Theoretical study of the acidity and basicity of uric acid and its interaction with water

The proton affinity (PA) of the O atoms and the deprotonation enthalpies (DPE) of the NH bonds of isolated uric acid are calculated using the density functional theory (B3LYP) combined with the 6-31++G(d,p) basis set. The optimized geometries, energies and harmonic vibrational frequencies of monohydrated uric acid are calculated at the same level of theory. The five stable complexes between uric acid and water are the ones in which the oxygen atom of water accepts the acidic NH proton while donating a proton to the carbonyl oxygens of uric acid. The binding energies of these complexes ranging from 24 to 37 kJ mol−1 are analysed in terms of PAs for O atoms and the acidity of the NH groups of isolated uric acid. A natural bond orbital analysis is performed to understand the nature of hydrogen bonding interaction in monohydrated uric acid and also to reveal the inter-relations between electronic structure and other properties.