193 NM LIGHT INDUCES SINGLE-STRAND BREAKAGE OF DNA PREDOMINANTLY AT GUANINE

Irradiation of DNA with 193 nm light results in monophotonic photoioni zation, with the formation of a base radical cation and a hydrated ele ctron (phi(P1) = 0.048-0.065). Although >50% Of the photoionization ev ents initially occur at guanine in DNA, migration of the ''hole'' from the other bases to guanine occurs to yield predominantly its radical cation or its deprotonated form. From sequence analysis, the data reve al that 193 nm light induces single strand breaks (ssb) in double-stra nded DNA preferential 3' to a guanine residue. However, it has previou sly been reported that 193 nm light yields very low yields of ssb (<2% of the yield of e(-)aq). The distribution of these ssb at guanine is nonrandom, showing a dependence on the neighboring base moiety, The ef ficiency of ssb formation at nonguanine sites is estimated to be at le ast one order of magnitude lower. The preferred cleavage at guanine is consistent with migration and localization of the electron loss cente r at guanine. It is argued that singlet oxygen and the photoionized ph osphate group of the sugar moiety are not major precursors to ssb. At present, the mechanisms of strand breakage are not known although a gu anine radical or one of its products remain potential precursors.