Quantum computations of the UV-visible spectra of uric acid and its anions

UV-visible spectra of uric acid as a function of pH have been studied and compared with experimental data. Different quantum methodologies have been employed: CIS-ZINDO, CIS-HF and TD-DFT. Solvent effects were modeled using the COSMO model. To account for the change in maximum absorbance along a variation of pH, different forms of uric acid have been taken into account: uric acid represented the major species at low pH, urate monoanions represented the major components of urate solution at pH 7, and urate dianions represented the major components of urate solution at pH above 10. Our computational results show CIS-ZINDO and CIS-HF quantum methods do not reproduce well experimental absorption spectra whatever the pH. In contrast, TD-DFT in conjunction with the COSMO continuum model gives very good agreement with experimental data and strongly indicates at medium and high pH uric acid solution is composed of a mixing of the different possible existing species. At neutral pH, uric acid exists as a mixture of a major monoanion component and a minor monoanion component, while at high pH, several dianion species seem to coexist.