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(center dot-) inverts the direction of the transfer, with respect to ET involving other ROS: the O-2(center dot-) becomes the electron donor and carotenoids (CAR) the electron acceptor. Therefore the "antioxidant'' activity of CAR when reacting with O-2(center dot-) 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(center dot-) is drastically different from their reactivity to other ROS. Asthaxanthin (ASTA) is predicted to be a better O-2(center dot-) quencher than LYC and the other CAR. The CAR + O-2(center dot-) reactions were found to be diffusion controlled. The agreement with available experimental data supports the density functional theory results from the present work.