Antiradical power of carotenoids and vitamin E: testing the hydrogen atom transfer mechanism

The antiradical capacities of 13 carotenoids (CAR) and vitamin E are explored, by assessing CAR-H bond dissociation energy. Density functional theory (DFT) calculations were performed, in order to evaluate the hydrogen atom transfer (HAT) antiradical mechanism. Results indicate that C4 or C4' is not always the reactive position when it is unsubstituted and also that CAR without H atoms in the 4 position may be as effective against free radicals as other CAR with H atoms in C4 and C4'. Lutein is the most effective antiradical for the purpose of hydrogen abstraction, whereas the least effective antiradical for this process is canthaxanthin, which is one of the reddest CAR. Vitamin E is not as effective as most of the yellow CAR but may be a better antiradical than canthaxanthin. In addition to the CAR-H bond dissociation energy, the number of reactive positions as we report in this paper represents another important aspect for consideration, when analyzing capacity for scavenging free radicals. Many additional aspects exist, which we do not consider here; thus we cannot attempt to reflect all the factors seen in vivo. However, our results provide comparative information on the relative ability of CAR to protect against free radicals, using the CAR-H bond dissociation energy, as one useful parameter. We hope that our theoretical results will contribute to the advancement of this complex research field.