PARENT SESSION

Ion Beams from Physics to Therapy

(SY012) Ion beam irradiation of DNA: LET effects on radiation production of sugar-phosphate backbone radicals.

Adhikary, Amitava¹, Sevilla, Michael¹, Becker, David^*,1, ¹ Chemistry, Rochester, MI, United States

ABSTRACT- The double strand break is a critical DNA lesion and it is well known that radicals produced on the DNA sugar-phosphate backbone lead to the formation of strand breaks. Argon or oxygen ion beam irradiation of hydrated salmon sperm DNA at 77 K results in the formation of base radicals and, in addition, of a complex mixture of sugar radicals. Among the sugar radicals are three and, perhaps, four that arise from the electron-loss radiation path, the C1'⋅, C3'⋅, and C5'⋅ and possibly C4'⋅ radicals. Direct strand break radicals, likely formed through dissociative electron attachment by low energy electrons are also found; in this case, a phosphorus-centered radical formed by rupture of a P—O bond and a complementary deoxyribose carbon-centered radical formed by rupture of a C—O bond are observed. These are radicals whose formation is accompanied by a direct, concomitant strand break. Analysis of the yields of both DNA base radicals and sugar-phosphate backbone radicals indicates that the sugar radicals are largely found in the densely ionized ion-beam track core and DNA base radicals largely in low LET like spurs. Because of this track structure, there is a possibility of spatial correlation of these free radicals and, thereby, formation of double strand breaks. The formation of backbone radicals in the high-energy ion-beam core also suggests that excited state processes may be involved in sugar radical formation. This is verified by studies which show that excitation of guanine cation radical in both DNA and model compounds leads to direct formation of sugar radicals.

Key words: DNA, Radicals, ESR, LET