Reactivity of chlorogenic acid toward hydroxyl and methyl peroxy radicals relative to trolox in nonpolar media

Abstract

The quantum mechanics-based test for overall free-radical scavenging activity was applied for the investigation of antioxidative capacity of chlorogenic acid (5-O-caffeoylquinic acid, 5CQA) relative to trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid, Tx) as a reference compound. Hydrogen atom transfer (HAT), radical adduct formation (RAF), electron transfer (ET), and proton loss (PL) reactions of 5CQA and Tx with HO• and CH3OO• radicals in benzene and pentyl ethanoate were examined. For this purpose, two theoretical models, M06-2X/6-311++G(d,p) in combination with the CPCM solvation model, and M05-2X/6-311++G(d,p) in combination with the SMD solvation model, were employed. It was found that M05-2X/6-311++G(d,p)—SMD failed to evaluate the influence of pentyl ethanoate, whereas M06-2X—CPCM in benzene and pentyl ethanoate and M05-2X—SMD in benzene proved to be operative and showed similar trends. Both compounds can react with HO• via HAT and RAF mechanisms, whereas HAT is the only reaction pathway with CH3OO•. 5CQA is more efficient scavenger of HO• than Tx, but less efficient scavenger of CH3OO•. Less reactive free radicals are more suitable for the determination of antioxidative activity of a compound relative to Tx. Highly reactive free radicals need to be included in the investigation of all potential reaction pathways of the compound, in which case the results from different approaches can be inconsistent.