Carotenoids can act as antioxidants by oxidizing the superoxide radical anion

The electron transfer (ET) reaction between carotenoids and the superoxide radical anion is found to be not only a viable process but also a very unique one. The nature of the O(2) (-) inverts the direction of the transfer, with respect to ET involving other ROS: the O(2) (-) becomes the electron donor and carotenoids (CAR) the electron acceptor. Therefore the "antioxidant" activity of CAR when reacting with O(2) (-) lies in their capacity to prevent the formation of oxidant ROS. This peculiar charge transfer is energetically feasible in non-polar environments but not in polar media. In addition the relative reactivity of CAR towards O(2) (-) is drastically different from their reactivity to other ROS. Asthaxanthin (ASTA) is predicted to be a better O(2) (-) quencher than LYC and the other CAR. The CAR + O(2) (-) reactions were found to be diffusion controlled. The agreement with available experimental data supports the density functional theory results from the present work.