INDUCTION OF STRAND BREAKS IN SINGLE-STRANDED POLYRIBONUCLEOTIDES ANDDNA BY PHOTOIONIZATION - ONE-ELECTRON OXIDIZED NUCLEOBASE RADICALS ASPRECURSORS

As a model for the direct effect of ionizing radiation, time-resolved changes of light-scattering intensity (LSI) of aqueous, oxic solutions of single-stranded DNA samples (calf thymus, Micrococcus lysodeikticu s, Clostridium perfingens), polyC, polyU, and polyA at pH 7.5-8 have b een studied following pulse irradiation with 193-nm laser light. A com parison of LSI changes (an index of single-strand break formation) wit h transient optical absorption changes (an index of nucleic acid base radical chemistry) following pulsed 193-nm irradiation shows that stra nd breakage occurs at rates similar to those for the decay of the nucl eic acid base radical species for DNA, polyC, and polyU but not for po lyA. With the exception of polyA, 193-nm light leads to a reduction in the LSI over similar to 0.2 s, indicating that strand breakage occurs by a nucleobase radical-mediated process, whereby the nucleobase radi cal site is transferred to the sugar moiety. With polyU and polyC, sig nificant changes in LSI also occur on a much faster time scale, (withi n 3 ms). In DNA, the one electron oxidized sites become localized pred ominantly at guanine and the resulting one electron oxidised guanine r adicals lead to strand breakage in competition with major, non-strand- breakage pathways.