Antioxidant Properties of Phenolic Compounds:  H-Atom versus Electron Transfer Mechanism

Abstract

Density functional theory (DFT) calculations were performed to evaluate the antioxidant activity of molecules commonly present in many Mediterranean foods. A series of interesting systems, including tyrosol, hydroxytyrosol, gallic, and caffeic acids, belonging to the family of phenols, resveratrol of the stilbenes family, epicatechin, kaempferol, and cianidin as examples of flavonoids and, at last, a simplified model of R-tocopherol (6-hydroxy-2,2,5,7,8-pentamethylchroman (HPMC)) were studied. Conformational behavior was examined at the B3LYP/6-311++G(3df,2p) level of theory, in the gas phase and in two solvents with different polarity (water and benzene), with the aim to compute the bond dissociation enthalpy (BDE) for the O-H bonds and the adiabatic ionization potentials (IP). BDE and IP for these systems do not follow the same trends in gas and solution phases: the major differences with respect to vacuum are found as when water computations are performed. On the basis of the computed BDE and IP values, the most active systems able to transfer an H-atom seem to be R-tocopherol followed by hydroxytyrosol, gallic acid, caffeic acid, and epicatechin. Instead, kaempferol and resveratrol appear to be the best candidates for an electron-transfer mechanism.

Antioxidant activities and lutein content of 11 marigold cultivars (Tagetes spp.) grown in Thailand

“…This result coincides with the observations of two following studies. Leopoldini et al (2004) compared the mechanism of hydrogen atom and electron transfer in phenolic compounds, and concluded gallic acid to be one of the most active systems in the transfer of hydrogen atoms. Supporting evidence was also provided by the research conducted by Rice-Evans et al (1996), which showed the effect of the chemical structure of flavonoids and phenolic acids on the antioxidant activity.…”

FREE RADICALS AND ANTIOXIDANTS: ANTIOXIDATIVE AND OTHER PROPERTIES OF SWISS CHARD (Beta vulgaris L. subsp. cicla)

“…The antioxidant activity of phenolics is mainly due to their redox properties, which allow them to act as reducing agents, hydrogen donators, and radical and singlet oxygen quenchers. In addition, they have a metal chelation potential (Kӓhkӧnen et al, 1999;Rice-Evans et al, 1995;Leopoldini et al, 2004).…”

Recent advances in 1,4-benzoquinone chemistry

“…241 The antifungal and antioxidant activities of flavonoids depend on the stability of these semiquinones and quinones. 242 The antioxidant activity is directly related to the ease of deprotonation of its OH groups. Consequently the structural properties, the bond dissociation energy and total energy of these compounds from epicatechin was determined using B3LYP/6-31G** level of calculation.…”