Independent Generation and Time-Resolved Detection of 2'-Deoxyguanosin-N2-yl Radicals

Guanine radicals are important reactive intermediates in DNA damage. Hydroxyl radical (HO^. ) has long been believed to react with 2'-deoxyguanosine (dG) generating 2'-deoxyguanosin-N1-yl radical (dG(N1-H)^. ) via addition to the nucleobase π-system and subsequent dehydration. This basic tenet was challenged by an alternative mechanism, in which the major reaction of HO^. with dG was proposed to involve hydrogen atom abstraction from the N2-amine. The 2'-deoxyguanosin-N2-yl radical (dG(N2-H)^. ) formed was proposed to rapidly tautomerize to dG(N1-H)^. . We report the first independent generation of dG(N2-H)^. in high yield via photolysis of 1. dG(N2-H)^. is directly observed upon nanosecond laser flash photolysis (LFP) of 1. The absorption spectrum of dG(N2-H)^. is corroborated by DFT studies, and anti- and syn-dG(N2-H)^. are resolved for the first time. The LFP experiments showed no evidence for tautomerization of dG(N2-H)^. to dG(N1-H)^. within hundreds of microseconds. This observation suggests that the generation of dG(N1-H)^. via dG(N2-H)^. following hydrogen atom abstraction from dG is unlikely to be a major pathway when HO^. reacts with dG.