The formation of DNA sugar radicals from photoexcitation of guanine cation radicals.
In this investigation of radical formation and reaction in gamma- 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 D(2)O) were prepared with Tl(3+) or Fe(CN)(3-)(6) as electron scavengers. Upon gamma 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.
Shukla
Lata I
LI
Department of Chemistry, Oakland University, Rochester, Michigan 48309, USA.
Pazdro
Robert
R
Huang
James
J
DeVreugd
Christopher
C
Becker
David
D
Sevilla
Michael D
MD
D000818:Animals; D002412:Cations / Q000737:chemistry / Q000528:radiation effects; D004247:DNA / Q000737:chemistry* / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005609:Free Radicals / Q000737:chemistry / Q000528:radiation effects; D005720:Gamma Rays; D006147:Guanine / Q000737:chemistry* / Q000528:radiation effects*; D008297:Male; D015394:Molecular Structure; D010777:Photochemistry / Q000379:methods; D011829:Radiation Dosage; D012474:Salmon; D013403:Sugar Phosphates / Q000737:chemistry* / Q000528:radiation effects*; D013737:Testis / Q000378:metabolism; D014466:Ultraviolet Rays
D002412:Cations; D005609:Free Radicals; D013403:Sugar Phosphates; D006147:Guanine; D004247:DNA