Highly Oxidizing Excited States of One-Electron-Oxidized Guanine in DNA: Wavelength and pH Dependence

Excited states of one-electron-oxidized guanine in DNA are known to induce hole transfer to the sugar moiety and on deprotonation result in neutral sugar radicals that are precursors of DNA strand breaks. This work carried out in a homogeneous aqueous glass (7.5 M LiCl) at low temperatures (77-175 K) shows the extent of photoconversion of one-electron-oxidized guanine and the associated yields of individual sugar radicals are crucially controlled by the photon energy, protonation state, and strandedness of the oligomer. In addition to sugar radical formation, highly oxidizing excited states of one-electron-oxidized guanine are produced with 405 nm light at pH 5 and below that are able to oxidize chloride ion in the surrounding solution to form Cl-2(center dot-) via an excited-state hole transfer process. Among the various DNA model systems studied in this work, the maximum amount of Cl-2(center dot-) is produced with ds (double-stranded) DNA, where the one-electron-oxidized guanine exists in its cation radical form (G(center dot+):C). Thus, via excited-state hole transfer, the dsDNA is apparently able to protect itself from cation radical excited states by transfer of damage to the surrounding environment.