PHOTOPHYSICS OF THE CAROTENOIDS ASSOCIATED WITH THE XANTHOPHYLL CYCLEIN PHOTOSYNTHESIS

Green plants use the xanthophyll cycle to regulate the flow of energy to chlorophyll a within photosynthetic proteins. Under conditions of l ow light intensity violaxanthin, a carotenoid possessing nine conjugat ed double bonds, functions as an antenna pigment by transferring energ y from its lowest excited singlet state to that of chlorophyll a withi n light-harvesting proteins. When the light intensity increases, viola xanthin is biochemically transformed into zeaxanthin, a carotenoid tha t possesses eleven conjugated double bonds. The results presented here show that extension of the pi conjugation of the polyene lowers the e nergy of the lowest excited singlet state of the carotenoid below that of chlorophyll a. As a consequence zeaxanthin can act as a trap for t he excess excitation energy on chlorophyll a pigments within the prote in, thus regulating the flow of energy within photosynthetic light-har vesting proteins.