A theoretical study of structures and electron affinities of radical anions of guanine-cytosine, adenine-thymine, and hypoxanthine-cytosine base pairs.

Adiabatic electron affinities (AEA) and structural perturbations due to addition of an excess electron to each of the neutral guanine-cytosine (G-C), adenine-thymine (A-T), and hypoxanthine-cytosine (HX-C) base pairs were studied using the self-consistent charge, density functional tight-binding (SCC-DFTB-D) method, augmented by the empirical London dispersion energy term. Performance of the SCC-DFTB-D method was examined by comparing the calculated results using it with those obtained from experiment as well as ab initio and other different density functional theoretical studies. An excellent agreement between the SCC-DFTB-D results and those obtained by the other calculations regarding the structural modifications, hydrogen bonding, and dissociation energies of the neutral and radical anion base pairs was found. It is shown that adiabatic electron affinity can be better predicted by considering reaction enthalpies of formation of the respective neutral and anionic base pairs from their respective molecular components instead of taking the difference between their total energies. The calculated AEAs of the base pairs were compared with those obtained by the bracketing method from Schaefer and coworkers, where a satisfactory agreement was found. It shows applicability of the SCC-DFTB-D method to study charged DNA models at a highly economical computational cost.