Radical scavenger competition of alizarin and curcumin: a mechanistic DFT study on antioxidant activity.

In vivo hydroxyl, peroxyl, and superoxide free radicals caused by oxidative stress can be toxic to molecules that are essential for the human body. However, there are natural compounds that can decrease the amount of these harmful species. In this work, we are focusing on two well-known compounds, alizarin (red) and curcumin, to study their interactions with these small radicals for a comparison between a rigid and a flexible structure. We made a mechanistic study and found the major and minor degradation products of curcumin as well as the autoxidation products of it based on a wide range of literature. We found several more favored pathways than those that were previously proposed. On the contrary, for degradation/oxidation of alizarin, only a few proposed mechanisms can be found which were performed in specific conditions. Our calculations predicted some favored rearrangements for the alizarin by peroxyl and superoxide radicals. Interaction of alizarin red and bright yellow curcumin with small radicals like hydroxyl, peroxyl, and superoxide radicals, such as the reaction between curcumin radicals and oxygen molecule, results in different species like epoxides or another kind of radical forms. The stability of epoxides is different in the case of rigid and flexible structures.