Specificity of light-harvesting systems of chromophytes in reference to their energy migration

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To understand why, among a vrieties of carotenoid species, fucoxanthin is used for the light-harvesting in chromophyte algae, an attempt was made to replace the fucoxanthin molecules in fucoxanthin-chlorophyll a/c protein with different species of carotenoids and to analyze their excitation energy transfer to chlorophyll a. Treatment of purified and lyophyrized fucoxanthin-chlorophyll a/c protein from Ectocarpus (pheophyte) with anhydrous diethylether, a fucoxanthin-depleted protein preparation was obtained, in which three of 4 fucoxanthins originally bound were removed. This protein retained 2 chlorophyll a's but the excitation energy migration both between fucoxanthin and chlorophyll a between chlorophyll c and chlorophyll a were hardly seen in this protein. When this fucoxanthin-depleted protein was incubated with fucoxanthin in 0.1% lauryl sucrose, fucoxanthin was incoporated into protein in the form competent in excitation energy migration to chlorophyll a, and simultaneously regained in the energetic coupling from chlorophyll c to a. beta-Carotene and zeaxanthin were also incorporated into this protein but without showing any recovery of these energetic coupling, while neither 19'-hexanoyl oxyfucoxanthin isolated from Ruttnera (haptophyte) nor peridinin isolated from Symbionidium (dinophyte) was incorporated in this protein, indicating the fucoxanthin-binding domain of this protein is not highly specific to fucoxanthin but is not structurally fitted to modifid forms of carotenoids.