Electron interaction with a DNA duplex: dCpdC:dGpdG

Abstract

Electron attachment to double-stranded cytosine-rich DNA, dCpdC:dGpdG, has been studied by density functional theory. This system represents a minimal descriptive unit of a cytosine-rich double-stranded DNA helix. A significant electron affinity for the formation of a cytosine-centered radical anion is revealed to be about 2.2 eV. The excess electron may reside on the nucleobase at the 5′ position (dC˙⁻pdC:dGpdG) or at the 3′ position (dCpdC˙⁻:dGpdG). The inter-strand proton transfer between the radical anion centered cytosine (N3) and the paired guanine (HN1) results in the formation of radical anion center separated complexes dC_1H˙pdC:dG_2-H⁻pdG and dCpdC_2H˙:dGpdG_1-H⁻. These distonic radical anions are found to be approximately 1 to 4 kcal mol⁻¹ more stable than the normal radical anions. Intra-strand cytosine π → π transition energies are below the electron detachment energy. Inter-strand π → π transitions of the excess electron from C to G are predicted to be less than 2.79 eV. Electron transfer might also be possible through the inter-strand base-jumping mode. An analysis of absorption visible spectra reveals the absorption bands ranging from 500 nm to 700 nm for the cytosine-rich radical anions of the DNA duplex. Electron attachment to cytidine oligomers might add color to the DNA duplex.