ABSTRACT:Geometries of the neutral and ionic tautomeric species of ascorbic acid were optimized at the density functional theory (DFT) level using the B3LYP functional. The radical species were evaluated using the unrestricted B3LYP method. Single-point energy calculations were also performed using the Møller-Plesset (MP2) and unrestricted MP2 (UMP2) methods for the closed-shell and open-shell systems, respectively. The effects of aqueous solution were evaluated using the conducting polarized continuum model (CPCM) and polarized continuum model (PCM). The geometries of most stable radicals in the respective groups were also optimized in the water solution using the CPCM model at the UB3LYP level. All calculation were performed using the 6-311ϩϩG(d,p) basis set. The nature of stationary points on the gas phase potential energy surfaces (PESs) was evaluated using vibrational frequency calculations; all geometries characterize local minima. The species obtained by the deprotonation of the O 3 site is the most stable monoanion of ascorbic acid. For the radical species, the structure obtained by the dehydrogenation of the O 3 site is the most stable monoradical. Among the radical anions, the species obtained by the deprotonation of the O 3 site and subsequent dehydrogenation of the O 2 site is the most stable in the gas phase and in an aqueous medium. The computed isotropic hyperfine coupling constants of this species were found to be in good agreement with the experimental data. Our investigation also supports the earlier findings that the oxidized species of ascorbic acid in water solution by the OH• radical is radical anion of the AAO 3• O 2 Ϫ form. The spin densities and molecular electrostatic potentials are also discussed.

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.

ABSTRACT:A detailed study of tautomeric properties of uric acid, its different anions, and radical species was performed at the DFT level, employing B3LYP functional and the 6-311ϩϩG(d,p) basis set. Single-point energy calculations were also performed at the MP2 level using B3LYP/6-311ϩϩG(d,p) optimized geometries and the cc-pVTZ and 6-311ϩϩG(d,p) basis sets. The effect of aqueous solvation on the relative stability of the neutral and anionic species was investigated using Tomasi's polarized continuum model. The keto form of the molecule was found to be the most stable in the gas phase and in aqueous medium. The proton transfer barrier height was also computed. The gas-phase barrier height is high; however, the inclusion of a water molecule in the proton transfer reaction path reduces the barrier significantly. Among monoanions of uric acid, the species obtained by the deprotonation of the N 3 site is the most stable, whereas among dianions, the anion obtained by deprotonation of both N 3 and N 9 sites is the most stable both in the gas phase and in aqueous medium. It appears that both forms of radicals UAN 3• and UAN 9• would exist. Among radical anions, the species obtained by the dehydrogenation of the N 3 site of the N 7 anion (the N 7 anion was obtained by deprotonation of the N 7 site of uric acid) is the most stable in the gas phase and in aqueous solution. The molecular electrostatic potential, ionization potential, and electron affinity are also reported.

The structures and interactions of different possible ascorbic acid anions (ascorbates) with Li, and Ca 2ϩ metal cations were studied at the density functional theory level employing the B3LYP exchange correlation functional and the 6-311ϩϩG(d,p) basis set. The interactions of the metal cations at the different basic sites of ascorbate were considered. The single-point energy calculations were also performed at the MP2/6-311ϩϩG(d,p) level. Tomasi's polarized continuum model (PCM) was used to evaluate the influence of aqueous solvent on the relative stability of different complexes. The interaction energies for the complexes were corrected for the basis set superposition error (BSSE) using the counterpoise method. The AIM theory was used to characterize the electron density distribution involved in the coordination of studied metal cations with ascorbate. In water, the most preferred position for the interaction of Li

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