Electronic structure of orotic acid II: Acidity, basicity, and interaction with water

The optimized geometries and energies of the four cyclic structures of the orotic acid-water complex are computed using density functional theory (B3LYP) combined with the 6-31++G(d,p) basis set. In the three most stable cyclic structures, the water molecule accepts the NH proton and donates a proton to the carbonyl oxygen atoms. Less stable cyclic complexes involving the C5H bond also formed. The proton affinity of the oxygen atoms and the deprotonation enthalpy of the NH bonds of orotic acid are computed at the same level and compared with reported data on uracil and thymine. The most stable cyclic complex is formed at the oxygen lone pair characterized by the lowest basicity and at the NH bond characterized by the highest acidity. Although intrinsic acidities are lower in orotic acid than in uracil and thymine, the binding energies with one water molecule do not differ markedly for uracil and orotic acid. Substituent of carboxylic group at the 6-position, however do not seem to have much effect on the proton affinities and the deprotonation enthalpy of uracil.