Low energy electron attachment to the adenosine site of DNA.

To evaluate the role of adenosine in low energy electron (LEE) induced DNA strand breaks, theoretical investigations of the LEE attachment induced C-O σ bonds and N-glycosidic bond breaking of 2'-deoxyadenosine-3',5'-diphosphate (3',5'-dAMP) were performed at the B3LYP/DZP++ level of theory. The results indicate that, although adenine-rich oligonucleotides are capable of capturing the near 0 eV electron to form the electronically stable radical anions in the gas phase, it is unlikely to undergo either C-O σ bond cleavage or the glycosidic processes due to the low electron detachment energy (VDE) of 3',5'-dADP(-) unit (0.26 eV). Instead, these radical anions should directly yield an electron detachment product in the gas phase. In the presence of polarizable surroundings, due to the large increase of the electron detachment energy (VDE increases to 1.59 eV), the adenine-centered radical anions could directly lead to strand breaks in the adenine-rich DNA single strands through either σ bond or N-glycosidic bond breaking. The values of activation energy for rupture of the C(5')-O(5') σ bond (22.5 kcal/mol), the N-glycosidic bond (20.2 kcal/mol), and the C(3')-O(3') σ bond (13.2 kcal/mol) indicate that C(3')-O(3') σ bond breaking should dominate. Moreover, along with the previous research, the predicted ratio of the activation energy barrier of the C(5')-O(5') σ bond breakage in 3',5'-dXDP(-) (X = G, A, T) partly explains the observation in the femtosecond time-resolved laser spectroscopic experiment that the C(5')-O(5') σ bond breaking only occurs in dGMP(-) and dTMP(-) not in dAMP(-). This study completes the series of LEE-induced DNA single strand breaking investigations for the four basic DNA units 3',5'-dXDP (X = G, A, T, C). The obtained results are vital for elucidating the experimental observations. Combined with the previous studies, the information revealed in this study is crucial for understanding the mechanisms of the interactions between the LEE and the DNA stands.