UV laser-induced DNA photochemistry

Previous studies examining the effects of UV laser irradiation of nucleosides and nucleotides have determined that qualitative and quantitative differences exist between irradiation at low and high intensities. Multi-photon events involving the singlet and triplet excited states of DNA bases occur following irradiation at high intensity, leading to degradation of bases due to intra-molecular bond cleavage; such events are not seen following irradiation at low intensity. This work extends these studies. Salmon sperm and plasmid DNA were irradiated at low (3.15 x 107 W/m2), intermediate (2.5 x 109 and 1.16 x 1010 W/m2), and high (1.25 x 1011 W/m2) intensities, using a KrF excimer laser emitting at 248 nm. DNA damage was then assayed, with the following findings; (1) pyrimidine cyclobutane dimer and bipyrimidine T(6-4)C photoadduct formation was reduced at high intensity relative to low intensity; (2) free thymine and thymine fragments were released from DNA at high intensity, but not at low intensity; (3) DNA strand break formation increased with increasing intensity; (4) double-stranded breaks occurred in DNA at high intensity. A mathematical model describing the effect of high intensity UV radiation on plasmid DNa conformation was developed and fit to experimental data on strand breaks. Using the model, dose constants for single- and double-stranded breaks were determined and found to increase with intensity. These results are consistent with the absorption of a second photon by long-lived triplet excited states of DNA following irradiation at high intensity, but not low intensity. Absorption of two photons leads to the depopulation of triplet excited states in DNA through ionization and fragmentation of bases, causing decreased levels of pyrimidine dimer formation and increased amounts of strand breakage in DNA components, and help extend our understanding of DNA-UV light interactions