Electron attachment to the 2-deoxythymidine-5-monophosphate-adenine pairs (5-dTMPH-A and 5-dTMP--
A) has been investigated at a carefully calibrated level of theory (B3LYP/DZP++) to investigate the electronaccepting
properties of thymine (T) in the DNA double helix under physiological conditions. All molecular
structures have been fully optimized in vacuo and in solution. The adiabatic electron affinity of 5-dTMPH-A
in the gas phase has been predicted to be 0.67 eV. Solvent effects greatly increase the electron capture ability
of 5-dTMPH-A. In fact, the adiabatic electron affinity increases to 2.04 eV with solvation. The influence
of the solvent environment on the electron-attracting properties of 5-dTMPH-A arises not only from the
stabilization of the corresponding radical anion through charge-dipole interactions, but also by changing the
distribution of the unpaired electron in the molecular system. The unpaired electron is covalently bound even
during vertical attachment, due to the solvent effects. Solvent effects also weaken the pairing interaction in
the thymidine monophosphate-adenine complexes. The phosphate deprotonation is found to have a relatively
minor influence on the capture of electrons by the 5-dTMPH-A species in aqueous solution. The electron
distributions, natural population analysis, and geometrical features of the models examined illustrate that the
influence of the phosphate deprotonation is limited to the phosphate moiety in aqueous solution. Therefore,
it is reasonable to expect that electron attachment to nucleotides will be independent of monovalent counterions
in the vicinity of the phosphate group in aqueous solution.