The optimized geometries, harmonic vibrational frequencies, and energies of the cyclic structures of monohydrated guanine and adenine are computed using density functional theory (B3LYP) combined with the 6-31+G(d,p) basis set. 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 level of theory. The results are 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 the amino group. For closed complexes where water interacts with the O 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.