The direct effects of ionizing radiation on DNA and its higher ordered structures
This thesis investigates the effects of ionizing radiation on frozen aqueous solutions of DNA using e.s.r. spectroscopy and a plasmid (pBR322) strand break assay. To elucidate the mechanisms subsequent to primary ionic radical formation (G˙+ and T˙¯), additives that influence the radiolytic processes were included prior to irradiation. The presence of hydrogen peroxide (Chapter Three) switched the mechanism from direct damage to a pathway in part mediated through oxygen centred radicals (˙OH, HO˙2) and resulted in a modest increase in the number of strand breaks (i.e. radiosensitization). E.s.r. observations showed the appearance of sugar radicals (strand break precursors) which were lost at temperatures well below those of base radicals. The inclusion of a variety of thiols (Chapter Four) resulted in no change to either G˙ + or T˙¯, However, on warming, the normal pattern of radical reactions was dramatically modified, the DNA radical centres being abruptly reduced in concentration. In anoxia this was concomitant with the appearance of RSSR ¯, and strand breaks were noted to decrease (i,e. radioprotection). Under oxic conditions the degree of repair was a function of the relative concentration of oxygen and thiol. E.s.r. indicated repair of DNA centred peroxyl radicals and also RSO˙2 formation. The latter may react with DNA and account for attenuation, by oxygen, of protection afforded by thiols at low concentrations. The effects of ionizing radiation on higher ordered DNA structures (nucleohistone, chromatin and cell nuclei) has been investigated (Chapter Five). Relative to DNA, all systems gave equivalent yields of G˙+, together with protein electron-loss centres (Hist)˙+. However, T˙¯ yields were enhanced, the increase being greatest for nuclei. For the protein component it was suggested that (Hist)˙+ are amide cations, readily trapped by loss of N-H protons, but that the electrons are Mobile and able to transfer to DNA. Mechanisms leading to strand breaks, involving intramolecular hydrogen atom abstraction by directly induced base radicals from neighbouring sugar residues, are proposed (Appendix B) and compared with those obtained for hydroxyl radical damage.