An understanding of the DNA strand breaks induced by low
energy electrons (LEE) is of importance for the advancement of
global models of cellular radiolysis and for the development of
efficient methods of radiotherapy.1 Both experimental investigations
and theoretical studies of different DNA fragment models have
demonstrated that, at near 0 eV energy, electron attachment may
induce strand breaks in DNA.2-14 Different DNA strand breaking
mechanisms have been proposed in order to understand the nature
of DNA strand breaks by LEEs.7,9,11,14
On the basis of the gas-phase model of the sugar-phosphatesugar
(S-P-S) moiety, Sevilla et al.7 have proposed that LEE may
be captured first by the phosphate, forming a phosphate-centered
radical anion. Subsequent C-O -bond breaking was estimated to
have an energy barrier of 10 kcal/mol. However, the studies of
Simons et al. suggested a base-hosted anion mechanism and a much
higher activation barrier (15 kcal/mol in the gas phase, 25 kcal/
mol in an aqueous solution) for C3-O3 bond cleavage.10-13,15
Moreover, recent experiments suggested that the DNA single strand
breaks might be initiated by electron attachment to the bases in
the gas phase.16
To elucidate the mechanism of DNA single strand breaks by
LEEs, a reliable description of the properties of the radical anions
of the nucleotides and the accurate determination of the activation
energy barrier of the corresponding bond rupture is necessary. With
the reliably calibrated DFT-based B3LYP/DZP++ approach, the
electron affinity of 2-deoxycytidine-3-monophosphate and its
phosphate deprotonated anion have been studied by Schaefer et
al.17 3-dCMPH is found to be able to capture near 0 eV electron
to form a stable radical anion in both the gas phase and aqueous
solution. With the same method, investigations of LEE attachmentinduced
C5-O5 -bond breaking of the pyrimidine nucleotides (5-
dCMPH and 5-dTMPH) reveal that the activation barriers are about
14 kcal/mol in the gas phase and 18 kcal/mol in an aqueous
solution.18 In light of these findings, here we report theoretical
investigations of LEE attachment-induced C3-O3 -bond breaking
of pyrimidine nucleotides. 2-Deoxycytidine-3-monophosphate and
2-deoxythymidine-3-monophosphate in their protonated forms (3-
dCMPH and 3-dTMPH, Scheme 1) have been selected as models.
These models complement our previous studies for 5-dCMPH and
5-dTMPH.18