Critical energies for ssb and dsb induction in plasmid DNA by vacuum-UV photons: an arrangement for irradiating dry or hydrated DNA with monochromatic photons

Abstract

Purpose : Theoretical modelling techniques are often used to simulate the action of ionizing radiations on cells at the nanometre level. Using monoenergetic vacuum-UV (VUV) radiation to irradiate DNA either dry or humidified, the action spectra for the induction of DNA damage by low energy photons and the role of water and can be studied. These data provide inputs for the theoretical models. Methods : Various combinations of monochromator, grating and VUV window have been used to obtain monochromatic photons from the 2 GeV electron synchrotron at the CLRC, Daresbury Laboratory. A sample chamber containing plasmid DNA is installed at the end of the beamline. The chamber can be evacuated or water can be introduced (as water vapour or humidified helium). In this way, DNA can be irradiated either dry or humidified. Results : An arrangement for irradiating dry or humidified DNA using monoenergetic photons from 7eV to 150eV has been developed. At the energies used, exposure rates vary from about 5x10 10 to 3x10 12 photons cm -2 s -1 over a 1 cm 2 sample area. At all but the lowest energies this is sufficient to produce significant levels of DNA damage in just a few minutes. The measured dose variation over the sample area is typically 30%, but this is reduced significantly using sample scanning techniques.