Further observations on the photooxidation of DNA in the presence of riboflavin.

Illumination of DNA in the presence of riboflavin results in an increase in buoyant density and a decrease in the temperature of the thermal helix-coil transition (Tm). The increase in buoyant density is maintained even after thermal denaturation, which indicates that it reflects a chemical alteration of a DNA base (presumably guanine). beta-Carotene, a quencher of singlet oxygen, inhibits the increase in buoyant density but it prevents only partially the decrease in Tm. This is taken as an indication that the photo-induced alteration of the DNA structure is due to more than one reaction. Illumination of DNA in the presence of methylene blue also causes an increase in buoyant density , but this increase is not retained upon thermal denaturation.