Excess-electron capture and energy transfer to bulk water for aqueous DNA nucleotide

Abstract

We performed QM/MM simulations to investigate excess-electron attachment to four aqueous DNA nucleotide anions (dRT⁻). The negative QM/MM vertical electron affinities (−0.86 to −0.59 eV) reveal that aqueous dRT⁻ anions improbably capture the excess electron near 0 eV. Comparing with the calculations in the gas phase and without the background charges, it can be found that first-shell water molecules have a larger contribution to the promotion of the ability of the excess-electron capture and the bulk-water polarization has a small effect on vertical electron affinities. The phosphate group hampers the attachment of the very low-energy excess electron to aqueous dRT⁻. The large adiabatic electron affinities (1.45–1.96 eV) and vertical detachment energies (1.92–2.44 eV) reveal that stable dRT²⁻ dianions could be formed after dRT⁻ anions catch the higher-energy excess electron (>0.59 eV). We computed the energy changes in the dRT²⁻ structural relaxations. The QM-region conformational changes cause small energy alterations (−0.28 to 0.35 eV). The QM/MM energy decreases are 2.31–2.73 eV which mainly come from QM computations (3.49–4.00 eV) embedded in the background charges. The analysis of excess-electron distributions indicates that the polarization of bulk water and structural relaxations of dianions induce the excess-electron redistributions in the QM region and produce large QM-energy decreases. The MM energy changes are −1.27 to −1.11 eV for four aqueous dianions. The negative values demonstrate that the energy of the MM region would increase in dRT²⁻ structural relaxations. In contrast with the values of the polarized QM computations, about 30% of the energy released by the QM region is transferred to bulk water in the MM region. The large energy dissipation probably suppresses DNA damage by the low-energy electron.