Protonation and Deprotonation Enthalpies of Guanine and Adenine and Implications for the Structure and Energy for 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 were computed using density functional theory (B3LYP) combined with the 6-31+G(d,p) basis set. The proton affinity of oxygen and nitrogen atoms and the deprotonation enthalpy of the different N-H bonds of guanine and adenine were computed at the same level of theory. The results were compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the five nucleobases are discussed. Complex formation with water results in a moderate change of the pyramidal character of amino group. For closed complexes, where water interacts with oxygen atom of the nucleobase, the intermolecular distances and the hydrogen bond energies are correlated to the proton affinities and deprotonation enthalpies of the sites involved in complex formation.