Correlations of computational ionization energy with experimental oxidation potential and with antioxidant efficiencies in catechins

Ionization energies of catechins were calculated by using B3LYP density functional method with 6-31G** basis set. Then, conformations of catechins were extensively explored. Geometry optimizations starting from the explored conformers were made not only for the neutral species but also for the radical cation. The ionization energy of each catechin was estimated as the energy difference between the most stable geometries obtained for the neutral species and radical cation. Tendency of experimental oxidation potential in catechins is well reproduced by this calculation, and the ionization energy correlates well with the singlet-oxygen quenching efficiency in case that stereochemistry of attachment at the 3-position is common. Electron transfer from catechins plays an important role in the singlet-oxygen quenching, free-radical scavenging and recycling from vitamin E radical to vitamin E. The reason for discrepancy among antioxidant efficiencies with regard to the ionization energy dependence is explained.