Modeling of interaction of alkali and alkaline earth metal cations with naturally occurring antioxidants (uric acid and ascorbic acid)

The results of theoretical investigations of structures and properties of neutral, anionic and radical forms of two natural antioxidants namely uric acid and ascorbic acid (vitamin C) and interaction of alkali and alkaline earth metal cations with different urate and ascorbate are reported. Molecular geometries of the all considered complexes were optimized at the Density Functional Theory (DFT) level utilizing the B3LYP functional and the 6-311++G(d,p) basis set. Nature of potential energy surfaces was ascertained by the harmonic vibrational frequency analysis. All geometries were found minima at the respective potential energy surfaces. Single point energy calculations were also performed at the MP2 level. Effect of aqueous environment was modeled using the PCM solvation model. Interaction energy of urate and ascorbate metal cation complexes were corrected for the basis set superposition error. The most probable forms of antioxidants in the gas phase and in water solution are discussed. Further the comparative role of relative stability of different anions and amount of interaction energy in deciding the stability of metal cation coordinated urate and ascorbate complexes are also discussed.