GEOMETRICAL ISOMERIZATION OF CAROTENOIDS MEDIATED BY CATION-RADICAL DICATION FORMATION

Electrochemical oxidation of all-trans-canthaxanthin and beta-carotene in dichloromethane leads to significant trans-to-cis isomerization, w ith cis isomers accounting for about 40% of the products formed. The e lectrochemically generated isomers were separated by reverse-phase hig h-performance liquid chromatography and identified as 9-cis, 13-cis, 1 5-cis, and 9,13-di-cis isomers of the carotenoids by H-1-NMR spectrosc opy and optical spectroscopy (Q ratio). The results of simultaneous bu lk electrolysis and optical absorption spectroscopy indicate the follo wing isomerization mechanism: the all-trans cation radicals and/or dic ations formed by electrochemical oxidation of all-trans-carotenoids ca n easily undergo geometrical isomerization to form cis cation radicals and/or dications. The latter are converted by the comproportionation equilibrium to cation radicals which are then transformed to neutral c is-carotenoids by exchanging one electron with neutral carotenoids. AM 1 molecular orbital calculations, which show that the energy barriers of configurational transformation from trans to cis are much lower in the cation radical and dication species than in the neutral molecule, strongly support the first step of this mechanism.