A Density Functional Theory Study on Aurantinidin

A complete conformational analysis on the isolated and PCM modeled aqueous solution cation, neutral, and anion forms of aurantinidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The most stable conformer of isolated cationic and neutral forms of aurantinidin are completely planar, whereas ring B and 3OH are significantly twisted with regard to the AC system in the anion. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle even in the most stable conformers of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for hydroxyls at 3 and 5 positions, whereas syn and anti orientation of hydroxyls at 7 and 4’ positions are nearly isoenergetic. The most stable tautomer of neutral aurantinidin is obtained by deprotonating hydroxyl at C5 in gas phase, but at C7 in water solution, according to PCM. Also the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM calculations for water solution (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Intramolecular hydrogen bond between O3 and H6' is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies are consistent with an important antioxidant activity.