In this work we investigate the effect of cis isomerizations and conformational changes on the antioxidant activity of beta-carotene, one of the most important pigments in nature. The electrodonating (omega(-)) and electroaccepting (omega(+)) powers of the most relevant isomers of beta-carotene are first evaluated in polar and nonpolar solvents using density functional theory (DFT), and these quantities are then used to establish an antioxidant scale of the isomers. The electrodonating power, which is directly related to the antioxidant activity, is shown to provide a very good correlation with the experimental data. Next, we compute the intermediate twisted structures of the beta-carotene isomers generated by partial rotation of every single bond in the polyenic chain. The electrodonating and electroaccepting powers are evaluated for each of these intermediate structures along with their maximum absorption wavelengths, which are computed using time-dependent DFT (TD-DFT). The trends observed for both the electrodonating power and the maximum absorption wavelength can be rationalized in terms of the effective conjugated chain length of the structure resulting from single bond rotations. The results obtained are used to analyze the conformational distribution of beta-carotene in the well-resolved photosystem I (PS-I) of purple cyanobacteria. It is then shown that the isomers present in this photosystem are those having the lowest calculated relative energies and that those with enhanced antioxidant activity are preferentially located in the inner core of the protein complex.