Grant-funded Project Nr. 175/1999/B-FYZ/MFF
Final Report

Project title:Exciton transfer and electron transport in photosynthesis
Research leader:Doc.RNDr. Jan Hála, DrSc.
Co-researcher: RNDr. Antonín Svoboda, CSc.; RNDr. Martin Vácha, CSc.; Jakub Pšenčík, Dr.; Tomáš Polívka, Dr.; RNDr. Roman Dědic, Ph.D.; RNDr. Miroslav Lovčinský, Ph.D.; Veronika Brázdová; Alexander Molnár; Pavel Klinger
Period of project:1999-2001
Overall grant:430 000 CZK

Project Results

Chlorophylls (Chl), pheophytines (Pheo) and carotenes (Car) are main photosynthetic pigments of the photosystem 2 reaction centers (RC PS2). They play a crucial role in excited energy transfer and in electron transport. In this project we report on both resonant and off-resonant hole burning low temperature absorption and fluorescence spectra. The resonant narrow zero phonon holes together with broad off-resonant holes were burnt into Qy absorption bands, while very broad off-resonant satellite holes were observed in Qx absorption transitions. Resonant spectral hole burning enabled to determine excited state lifetimes calculated from widths of resonant zero phonon holes. These values reflected ultrafast exciton migration within the RC PS2 containing (5 and 6) Chl a, (2) Pheo a and (1) beta-Car. This technique enabled to distinguish between satellite holes of Chl a and Pheo a and consequently to localize spectral bands of both Pheo a in the complex Qy absorption spectrum of RC PS2. The resonant holes were observed together with their vibronic satellites burnt both into absorption and fluorescence spectra of the RC. This technique provided moreover Debye-Waller factor and mean frequency of protein phonons, which characterized the pigment-protein interactions. The phenomenon of laser induced hole-filling of primary hole after burning of secondary holes at different wavelengths was used to study fast deexcitation processes.
The synthesis of supplementary chlorophyll-binding proteins is a common response of various microorganisms to environmental stresses. The cyanobacteria Synechococcus elongatus grown under the lack of iron produces the protein called CP34 which is generally considered to act as an auxiliary light harvesting complex in primary  photosynthetic processes. A study of vibrations, electron-phonon coupling, and excitation energy transfer in CP34 was based on systematic measurements of low temperature site selection fluorescence spectra. The spectral data obtained under nonselective blue excitation (into B absorption band of Chl a) were compared to selectively red excited (into Qy absorption band of Chl a) ones. The frequencies of normal vibrations were determined from site selection spectra excited at different wavelengths between 660 and 690 nm. Their values obtained in CP34 were compared to those observed in chlorophyll-protein complexes of RC PS2.
The role of Car in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll e (BChl e) and the Car isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. It was shown that the lifetime of Car S1 state was ~10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was attained for the presence of an ultrafast (<100 fs) energy transfer process from the S2 state of Cars to BChls in control chlorosomes. Using two time-resolved techniques, we further found that the absence of Cars leads to overall slower decay kinetics probed within the Qy band of BChl e aggregates, and that two time constants are generally required to describe energy transfer from aggregated BChl e to baseplate BChl a.
Time resolved measurements of the singlet oxygen (1269 nm) luminescence were used to follow the kinetics and efficiency of excitation energy transfer between Chl derivatives and oxygen. The results have shown that the efficiency of excitation energy transfer from Chl to oxygen is highly dependent on the central ion in the pigment. The results are discussed in the context of the oxidative stress accompanying heavy metal induced stress in plants.