Conformations of Flavan-3-ols in Water: Analysis Using Density Functional Theory

To elucidate the conformations and their relative stabilities of flavan-3-ols in water, the optimized structures and energies were calculated by density functional theory, in which M06-2X, ωB97X-D, B3LYP, and CAM-B3LYP were examined as exchange-correlation functionals. The results were evaluated by comparing calculated spin-spin coupling constants of vicinal protons with the corresponding values observed in experimental ¹H NMR spectra in D₂O. The M06-2X and ωB97X-D results showed good agreement with the experimental NMR data and revealed that the B-ring (pseudoequatorial)/3-O (pseudoaxial) conformers were more stable [5.4-8.7 kJ/mol (1.3-2.1 kcal/mol)] than the B-ring (pseudoaxial)/3-O (pseudoequatorial) conformers in the 2,3-cis-flavan-3-ols, while the B-ring (pseudoequatorial)/3-O (pseudoequatorial) conformers and the B-ring (pseudoaxial)/3-O (pseudoaxial) conformers had similar stabilities in the 2,3-trans-flavan-3-ols.

Molecular interactions between caffeine and catechins in green tea

Migration of green tea components from an active packaging material containing green tea extract was performed in water and 3% acetic acid in water. The migration values for acid simulant were much higher than the values obtained in water. The influence of the acidic media in solutions of catechin standards and green tea extract was evaluated by liquid chromatography. Catechin, epicatechin, and caffeine from the green tea extract exhibited major variation in their concentrations values, with increases of 29.90, 20.75, and 15.95%, respectively, in acidic medium. The results suggested that catechins and caffeine form complexes through intermolecular interactions in neutral media and that these interactions are broken in acidic media. The continuous variation method was also performed to confirm the stoichiometry of the complexes between catechins and caffeine. Finally, a computer simulation was applied by Chem Pro 12.0, and the energies involved were calculated to confirm the experimental results obtained.