SIMULTANEOUS ELECTROCHEMICAL AND ELECTRON-PARAMAGNETIC-RESONANCE STUDIES OF KETO AND HYDROXY CAROTENOIDS

Cyclic voltammogram (CV) and simultaneous electrochemical and electron paramagnetic resonance (SEEPR) measurements have been carried out on the oxygenated carotenoids: echinenone, canthaxanthin, isozeaxanthin a nd rhodoxanthin in dichloromethane. The CV displays are markedly diffe rent from that of beta-carotene. Comproportionation constants, deduced from EPR spin concentration measurements of electrochemically oxidize d dihydroxy beta-carotene and several keto carotenoids vary by four or ders of magnitude. DELTAH(pp) values deduced from the SEEPR spectra of the cation radicals are in the range 13.2 to 14.5 G and the g-factors are 2.0027 +/- 0.0002. These EPR parameters are in accordance with a polyene pi-cation radical structure. Theoretical CVs are calculated us ing DigiSim, a CV simulation program, and the proposed mechanism invol ves three electrode and two homogeneous reactions.