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Temperature dependence of the triplet-triplet energy transfer in photosynthetic light-harvesting complexes
Vinklárek, Ivo ; Pšenčík, Jakub (advisor)
Toxic singlet oxygen can be populated by the quenching of triplet states of chlorophyll (Chl). In photosynthetic light-harvesting complexes (LHCs), the gen- eration of singlet oxygen is prevented by a photoprotective mechanism based on an energy transfer from Chl triplets to carotenoids, which occurs via a Dexter mechanism (DET). The temperature dependence of the DET was studied in three selected LHCs by means of transient absorption spectroscopy. The emphasis was on a chlorophyll a-chlorophyll c2-peridinin-protein complex (acpPC) of Dinoflagel- late Amphidinium carterae. The results obtained from acpPC were compared with those for LHC-II from pea and chlorosomes of Chloroflexus aurantiacus. All three antennas exhibit high efficiency and fast rate of chlorophyll triplet quenching by carotenoids at room temperature, which prevents the accumulation of Chl triplets. The fast rate of quenching persists at low temperatures (≥77 K) in the case of LHC-II. However, the efficiency of the Chl triplets quenching is lower as proved by a detection of long-lived Chl triplets with a millisecond lifetime. These triplets were assigned to peripheral Chls that are not neighbouring with carotenoids active at 77 K. A similar population of long-lived Chl triplets was detected in the acpPC complex. In acpPC, the rate of the...
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Temperature dependence of the triplet-triplet energy transfer in photosynthetic light-harvesting complexes
Vinklárek, Ivo ; Pšenčík, Jakub (advisor) ; Polívka, Tomáš (referee)
Toxic singlet oxygen can be populated by the quenching of triplet states of chlorophyll (Chl). In photosynthetic light-harvesting complexes (LHCs), the gen- eration of singlet oxygen is prevented by a photoprotective mechanism based on an energy transfer from Chl triplets to carotenoids, which occurs via a Dexter mechanism (DET). The temperature dependence of the DET was studied in three selected LHCs by means of transient absorption spectroscopy. The emphasis was on a chlorophyll a-chlorophyll c2-peridinin-protein complex (acpPC) of Dinoflagel- late Amphidinium carterae. The results obtained from acpPC were compared with those for LHC-II from pea and chlorosomes of Chloroflexus aurantiacus. All three antennas exhibit high efficiency and fast rate of chlorophyll triplet quenching by carotenoids at room temperature, which prevents the accumulation of Chl triplets. The fast rate of quenching persists at low temperatures (≥77 K) in the case of LHC-II. However, the efficiency of the Chl triplets quenching is lower as proved by a detection of long-lived Chl triplets with a millisecond lifetime. These triplets were assigned to peripheral Chls that are not neighbouring with carotenoids active at 77 K. A similar population of long-lived Chl triplets was detected in the acpPC complex. In acpPC, the rate of the...
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Dynamika tripletních stavů pigmentů ve fotosyntetických světlosběrných komplexech
Kvíčalová, Zuzana ; Pšenčík, Jakub (advisor) ; Vácha, František (referee)
Chlorophyll molecules in their triplet excited state can react with the ground state oxygen, producing oxygen in a singlet excited state, which is very reactive and thus very harmful to the light-harvesting complex. Photosynthetic organisms employ carotenoids to prevent the damage by quenching both excited (singlet) states of oxygen and excited triplet states of chlorophyll. In this work, we use ns transient absorption spectroscopy and global analysis to study the dynamics of carotenoid and chlorophyll triplet states in two light-harvesting complexes of Amphidinium carterae, the Peridinin-Chlorophyll a-Protein complex (PCP) and the main light-harvesting complex (LHCP). It appears that at room temperature all triplets are transferred from chlorophylls to carotenoids within ~ 5 ns, providing a very efficient protection against formation of singlet oxygen. One carotenoid triplet with a lifetime of ~ 10.2 µs participating in the chlorophyll triplet quenching was observed in the PCP sample, while results from LHCP suggest that two carotenoid triplets with a similar lifetime of ~ 2.5 µs contribute to quenching of chlorophyll triplet states. The two carotenoid triplets are attributed to peridinin placed in a polar environment and peridinin placed in a non-polar environment in the LHCP complex.
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Excited States of Carotenoids and Their Roles in Light Harvesting Systems
KEŞAN, Gürkan
Carotenoids are an extensive group of natural pigments employed by a majority of organisms on earth. They are present in most organisms, including humans, but can be synthesized only by plants and microorganisms. They perform two major roles in photosynthesis, often in partnership with the more prevalent chlorophylls (Chls) / bacteriochlorophylls (BChls): light-harvesting and photoprotection. Carotenoids absorb radiation in a spectral region inaccessible to Chls and BChls and transfer the absorbed energy to (B)Chls which, in turn, funnel it into the photosynthetic reaction center (RC). It is determined that the structures and dynamics of excited states of carotenoids found in photosynthetic proteins provide an explanation of their roles as light-harvesting and photoprotective agents. The conjugation length and the type of carotenoids play a big part in understanding the energy transfer from carotenoids to Chls and BChls, because excited-state properties of carotenoids are affected by number of conjugated C=C bonds and their structures. An accurate description of these states is, therefore, the crucial first step in explaining carotenoid photochemistry and understanding the interactions between carotenoids and other molecules in photobiological processes. The research in this thesis, femtosecond ultrafast transient absorption spectroscopy was used to study the light-harvesting function of carotenoids both in solvent and protein environment. The findings were supported with computational methods. Based on spectroscopic indications, the light-harvesting function of carotenoids has a bearing on their structures, and specific light-harvesting strategies are explicitly dependent with the structure of the light-harvesting complexes.
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