A DFT and PM6 study of free radical scavenging activity of ellagic acid

Reaction enthalpies related to mechanisms of free radical scavenging activity of ellagic acid and its phenoxide anions were calculated by density functional theory and the semiempirical PM6 method. In addition to the gas phase, calculations are performed for water and benzene as the solvents, which may represent biological liquids and the membrane lipids, i.e., a natural environment for antiradical action. The thermodynamically favored mechanism depends on the polarity of reaction media, deprotonation degree of ellagic acid as well as the properties of scavenging radicals. The most acidic 3-OH group of ellagic acid is the active site for radical inactivation. The ellagate monoanions and dianions possess progressively better scavenging potency than unionized ellagic acid. The sequential proton loss electron transfer mechanism is the preferred reaction pathway for the ellagate monoanion and dianion in water. In benzene, ellagic acid inactivates free radicals by the hydrogen atom transfer mechanism. In the gas phase the latter mechanism is favored for all ellagic acid species. .