What is important to prevent oxidative stress? A theoretical study on electron-transfer reactions between carotenoids and free radicals.

Oxidative stress is related to the development of a large number of health disorders. Therefore, the study of molecular systems capable of preventing its onset by fighting free radicals is a crucial area of research. Carotenoids are one of the most efficient families of compounds fulfilling this purpose. In the present work, the free-radical-scavenger efficiency, expressed as the one-electron-donating capability, of different carotenoids has been studied using density functional theory. A large number of free radicals were considered, as well as environments of different polarity. A new donor-acceptor map is proposed that allows a rapid evaluation of full electron-transfer processes. Its efficiency for predicting the feasibility of electron transfer (ET) between carotenoids and free radicals was tested and validated through comparison with the corresponding Gibbs free energies of reaction. Our results demonstrate that ET reactions between carotenoids and free radicals are strongly influenced by the nature of the latter. Moreover, it is proposed that the electron affinity (EA) of the reacting free radical has an important effect on the viability of these reactions. The reactions were found to become exergonic when the EA of the free radical involved reaches a value of approximately 5 eV.