DNA nucleosides and their radical anions: molecular structures and electron affinities

The deoxyribonucleosides have been studied to determine the properties of combinations of 2-deoxyribose with each of the isolated DNA bases for both neutral and anionic species. We have used a carefully calibrated theoretical method [Chem. Rev. 2002, 102, 231], employing the B3LYP hybrid Hartree-Fock/DFT functional with the DZP++ basis set. Predictions are made of the geometric parameters, adiabatic electron affinities, charge distributions based on natural population analysis, and decomposition enthalpy for the neutral and anionic forms of the four 2'-deoxyribonucleosides in DNA: 2'-deoxyriboadenosine (dA), 2'-deoxyribocytidine (dC), 2'-deoxyriboguanosine (dG), and 2'-deoxyribothymidine (dT). Geometric changes in the anions show that the glycosidic bond exhibits little change with excess charge for the guanosine but significant shortening for the adenosine and for the pyrimidines. The zero-point corrected adiabatic electron affinities in eV for each of the 2'-deoxyribonucleosides are as follows: 0.06, dA; 0.09, dG; 0.33, dC; and 0.44, dT. These values are uniformly greater than those of the corresponding isolated bases (-0.28, A; -0.07, G; 0.03, C; 0.20, T) and the isolated 2-deoxyribose (-0.38) at the same level of theory. The vertical detachment energies of dT and dC are substantial, 0.72 and 0.94 eV, and these anions should be observable. A high VDE, 0.91 eV, is also found for dA but its anion is unlikely to be stable due to the small AEA of 0.06 eV. The high VDE reflects the fact that the molecular structures of the anions and the corresponding neutral species are quite different. Valence character is displayed for the SOMOs of dA, dC, and dT, while some component of diffuse character is visible in the SOMO of dG. Further analysis of electronic changes upon electron attachment include an examination of the NPA charges, which show that in the neutral 2'-deoxyribonucleosides the sum of NPA charges for every base is the same, -0.28 with the sum of 2-deoxyribose charges being positive, +0.28. In the anions, the trend in charge division varies based on the nature of the excess electron in the anions. Thermodynamically, the overall enthalpy change for the reaction of water with the neutral nucleosides to give bases and ribose is approximately zero. The analogous decomposition is exothermic by 8 to 11 kcal mol-1 for the anions, indicating possible challenges for anionic gas-phase nucleoside exploration in the presence of water.