The Theoretical and Experimental Insights into the Radical Scavenging Activity of Rubiadin

Rubiadin (RBD), an anthraquinone derivative, is obtained from Rubia cordifolia, a plant species classified under the Rubiaceae family. Rubiadin has proven beneficial properties, such as anticancer, neuroprotective, anti-inflammatory, and antidiabetic activity. The antioxidant activity of this molecule was suggested by some experimental results but has not been clearly established thus far. In this study, we employ DFT calculations to comprehensively assess the mechanism and kinetics of the HO^•/HOO^• radical scavenging activity of this compound in relation to solvents. RBD showed moderate HO^• radical scavenging activity, with rate constants of 2.95 × 10⁸ and 1.82 × 10¹⁰ M^-1 s^-1 in lipid and polar media, respectively. In the aqueous solution, the compound exhibited remarkable superoxide anion radical scavenging activity (k = 4.93 × 10⁸ M^-1 s^-1) but modest HOO^• antiradical activity. RBD also showed promising antiradical activity against a variety of radicals (CCl₃O^•, CCl₃OO^•, NO₂, SO₄^•-, and N₃^•), while experimental and computational results confirmed that RBD has moderate activity in DPPH/ABTS^•+ assays. Thus, RBD is predicted to be a good, albeit selective, radical scavenger.