Strand Breaks Induced by Very Low Energy Electrons: Product Analysis and Mechanistic Insight into the Reaction with TpT.

Abstract

Numerous experimental studies show that 5-15 eV electrons induce strand breaks in DNA at energies below the ionization threshold of DNA components. In this energy range, DNA damage arises principally by the formation of transient negative ions, decaying into dissociative electron attachment (DEA) and electronic excitation of dissociative states. Here, we carried out LC-MS/MS analysis of the degradation products arising from bombardment of TpT, a DNA model compound, irradiated with very low energy electrons (vLEEs; ∼1.8 eV). The formation of thymidine 5'-monophosphate (TMP5') together with 2',3'-dideoxythymidine (ddT3') can be explained by cleavage of the C3'-O bond of TpT, whereas thymidine 3'-monophosphate (TMP3') and 2',5'-dideoxythymidine (ddT5') are formed by cleavage of the C5'-O bond. The formation of ddT3' and ddT5' decreased upon irradiation of either TMP5' or TMP3', and even further in the case of thymidine, underlining the critical role of the phosphate group. Interestingly, the yield of TMP5' and TMP3' was higher than that of the corresponding ddT3' and ddT5' products, suggesting alternative fates of C3' and C5'-centered sugar radicals. In contrast, the release of thymine from TpT was minor (<20%) and did not result in the formation of expected products from DEA-mediated cleavage at the N-glycosidic bond. Lastly, vLEE induced the conversion of thymine to 5,6-dihydrothymine (5,6-dhT) within TpT, a reaction likely involving thymine anion radicals. In summary, we show that a major pathway of vLEEs involves DEA-mediated cleavage of the C3'-O and C5'-O bonds of TpT, resulting in the formation of specific fragments, which represent a prompt single strand break in DNA.