Abstract

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.

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