Is Usnic Acid a Promising Radical Scavenger?

Usnic acid (UA) is a natural product found in the lichen genera. Because of the phenolic groups in its structure, UA is suspected to be an antioxidant. Therefore, in this study, the radical scavenging of UA was investigated in physiological environments in silico by using kinetic calculations. It was found that the overall rate constant for the hydroxyl radical scavenging activity was approximately 10⁹ M^–1 s^–1 in all environments, whereas the HOO^• and CH₃OO^• radical scavenging activities were only significant in the polar environments with k in the range of 10³–10⁴ M^–1 s^–1. The results also revealed that the HO^• scavenging activity followed the single electron transfer (SET) and radical adduct formation mechanisms; however, the SET pathway (for the dianion HU^2–) played a dominant role in the scavenging of other studied radicals, including CH₃O^•, CCl₃O^•, CCl₃OO^•, NO₂, SO₄^•–, and N₃^•. The activity of UA against these radicals was as high as that of typical phenolic acids such as ferulic acid, p-coumaric acid, caffeic acid, dihydrocaffeic acid, and sinapinic acid (k_f ∼ 10⁸ M^–1 s^–1) in polar solvents. Thus, UA is a promising natural antioxidant in aqueous environments.