Chapter 4. Photophysical Properties and Light-Harvesting and Photoprotective Functions of Carotenoids in Bacterial Photosynthesis: Structural Selections

The reasons for the natural selection of the length and the configuration of the conjugated chain of carotenoids bound to the antenna and the reaction-center complexes, in purple photosynthetic bacteria, are presented. In the antenna complexes, carotenoids select a shorter conjugated chain in the all-trans configuration whereas the reaction center has a longer conjugated chain in the 15-cis configuration, for the light-harvesting and the photoprotective functions, respectively. Natural selection of the carotenoid structures by the pigment–protein complexes is explained in terms of the energetics and dynamics of the carotenoid and bacteriochlorophyll molecules in the singlet- and triplet-excited states: a shorter conjugated chain in the all-trans configuration is advantageous for the carotenoid-to-bacteriochlorophyll singlet-energy transfer through multiple channels, whereas a longer conjugated chain in the 15-cis configuration is favored in quenching triplet bacteriochlorophyll and dissipating the triplet energy through the rotational motion around the central-cis double bond.