. Polyhydric alcohols showed an inverse correlation between the inhibitory effect on DNA strand-breaking activity and the octanol/water partition coefficient (logP).