Structures, Excited-State Properties and Functions of Carotenoids in Photosynthesis

Abstract

Carotenoids in photosynthetic systems have two functions of photo-protection and light-harvesting [1,2]. In the photo-protective function, carotenoids quench triplet chlorophylls, a sensitizer to generate harmful singlet oxygen. In the light-harvesting function, carotenoids capture the light energy and transfer the singlet energy to chlorophylls. Thus, chlorophyll-to-carotenoid triplet energy transfer and dissipation of the triplet energy by carotenoids are involved in the photo-protective function, while carotenoid-to-chlorophyll singlet energy transfer is involved in the light-harvesting function. In relation to the above two functions, natural selection of the carotenoid configurations has been found in purple photosynthetic bacteria (reviewed in ref. [2]): The 15-cis configuration is selected by the reaction center (RC) where the photo-protective function is most important, particularly when too much light energy is supplied. The all-trans configuration is selected by the light-harvesting complex (LHC) where the light-harvesting function is most important, when the supplied light energy is very low. Therefore, the 15-cis and all-trans configurations must have advantage, over the other cis-trans configurations, in the photo-protective and light-harvesting functions, respectively. Since the natural selection should originate from the triplet and singlet excited-state properties of the particular configurations, we have attempted to determine the configurational dependence of those excited-state properties by means of Raman, electronic absorption and fluorescence spectroscopy. The results obtained so far will be summarized in this presentation.