Excited States of Carotenoids and Their Functions in Light Harvesting

Abstract: The research presented in this thesis is focused on characterization of photophysical properties of lowest singlet excited states of carotenoids and their functions in the process of photosynthetic light harvesting. It is shown that the various light-harvesting complexes utilize different strategies to achieve efficient energy transfer from carotenoids to (bacterio)chlorophylls. Any study of energy transfer processes in carotenoids is complicated by the fact that the lowest excited singlet state (S1) is a dark state. To overcome this problem, we have developed a technique for direct determination of the S1 state energy, by employing near-IR pump-probe transient absorption spectroscopy to measure the allowed S1 - S2 transition. Using this technique we have successfully determined the S1 energy of a number of biologically active carotenoids. Spectroscopic and dynamical properties of excited states of the carotenoids spheroidene and peridinin were studied in detail. For spheroidene in solution, it was shown that different S1 state conformations can coexist, which results in a measurement-dependent discrepancy in the S1 state energy estimates. Properties of the lowest singlet excited states of the highly substituted carotenoid peridinin proved to be very different from other carotenoids. Studies of peridinin in different solvents revealed a strong lifetime dependence on solvent polarity, suggesting the presence of an intermolecular charge transfer (ICT) state in the excited state manifold. We have discovered a specific probe to study the ICT state dynamics, as the ICT state is characterized by a strong emission band in the near-infrared region centered at 950 nm. The dependence of peridinin excited state dynamics on excitation wavelength, viscosity and temperature was also investigated and incorporated into a new model of the kinetics and energy levels. Data obtained from the carotenoid studies in solution was applied to investigations of the role of singlet excited states of carotenoids in light-harvesting complexes. For the peridinin-chlorophyll-a-protein (PCP) complex it was shown that all singlet excitedstates of peridinin including the ICT state are involved in energy transfer from carotenoid to chlorophyll-a molecules. Furthermore, we have demonstrated that the efficiency of the energy transfer from the S1 state of carotenoids in LH2 complexes in photosynthetic bacteria is directly related to the energy of the S1 state. Finally, we have shown that it is changes in protein conformation induced by specific binding of carotenoids rather than their individual photochemical properties that are involved in energy quenching processes in plant light-harvesting complexes.