Green plants use the xanthophyll cycle to regulate the flow of energy to chlorophylla 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 chlorophylla 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 [Symbol: see text] conjugation of the polyene lowers the energy of the lowest excited singlet state of the carotenoid below that of chlorophylla. As a consequence zeaxanthin can act as a trap for the excess excitation energy on chlorophylla pigments within the protein, thus regulating the flow of energy within photosynthetic light-harvesting proteins.

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The spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. The spheroidene analogs studied here were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene and taken together with data from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biophys. Acta 1992, 1102, 107) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. All of the molecules were purified by high-pressure liquid chromatographic techniques prior to the spectroscopic experiments. The absorption spectra of the molecules were observed to red-shift with increasing extent of π-electron conjugation. The room temperature fluorescence data show a systematic crossover from dominant S1 → S0 (2Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The S2 fluorescence quantum yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon−carbon double bonds has an S2 quantum yield of (2.7 ± 0.3) × 10-4 which is the highest value in the series. The lifetimes of the S1 states of the molecules were determined from time-resolved transient absorption spectroscopy and found to decrease as the conjugated chain length increases. The transient data are discussed in terms of the energy gap law for radiationless transitions which allows a prediction of the S1 energies of the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.