Protonation and deprotonation enthalpies of guanine and adenine and implications for the structure and energy of their complexes with water: Comparison with uracil, thymine, and cytosine

The optimized geometries, harmonic vibrational frequencies, and energies of the cyclic structures of monohydrated guanine and adenine are computed usi ng density functional theory (B3LYP) combined with the 6-31+G(d,p) basis se t. The proton affinity of the O and N atoms and the deprotonation enthalpy of the different NH bonds of guanine and adenine are computed at the same l evel of theory. The results are compared with recent data on uracil, thymin e, and cytosine. The intrinsic acidities and basicities of the five nucleob ases are discussed. Complex formation with water results in a moderate chan ge of the pyramidal character of the amino group. For closed complexes wher e water interacts with the O atom of the nucleobase, the intermolecular dis tances and the hydrogen bond energies are correlated to the proton affiniti es and deprotonation enthalpies of the sites involved in complex formation.