The radical scavenging activity of monocaffeoylquinic acids: the role of neighboring hydroxyl groups and pH levels

Abstract

Caffeoylquinic acids (CQAs) are well-known antioxidants. However, a key aspect of their radical scavenging activity – the mechanism of action – has not been addressed in detail thus far. Here we report on a computational study of the mechanism of activity of CQAs in scavenging hydroperoxyl radicals. In water at physiological pH, the CQAs demonstrated ≈ 10⁴ times higher HOO˙ antiradical activity than in lipid medium (k_(lipid) ≈ 10⁴ M⁻¹ s⁻¹). The activity in the aqueous solution was determined by the hydrogen transfer mechanism of the adjacent hydroxyl group (O6′–H) of the dianion states (Γ = 93.2–95.2%), while the single electron transfer reaction of these species contributed 4.8–6.8% to the total rate constants. The kinetics estimated by the calculations are consistent with experimental findings in water (pH = 7.5), yielding a k_calculated/k_experimental = 2.4, reinforcing the reliability and precision of the computational method and demonstrating its utility for evaluating radical reactions in silico. The results also revealed the pH dependence of the HOO˙ scavenging activity of the CQAs; activity was comparable for all compounds below pH 3, however at higher pH values 5CQA reacted with the HOO˙ with lower activity than 3CQA or 4CQA. It was also found that CQAs are less active than Trolox below pH 4.7, however over pH 5.0 they showed higher activity than the reference. The CQAs had the best HOO˙ antiradical activity at pH values between 5.0 and 8.6. Therefore, in the physiological environment, the hydroperoxyl antiradical capacity of CQAs exhibits similarity to renowned natural antioxidants including resveratrol, ascorbic acid, and Trolox.