Electronic structure of hydroxylated derivatives of the flavylium cation

Antioxidant Activity of Anthocyanins and Anthocyanidins: A Critical Review

Anthocyanins are the main plant pigments responsible for the color of flowers, fruits, and vegetative organs of many plants, and are applied also as safe food colorants. They are efficient antioxidants. In this review, the reactivity of anthocyanins and their aglycones, anthocyanidins, in the main antioxidant assays, and their reactions with reactive oxygen and nitrogen species, effects of interactions with other compounds and metal ions on the antioxidant activity of anthocyanins and the electrochemical properties of anthocyanins are presented. Numerous cases of attenuation of oxidative stress at the cellular and organismal levels by anthocyanins are cited. The direct and indirect antioxidant action of anthocyanins, the question of the specificity of anthocyanin action in complex extracts, as well as limitations of cellular in vitro assays and biomarkers used for the detection of antioxidant effects of anthocyanins, are critically discussed.

Resonance structure contributions, flexibility, and frontier molecular orbitals (HOMO-LUMO) of pelargonidin, cyanidin, and delphinidin throughout the conformational space: application to antioxidant and antimutagenic activities

This research refers to the study and understanding of the conformational space of the positive-charged anthocyanidin structures in relation with the known chemical reactivities and bioactivities of these compounds. Therefore, the planar (P) and nonplanar (Z) conformers of the three hydroxylated anthocyanidins pelargonidin, cyanidin, and delphinidin were analyzed throughout the conformational space at the B3LYP/6-311 ++ G** level of theory. The outcome displayed eleven new conformers for pelargonidin, fifty-four for cyanidin, and thirty-one for delphinidin. Positive-charged quinoidal structures showed a significant statistical weight in the conformational space, thus coexisting simultaneously with other resonance structures, such that under certain reaction conditions, the anthocyanidins behave as positive-charged quinoidal structures instead of oxonium salts. The calculations of the permanent dipole moment and the polarizability showed relationships with the quantity and arrangement of hydroxyls in the structure. In addition, theoretical calculations were used to analyze the frontier molecular orbitals (HOMO-LUMO) of the three anthocyanidins. The novel conception of this work lies in the fact that dipole moment, polarizability, and HOMO-LUMO values were related to the reactivity/bioactivity of these three anthocyanidins. HOMO-LUMO energy gaps were useful to explain the antioxidant activity, while the percent atom contributions to HOMO were appropriate to demonstrate the antimutagenic activity as enzyme inhibitors, as well as the steric and electrostatic requirements to form the pharmacophore. Delphinidin was the strongest antioxidant anthocyanidin, and pelargonidin the best anthocyanidin with antimutagenic activity.

Structural conformations and vibrational spectral study of chloroflavone with density functional theoretical simulations

NIR-FT Raman and FT-IR spectra of 6-chloroflavone were recorded and analyzed. The vibrational wavenumber of the compound have been computed using B3LYP/6-31++G(d,p) level to derive the equilibrium geometry, conformational stability, molecular orbital energies and vibrational wavenumbers. The carbonyl stretching vibrations have been lowered due to conjugation and hydrogen bonding in the molecules. The assignment of fundamental vibrations agrees well with the calculated wavenumbers.

Molecular structure and antioxidant properties of delphinidin

Density functional theory calculations were performed to evaluate the antioxidant activity of delphinidin, taking into account its acid/base equilibrium. The conformational behavior of both the isolated and the aqueous solvation species (simulated with the polarizable continuum model) were analyzed at the B3LYP/6-31++G(d,p) level, considering the cationic, neutral, and anionic forms, the latter two forms consisting of diverse tautomers. The analysis of their electron density distributions, using the quantum theory of atoms in molecules, reveals several facts that are not in line with their usual Lewis structures. The prototropic preferences observed in the gas phase and in solution are similar. Thus, in both phases, most stable tautomer of neutral delphinidin is obtained by deprotonating the hydroxyl at C4', and the most stable tautomer of the anion is obtained by deprotonating the hydroxyls at C4' and C5. All the planar conformers obtained display an intramolecular hydrogen bond (IHB) between O3 and H6'. Furthermore, the most stable tautomers of the neutral and anionic forms display two IHBs between O4' and H3' and H5'. To obtain ionization potentials (IPs) and homolytic O-H bond dissociation enthalpies (BDEs), the corresponding radical species were optimized at the UB3LYP level. Heterolytic O-H bond dissociation enthalpies (proton dissociation enthalpies, PDEs) were also computed. The expected important antioxidant activity can be justified from these results. IP, O-H BDE, and O-H PDE values suggest that one-step H atom transfer rather than sequential proton loss-electron transfer or electron transfer-proton transfer would be the most favored mechanisms for explaining the antioxidant activity of delphinidin in nonpolar solvents as well as in aqueous solution.

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer 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, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 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 simulation (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. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.