Comparative study of the antioxidative activities of caffeoylquinic and caffeic acids.

A detailed conformational analysis was performed to determine the most stable conformers of chlorogenic, cryptochlorogenic, and neochlorogenic acids. The simulated and experimental NMR spectra of caffeoylquinic acids are in excellent agreement. The bond dissociation enthalpies, proton affinities, electron transfer enthalpies, ionisation potentials, and proton dissociation enthalpies for these compounds and caffeic acid in benzene, methanol, and water were used for thermodynamic consideration of the major antioxidative mechanisms: HAT (Hydrogen Atom Transfer), SPLET (Sequential Proton-Loss Electron-Transfer), and SET-PT (Single Electron Transfer - Proton Transfer). All compounds are characterised with very similar values of each enthalpy, suggesting that they will exhibit comparable antioxidative activities. This assumption is in perfect accord with the experimental findings. It was suggested that HAT may be the predominant mechanism in nonpolar solvents, while HAT and SPLET are competitive pathways in polar media. All calculations were performed using the B3LYP-D2/6-311++G(d,p) and M06-2X/6-311++G(d,p) levels of theory and CPCM solvation model.