New trends in photobiology: Structures and functions of carotenoids in photosynthetic systems

Abstract

Recent achievements in the investigation of the structures and excited-state properties of carotenoids, in relation to their photoprotective and light-harvesting functions in bacterial photosynthesis, are reviewed. In purple bacteria, natural selection of the carotenoid configurations has been observed. The 15-cis configuration is selected by the reaction centers (RCs), whereas the all-trans configuration is selected by the light-harvesting complexes (LHCs). In the photoprotective function in the RCs, involvement of one of the accessory bacteriochlorophylls in the triplet energy transfer has been demonstrated. The extremely efficient isomerization of the 15-cis T 1 carotenoid in vitro, together with indications of isomerization and two forms of carotenoids in vivo, have led to the proposal of a mechanism of energy dissipation in which isomerization of the carotenoid is involved. In the energy transfer function in the LHCS, evidence for the 2A g (S 1) state for carotenoids in vitro and in vivo has been obtained. Its lifetime of approximately 10 ps in vitro and an energy transfer time of 1–10 ps in vivo, determined by picosecond absorption spectroscopy, make the electron-exchange mechanism via the 2A g state likely. However, the B u (S 2) lifetime of approximately 200 fs in vitro and an energy transfer time of the same order of magnitude in vivo, determined by femtosecond absorption spectroscopy, suggest that the dipole mechanism via the B u state is also possible.