GUANINE RADICAL CATIONS ARE PRECURSORS OF 7,8-DIHYDRO-8-OXO-2'-DEOXYGUANOSINE BUT ARE NOT PRECURSORS OF IMMEDIATE STRAND BREAKS IN DNA

Biphotonic photoionization of frozen aqueous solutions of DNA at 248 n m has been shown by EPR spectroscopy to lead selectively to the guanin e cation. In (H2O)-O-18 under these conditions high levels of [O-18]-7 ,8-dihydro-8-oxo-2'-deoxyguanosine are produced in a dose-dependent ma nner, confirming direct formation of this oxidation product by hydrati on of the guanine cation. Photoionization of defined oligonucleotides did not give rise to significant levels of immediate strand breaks but generated G-specific alkali-labile sites that are readily cleaved by piperidine treatment, Authentic oligonucleotides containing 7,8-dihydr o-8-oxo-2'-deoxyguanosine (8-oxodG, 5) sites are slowly cleaved at the se sites on treatment with piperidine but at rates inconsistent with t his being the source of the alkali-labile site, Photoionization of 7,8 -dihydro-8-oxo-2'-deoxyguanosine-cont oligonucleotides demonstrated th at this residue is highly susceptible to secondary oxidation and leads to formation of a markedly more alkali-labile lesion. Photoionization of 7,8-dihydro-8-oxo-2'-deoxyguanosine leads to release of 7,8-dihydr o-8-oxoguanine suggesting that the alkali-labile site on photoionizati on of 7,8-dihydro-8-oxo-2'-deoxyguanosine-containing oligonucleotides is an apurinic site. However, the lack of significant 7,8-dihydro-8-ox oguanine release on photoionization of the parent oligonucleotides rul es out secondary oxidation of 7,8-dihydro-8-oxo-2'-deoxyguanosine as t he mechanism of formation of alkali-labile sites.