Intrinsic fluorescence of UV-irradiated DNA

In this report, we show that UV irradiation of natural and synthetic DNA results in a fluorescent photoproduct. Calf thymus DNA, poly(dAdT)2, poly(dGdC)2 and poly(dA).poly(dT) in phosphate buffer at pH 7 were investigated with UV-Vis and fluorescence spectroscopy before and after irradiation with UV light from an Hg lamp. Exposure of calf thymus DNA and poly(dA).poly(dT) to UV light for select amounts of time resulted in a fluorescent photo-product. Excitation of the samples at 298 nm after irradiation with UV light produced an emission spectrum with a maximum at approximately 402 nm. The intensity of the emission increased with increasing exposure time to UV light prior to the fluorescence measurement. Poly(dAdT)2 shows some fluorescence after irradiation with UV light, but to a much lesser degree than calf thymus DNA and poly(dA).poly(dT). Poly(dGdC)2 shows no fluorescence after irradiation with UV light, however, the UV-Vis data shows that poly(dGdC)2 does undergo photochemical changes. The results show that under the conditions employed, fluorescence occurs when thymine and adenine base pairs are present and is enhanced when the thymine nucleobases are adjacent to each other on the same strand.