To elucidate the mechanism of DNA strand breaks by low-energy
electrons (LEE), theoretical investigations of the LEE attachmentinduced
C5OO5  bond breaking of pyrimidine nucleotides (5-
dCMPH and 5-dTMPH) were performed by using the B3LYP
DZP approach. The results indicate that the pyrimidine
nucleotides are able to capture electrons characterized by near-
0-eV energy to form electronically stable radical anions in both the
gas phase and aqueous solution. The mechanism of the LEEinduced
single-strand bond breaking in DNA might involve the
attachment of an electron to the bases of DNA and the formation
of base-centered radical anions in the first step. Subsequently,
these radical anions undergo either COO or glycosidic bond breaking,
yielding neutral ribose radical fragments and the corresponding
phosphoric anions or base anions. The COO bond cleavage is
expected to dominate because of its low activation energy. In
aqueous solutions, the significant increases in the electron affinities
of pyrimidine nucleotides ensure the formation of electronically
more stable radical anions of the nucleotides. The low activation
energy barriers for the C5OO5 bond breaking predicted in
this work are relevant when the counterions are close enough to
the phosphate moiety of DNA.