Electron affinities of the DNA and RNA bases

Adiabatic electron affinities (AEAs) for the DNA and RNA bases are predicte d by using a range of density functionals with a double-zeta plus polarizat ion plus diffuse (DZP++) basis set in an effort to bracket the true EAs. Al though the AEAs exhibit moderate fluctuations with respect to the choice of functional, systematic trends show that the covalent uracil (U) and thymin e (T) anions are bound by 0.05-0.25 eV while the adenine (A) anion is clear ly unbound. The computed AEAs for cytosine (C) and guanine (G) oscillate be tween small positive and negative values for the three most reliable functi onal combinations (BP86, B3LYP, and BLYP), and it remains unclear if either covalent anion is bound. AEAs with B3LYP/TZ2P++ single points are 0.19 (U) , 0.16 (T), 0.07 (G), -0.02 (C), and -0.17 eV (A.). Favorable comparisons a re made to experimental estimates extrapolated from photoelectron spectra d ata for the complexes of the nucleobases with water. However, experimental values scaled from liquid-phase reduction potentials are shown to overestim ate the AEAs by as much as 1.5 eV. Because the uracil and thymine covalent EAs are in energy ranges near those of their dipole-bound counterparts, pre paration and precise experimental measurement of the thermodynamically stab le covalent anions may prove challenging.