Electrochemical properties of natural carotenoids

Carotenoids are widely distributed among plants, certain bacteria and animals, serving for light harvesting and photoprotection in photosynthesis and as antioxidants. Electrochemical data (such as oxidation potentials, reaction rate constants, kinetic equilibrium constants) for 12 naturally occurring carotenoids are summarized in this paper. Electrochemical parameters were estimated by simulating experimental cyclic voltammograms (CVs). Conventional electrochemical techniques (cyclic voltammetry and Osteryoung square-wave voltammetry) were used in these studies. CVs of some natural carotenoids are presented and discussed. The dependence of redox potentials and other kinetic parameters on the structures of carotenoids is summarized. The oxidation potential of neutral carotenoid (E(1)degrees), which corresponds to the formation of cation radicals, varies from 0.50 to 0.72 V versus SCE. The oxidation potential of carotenoid cation radical (E(2)degrees), which corresponds to the formation of a dication, usually lies in the region of 0.52-0.95 V versus SCE. The electrochemical data presented in this article should be helpful in the study of reaction processes in model systems or actual photosynthesis reaction centers. (C) 2000 Elsevier Science S.A. All rights reserved.