SIMULTANEOUS ELECTROCHEMICAL AND ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF CAROTENOID CATION RADICALS AND DICATIONS

Comproportionation equilibrium constants have been determined from simultaneous electrochemical and EPR measurements for the carotenoid cation radicals (CAR.+) and dications (CAR2+) of beta-carotene (I), beta-apo-8'-carotenal (II), and canthaxanthin (III). K(I)com = 2.4 x 10(-2), K(com)(II) = 1.8 x 10(-2), K(com)(III) = 2.1 x 10(3). These indicated that, upon oxidation of III, 96% CAR.+ would be formed while 99.7% CAR2+ would be formed for I and II if the oxidation potential was 100 mV anodic of the first observed voltammetric wave. This explains the reason for the strong EPR spectrum observed for III and the weak EPR spectra observed for I and II. Rotating disk experiments confirm that oxidation of carotenoids occurs by an EE rather than by an ECE mechanism and are highly quasireversible systems. The second oxidation peak in the CV spectrum of II has been shown not to be due to a dication analogous to the CV of III but to a radical apparently from the oxidation of a decay product of the dication from II.