Shukla, L. I., Pazdro, R., Huang, J., DeVreugd, C., Becker, D. and Sevilla, M. D. The Formation of DNA Sugar Radicals from Photoexcitation of Guanine Cation Radicals. Radiat. Res. 161, 582–590 (2004).
In this investigation of radical formation and reaction in γ- irradiated DNA and model compounds, we report the conversion of the guanine cation radical (one-electron oxidized guanine, G·+) to the C1′ sugar radical and another sugar radical at the C3′ or C4′ position (designated C3′·/C4′·) by visible and UV photolysis. Electron spin resonance (ESR) spectroscopic investigations were performed on salmon testes DNA as well as 5′-dGMP, 3′-dGMP, 2′-deoxyguanosine and other nucleosides/nucleotides as model systems. DNA samples (25– 150 mg/ml D2O) were prepared with Tl3+ or Fe(CN)3−6 as electron scavengers. Upon γ irradiation of such samples at 77 K, the electron-gain path in the DNA is strongly suppressed and predominantly G·+ is found; after UV or visible photolysis, the fraction of the C1′ sugar radical increases with a concomitant reduction in the fraction of G·+. In model systems, 3′- dGMP+·; and 5′-dGMP+· were produced by attack of Cl·−2 on the parent nucleotide in 7 M LiCl glass. Subsequent visible photolysis of the 3′-dGMP+· (77 K) results predominantly in formation of C1′· whereas photolysis of 5′-dGMP+· results predominantly in formation of C3′·/C4′·. We propose that sugar radical formation is a result of delocalization of the hole in the electronically excited base cation radical into the sugar ring, followed by deprotonation at specific sites on the sugar.