Energy Requirements for Damaging DNA Molecules

Strand break formation and biological inactivation of infectious DNA of bacteriophage phi chi 174 exposed to vacuum-ultra-violet radiation of 4-9 to 21-2 eV quantum energy is investigated. At 21-2 eV as many as 84 per cent of the DNA molecules are inactivated by breaks whereas breaks do not contribute to inactivation at 4-9 eV. The quantum yield of break formation increases from 1-7 X 10(-5) (4-9 EV) to 0-55 (21-2 eV) and shows a dependence on energy similar to that of electron emission (due to ionization) above 8 eV. The mechanisms leading to break formation and inactivation are discussed taking the absorption spectrum of DNA in the vacuum-ultra-violet region into consideration.