Photophysics of the carotenoids associated with the xanthophyll cycle in photosynthesis

Green plants use the xanthophyll cycle to regulate the flow of energy to chlorophyll a within photosynthetic proteins. Under conditions of low light intensity violaxanthin, a carotenoid possessing nine conjugated double bonds, functions as an antenna pigment by transferring energy from its lowest excited singlet state to that of chlorophyll a within light-harvesting proteins. When the light intensity increases, violaxanthin is biochemically transformed into zeaxanthin, a carotenoid that possesses eleven conjugated double bonds. The results presented here show that extension of the {squared rising diagonal slash} conjugation of the polyene lowers the energy of the lowest excited singlet state of the carotenoid below that of chlorophyll a. As a consequence zeaxanthin can act as a trap for the excess excitation energy on chlorophyll a pigments within the protein, thus regulating the flow of energy within photosynthetic light-harvesting proteins.