Damage to DNA induced by low energy electrons and photons: mechanisms and analysis at the molecular level

In this thesis studies of interactions of both VUV light and low energy (below 20 eV) electrons with dry DNA films and DNA samples in aqueous solutions are reported. The damage induced by such radiation is quantified by monitoring both loss of supercoiled DNA and formation of single and double strand breaks using agarose gel electrophoresis. In addition VUV spectra of DNA were recorded and, for the first time, spectral signatures of both the structural water in DNA and individual contributions of separate DNA nucleotides were resolved. Such investigations required the assembly and calibration of new apparatus. New protocols for the synthesis and purification of high quality samples of plasmid DNA were also developed. Alternative methods for analysis of DNA damage were explored, such as the atomic force microscopy technique, which allows for lesion imaging, and a new method, based on the ELISA assay was developed for labelling damage sites. VUV irradiation experiments revealed enhanced damage upon irradiation with 170 nm photons that was shown, via VUV absorption experiments, to be due to the excitation of structural water leading to increased hydroxyl (OH) radical formation causing chemical damage to the plasmid DNA. In agreement with earlier work both single (SSBs) and double (DSBs) strand breaks were induced during by electron irradiation at 4.5 eV, 5 eV and 10 eV but only SSB were observed for electron irradiation at 1 eV