Electrochemical oxidation of carotenoids containing donor/acceptor substituents

Cyclic voltammetry and square wave voltammetry of the carotenoids beta-carotene (I), 7',7'-dimethyl-7'-apo-beta-carotene (II), 7',7'-hexadeuterodimethyl-7'-apo-beta-carotene (III), 7'-cyano-7'-ethoxycarbonyl-7'-apo-beta-carotene (IV), 7',7'-dicyano-7'-apo-beta-carotene (V) and canthaxanthin (VI) show that the electrochemical properties of the synthetic dimethyl and hexadeuterodimethyl compounds are similar to those of the naturally occurring beta-carotene, while the properties of the synthetic dicyano carotenoid are similar to those of the naturally occurring canthaxanthin, and those of the cyanoester compound are of an intermediate nature. The difference in oxidation potentials for the formation of the cation radical and the dication is small for the first set of compounds (less than 36 mV) and large (up to about 200 mV) for the second. Simulation of the CVs with the DigiSim(R) program led to evaluation of the half-wave potentials, heterogeneous rate constants and homogeneous equilibrium constants which describe the oxidation properties of the carotenoids.