Charged carotenoid species

Abstract

Abstract Carotenoids constitute a class of bioactive natural products. A short overview of naturally occurring carotenoids is presented. Although carotenoids are generally yellow-red isoprenoid polyene pigments, their light absorption range includes the UV-VIS-NIR region with charged carotenoids in the NIR region. The bioactivity of carotenoids are treated briefly as provitamins A and as proretinoids more generally, participation in photosynthesis and in photoprotection in a wider context, as well as in cell membrane stabilisation. Their actions as antioxidants and in health aspects receive much current interest. Some structure/activity relationships are considered. Carotenoid cation radicals are involved in photosynthesis and in antioxidant reactions. Charged carotenoids are treated in a systematic manner in this first comprehensive survey of the topic. Carotenoids with charged functional groups comprise naturally occurring carotenoid sulphates, which have been examined in detail, well known carotenoid carboxylic acids / carboxylates, synthetic phosphates, blue carotenoid oxonium ions studied in some detail, and blue carotenoproteins continuously investigated. Carotenoid cation radicals have received much interest in recent years. Theoretical calculations, their involvement in photosynthesis and in vitro preparation methods (electrochemically, via excited states, by Lewis acids or by oxidation with iodine) are referred to. Methods for characterisation include CV, NIR, EPR, ENDOR and resonance Raman spectroscopy. Properties and reactions of cation radicals are considered. For carotenoid dications are treated theoretical calculations, preparation by electrochemical oxidation or by treatment with FeCl 3 of BF 3 as suitable Lewis acids and their characterisation by NIR IR, and also recently by detailed NMR spectroscopy. Reactions of carotenoid dications with suitable nucleophiles are illustrated. Spinless carotenoid monocations are treated seperately. Included are theoretical calculations, hypothetical monocation intermediates in reactions with carotenols and 5,6-epoxides in acidic media, and characterised (NIR, NMR) monocations. Carotenoid anions including anion radicals and dianions are less studied. Theoretical calculations, preparation and spectral properties (NIR, Raman, EPR) are referred to. Relative to common carotenoids are carotenoids carrying a charge on the polyene chain particularly unstable and represent special challenges. This review gives 158 literature references, including 126 references since 1990.