SINGLE-STRAND AND DOUBLE-STRAND BREAKS IN PBR322 DNA BY VACUUM-UV FROM 8.3 TO 20.7 EV

Solid pBR322 DNA was irradiated in a Vacuum by monochromatic photons from 8.3 eV (150 nm) to 20.7 eV (60 nm), and the formation of single- and double-strand breaks (ssb and dsb) was determined using agarose gel electrophoresis. The action cross sections increased 20 times (ssb) and 43 times (dsb) from 8.3 eV to 20.7 eV; the quantum yields increased 5.4 times (ssb) and 12 times (dsb). The cross sections for dsb were 0.0059 at 8.3 eV and 0.013 at 20.7 eV of those of ssb; these values were smaller than about 0.04 for 2.1 keV photons, or about 0.18 for Co-60 gamma-rays. Although vacuum UV photons were proved to induce dsb, they had a lower efficiency than when using soft X-rays or gamma-rays.