The Study Of Photoinduced Ultrafast Dynamics Of Peripheral Light Harvesting Complexes (LH2) In Primary Process Of Purple Bacteria Photosynthesis

The photosynthesis is the most important and highest efficient photochemical process found in photosynthetic organisms in the nature. Peripheral light-harvesting complexes (LH2), which serve as pigment antenna to efficiently absorb sunlight and transfer excitation energy to reaction centers (RCs), are indispensable part of the photosynthetic unit in purple bacteria. The high-resolution X-ray crystal structures of LH2 show that nine monomeric bacteriochlorophylls (BChls) with absorption peak at 800 nm form a ring, labeled as B800, and eighteen strongly interacting BChls form B850 ring, which absorbs at 850 nm, the carotenoid in LH2 spans the entire width of the membrane and is in contact with BChl α-pigments of the B800 and B850 rings. The primary process of photosynthesis involves the capture of light energy, energy transfer which all take place in the picosecond or subpicosecond time region. The development of ultrafast spectroscopic techniques in the past decades provided us a significant experimental approach to investigate the ultrafast processes in the primary process of photosynthesis. Meanwhile, the fast developments of molecular biology, genetic engineering and the site mutation technique establish the fundamental molecular basis for us to study the mechanisms of photophysics and photochemistry in the primary processes involved. In this Ph.D thesis, we studied the photoinduced ultrafast dynamics taking place in peripheral light-harvesting complex LH2 from purple bacteria Rb. sphaeroides 601 by using ultrafast spectroscopic techniques. The obtained results in our experiments can be concluded as follows:1. Photodynamics of two kinds of peripheral antenna complexes LH2 of Rd sphaeroides 601, native LH2 (RS601) and B800-released LH2 where B800-BChls were partially or completely removed with different pH treatments, were studied using femtosecond pump-probe technique at different laser wavelengths. The obtained results for these samples with different B800/B850 ratios demonstrate that under the excitation around B800 nm, the photoabsorption and photobleaching dynamics are caused by the direct excitation of upper excitonic levels of B850 and excited state of B800 pigments, respectively. Furthermore, the removal of B800 pigments has little effect on the energy transfer processes of B850 interband / intraband transfer.2.Energy transfer in two kinds of peripheral antenna complexes LH2 from Rd sphaeroides 601 was studied by femtosecond transient absorption spectroscopy at room temperature. These two complexes are LH2 (RS601) and green carotenoid mutated LH2 (GM309). The obtained results demonstrate that compared with spheroidenes which have ten carbon-carbon double bonds in native RS601, carotenoids in GM309 were identified as containing neurosporenes with nine carbon-carbon double bonds, which show a significant blue shift of ～20 nm in the three absorption peaks because of the higher energy levels of neurosporene than those of spheroidene; the higher energy levels of neurosporene in GM309 induce a lower B800→B850 energy transfer rate and efficiency as compared to that in RS601 resulting from the relatively higher band gap between the donor of B800 and the bridge of the carotenoids ; the same lifetime of the B850 excited singlet state is observed in these two LH2 complexes.3.The oxidation of bacteriochlorophylls (BChls) in peripheral light harvesting complexes (LH2) from Rb. Sphaeroides 601 was investigated by spectroelectrochemistry of absorption, fluorescence emission and femtosecond (fs) pump-probe, with the aim at obtaining information about the effect of in-situ electrochemical oxidation on the pigment-protein arrangement and energy transfer within LH2. The experimental results revealed that the generation of the BChl radicalcation in both B800 and B850 rings dramatically induced bleaching of the characteristic absorption in the NIR region and quenching of the fluorescence emission from the B850 ring for the electrochemical oxidized LH2; the BChl-B850 radical cation might act as an additional channel to compete with the unoxidized BChl-B850 molecules for rapidly releasing the excitation energy, however the B800-B850 energy transfer rate remained almost unchanged during the oxidation process.In conclusion, we studied the photo-induced ultrafast photophysical and photochemical dynamics occurring in the pigment-protein complexes of photosynthetic primary reaction using ultrafast spectroscopic techniques. In order to elucidate the relationship between structure and function in photosynthesis, more experimental and theoretical research work is needed in the future.