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

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 pi-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.