Cis carotenoids: colorful molecules and free radical quenchers.

We present a density functional theory (DFT) and time-dependent density functional theory (TD-DFT) study on the stability, antioxidant properties with respect to the single electron transfer mechanism, and electronic absorption spectra of some isomers (9-cis, 13-cis, and 15-cis) of carotenoids such as astaxanthin, lycopene, and those present in virgin olive oil (lutein, β-carotene, neoxanthin, antheraxanthin, violaxanthin, neochrome, luteoxanthin, mutatoxanthin, and violaxanthin). In general, the calculated relative stability of the cis isomers appears to be in line with experimental observations. It is predicted that the above-mentioned carotenoids (cis and trans isomers) will transfer one electron to the (•)OH radical. However, this transference is not plausible with radicals such as (•)OOH, (•)OC2H5, (•)OOC2H5, (•)NO2, and (•)OOCH2CH═CH2. On the other hand, some carotenoids (β-carotene, lycopene, lutein, astaxanthin, violaxanthin, and antheraxanthin) will likely accept, in a medium of low polarity, one electron from the radical (•)O2(-). However, neoxanthin, auroxanthin, mutatoxanthin, luteoxanthin, and neochrome would not participate in such an electronic transfer mechanism. The TD-DFT studies show that neutral species of the cis and trans isomers maintain the same color. On the contrary, the ionic species undergo a "bleaching" process where the absorption wavelengths shift to longer values (>700 nm). Additionally, the formation of a complex between astaxanthin and Cu(2+) is explored as well as the effect that the metal atom will have in the UV-vis spectrum.