Insights on the kinetics and mechanisms of the peroxyl radical scavenging capacity of caftaric acid: the important role of the acid–base equilibrium

Abstract

Caftaric acid (CA), also known as cafeoyltartaric acid, is a molecule formed by linking caffeic acid (3,4-dihydroxy cinnamic acid) and tartaric acid through an ester moiety. This compound is found mainly in the juice of grapes and affects the color of white wine. CA exhibits several bioactivities, including anticholinesterase, antioxidant, anti-inflammatory, antidiuretic, antiapoptotic, and antiradical activities. Of these, the radical scavenging activity, particularly in the physiological environment, attracted little attention thus far. In this study, the radical scavenging activity of CA was investigated by DFT calculations. It was found that CA has good HOO˙ radical scavenging activity in water at physiological pH (k_overall > 10⁸ M⁻¹ s⁻¹), but it is only a weak antioxidant in lipid media. The HOO˙ radical scavenging activity follows the formal hydrogen transfer mechanism in lipid media, whereas in water at physiological pH both the single electron transfer and hydrogen transfer mechanisms of the trianion state make contributions. In the aqueous physiological environments, CA exhibits faster HOO˙ radical scavenging activity than the well-known antioxidants Trolox, ascorbic acid, resveratrol, cannabidiolic acid, cannabidiol, syringic acid, and 5-O-methylnorbergenin. Thus CA is predicted to be one of the best antioxidants among phenolic acids, particularly in the aqueous physiological environment.