Antioxidative mechanisms in chlorogenic acid

- 5CQA was selective towards DPPH, HO, and O2− in the ESR measurements.
- The HAT, RAF, SPLET, and SET-PT mechanisms were investigated.
- Thermodynamic approach included the O2−, HOO, HO, and DPPH radical species.
- Mechanisms of all antioxidative paths of 5CQA and HO were studied for the first time.
- HAT, RAF (neutral and acidic media), and SPLET (basic media) are likely pathways.

Although chlorogenic acid (5CQA) is an important ingredient of various foods and beverages, mechanisms of its antioxidative action have not been fully clarified. Besides electron spin resonance experiment, this study includes thermodynamic and mechanistic investigations of the hydrogen atom transfer (HAT), radical adduct formation (RAF), sequential proton loss electron transfer (SPLET), and single electron transfer - proton transfer (SET-PT) mechanisms of 5CQA in benzene, ethanol, and water solutions. The calculations were performed using the M06-2X/6-311++G(d,p) level of theory and CPCM solvation model. It was found that SET-PT is not a plausible antioxidative mechanism of 5CQA. RAF pathways are faster, but HAT yields thermodynamically more stable radical products, indicating that in acidic and neutral media 5CQA can take either HAT or RAF pathways. In basic environment (e.g. at physiological pH) SPLET is the likely antioxidative mechanism of 5CQA with extremely high rate.