Detailed Investigation of the OH Radical Quenching by Natural Antioxidant Caffeic Acid Studied by Quantum Mechanical Models.

The effectiveness of naturally occurring antioxidant caffeic acid in the inactivation of the very damaging hydroxyl radical has been theoretically investigated by means of hybrid density functional theory. Three possible pathways by which caffeic acid may inactivate free radicals were analyzed: hydrogen abstraction from all available hydrogen atoms, hydroxyl radical addition to all carbon atoms in the molecule, and single electron transfer. The reaction paths were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory including the contribution of tunneling. The more reactive sites in caffeic acid are the C4OH phenolic group and the C4 carbon atom, for the hydrogen abstraction and radical addition, respectively. The single electron transfer process seems to be thermodynamically unfavored, in both polar and nonpolar media. Both hydrogen abstraction and radical addition are very feasible, with a slight preference for the latter, with a rate constant of 7.29 × 10(10) M(-1) s(-1) at 300 K. Tunnel effects are found to be quite unimportant in both cases. Results indicate caffeic acid as a potent natural antioxidant in trapping and scavenging hydroxyl radicals.

The inactivation of lipid peroxide radical by quercetin. A theoretical insight.

The effectiveness of naturally occurring antioxidant quercetin in the inactivation of the damaging lipid peroxide radical was investigated by means of hybrid density functional based approach, using the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. H-atom abstraction in quercetin by CH(3)OO peroxide occurs preferentially at the 4'OH phenolic site, from both kinetic and thermodynamic points of view. In principle, the narrowness of the obtained adiabatic potential-energy profile makes the occurrence of a significant tunnelling contribution possible. In fact, this contribution enhances the value of the computed rate constant at 300 K from 1.94 x 10(1) to 9.63 x 10(3) M(-1) s(-1) indicating that quercetin is a potent natural antioxidant in trapping and scavenging free radicals.

One-electron spin-orbit contribution by effective nuclear charges.

Effective nuclear charges of the main group elements from the second up to the fifth row have been developed for the one-electron part of the spin-orbit (SO) coupling Hamiltonian. These parameters, suitable to be used for SO calculations of large molecular systems, provide a useful and remarkably good approximation to the full SO Hamiltonian. We have derived atomic effective nuclear charges by fitting procedure. Computed fine-structure splitting (FSS) of the doublet and triplet II states of AH species (A is one of the abovementioned elements) have been chosen for this purpose. We have adopted the noniterative scheme, previously reported, according to which SO contributions can be calculated through direct coupling between the II states. The latter have been optimized at B3LYP level using DZVP basis sets. As surrogates for a large number of possible applications, we have widely employed the empirical parameters to compute II-FSSs of diatomic species for which experimental data are available.

Reaction mechanism of molybdoenzyme formate dehydrogenase.

Formate dehydrogenase is a molybdoenzyme of the anaerobic formate hydrogen lyase complex of the Escherichia coli microorganism that catalyzes the oxidation of formate to carbon dioxide. The two proposed mechanisms of reaction, which differ in the occurrence of a direct coordination or not of a SeCys residue to the molybdenum metal during catalysis were analyzed at the density functional level in both vacuum and protein environments. Some DF functionals, in addition to the very popular B3LYP one, were employed to compute barrier heights. Results revealed the role played by the SeCys residue in performing the abstraction of the proton from the formate substrate. The computation of the energetic profiles for both mechanisms indicated that the reaction barriers are higher when the selenium is directly coordinated to the metal, whereas less energy is required when SeCys is not a ligand at the molybdenum site.