Facile method for spectroscopic examination of radical ions of hydrophilic carotenoids

Hydrophilic carotenoids, unusual members of an intrinsically hydrophobic family, and their radical ions are important reactants. An all-optical method for generating singly charged radical ions of a hydrophilic carotenoid (Car) is described. It relies on photolyzing an aqueous mixture of Car and a photoionizable auxiliary solute (A), and making conditions conducive to the capture, by Car, of the hydrated electron (e(aq)(-)) or the positive hole in A(*)(+) or both. When A is Trolox (TOH), only e(aq)(-) can be captured, since TOH (*)(+) deprotonates too rapidly to be a hole donor; when A is Trolox methyl ether (TOMe), both Car(*)(-) and Car(*)(+) are formed, since TOMe (+) lives long enough to transfer its positive hole to Car; formation of Car(*)(-) is prevented under aerobic conditions.