UVB and UVA radiation-mediated damage to isolated and cellular DNA
The effects of solar light on cellular DNA are mostly explained by both direct excitation of nucleobases and photosensitized reactions that are mediated by UVB and UVA radiation, respectively. A large body of information is now available on the main photodynamic reactions to DNA, which involve guanine as the preferential target of both one-electron oxidation and singlet oxygen oxidation, as the result of type I and type II mechanisms. Most of the final products of the photosensitized reactions of guanine base in isolated DNA have been characterized, and comprehensive mechanisms for their formation have been proposed. Further insights into the mechanisms of solar radiation-induced modifications within cellular DNA have been gained from accurate measurement of the main photoproducts using recently designed sensitive chromatographic and biochemical methods. Thus, the distribution pattern of the 12 possible bipyrimidine photoproducts has been shown to be similar in the DNA of UVB-irradiated rodent and human cells. Cyclobutyl pyrimidine dimers are also generated at di-thymine and thymine-cytosine sites within nuclear DNA upon exposure to UVA radiation as the likely result of triplet energy transfer. In addition, oxidative reactions that involved mostly singlet oxygen, and to a lesser extent, .OH radicals are also implicated, although less efficiently.