Energy Requirements for Damaging DNA Molecules

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Abstract
DNA of bacteriophage ΦX174 freeze-dried in extremely thin layers was exposed to metastably-excited gases. The loss of infectivity and the frequency of strand breaks was measured in the spheroplast system and by centrifugation in an alkaline sucrose gradient, respectively. The excitation energies of the various gases used amount to 4·3 eV for H2*, 6·2 eV for N2*, 11·6 eV for Ar*, and 19·8 eV for He*. These energies are transferred to the DNA molecules by ‘collisions of the second kind’. The experiments show that 76·5 ± 9·4 per cent of the inactivated DNA molecules carry at least one strand break, a value found not to depend on the amount of energy primarily transferred. Thus, the action of excited gases is entirely different from the effects of U.V. quanta and slow electrons even if identical energies are transferred in all cases.