Excited State Dynamics Of Carotenoid Fucoxanthin And Photosynthetic Antenna Complex LH2 Of Thermochromatium Tepidum

As the most abundant naturally occurring carotenoid(Car), fucoxanthin(Fuc) has a carbonyl group conjugated to its polyene backbone, and, therefore, bears rather unique excited-state properties. On the other hand, the light-harvesting complex 2 (LH2) from the thermophilic photosynthetic purple bacteria Thermochromatium (Tch.) tepidum has heterogeneous polypeptide and Car compositions, and the detailed structural information are unavailable yet. This thesis is intended to study, by the use of ultrafast time-resolved spectroscopies, the excited-state properties and the free radical reaction mechanism of fucoxanthin, as well as the excitation energy transfer dynamics of both isolated and aggregated LH2 complexes. The results are helpful in understanding the structure-function relationship of the aforementioned photosynthetic Car and antenna complex.1. Photoinduced formation Fuc radical cation in organic solventsThe formation of Fuc radical cation (Fuc?+) in chloroform and in methanol was comparatively investigated by means of nanosecond time-resolved absorption spectroscopy combined with spetroelectrochemistry. By comparing the mechanisms of Fuc?+ generation under direct photoexcitation and anthracene-sensitized triplet excitation, it was concluded that in chloroform Fuc?+ was mainly derived from the second lowest singlet excited state of Fuc (S2), whereas in methanol mainly from the lowest triplet excited state (T1). The results are helpful in understanding the antioxidation or photoprotecting roles of Fuc in complex micro-environment.2. Solvent effect on the low-lying singlet excited states of FucThe excited state properties of the Fuc in methanol, n-hexane, tetrahydrofuran and benzene were examined, respectively, by means of steady-state and femtosecond time-resolved spectroscopies. The complete spectra of broad-band fluorescence spectra covering 600~1100 nm were recorded, which were ascribed to the S1/ICT state emission. In non-hydrogen bond solvent the state energy and lifetime of the S1/ICT state are independent on the solvent polarizability, however, those of other higher-lying excited states are dependent. In hydrogen bond solvent the S1/ICT state lifetime depends linearly on solvent polarizability, whereas the excited state energy deviates from linear dependence, indicating the prominent conformational change of Fuc molecule.3. Excitation Dynamics of LH2 from Tch. tepidumWe have examined, by means of femtosecond time-resolved absorption and dynamic Stark spectroscopy, the Car-to-Car excitation energy transfer(EET) among Cars of different conjugated lengths (N), the Car-to-bacteriochlorophyll(BChl) EET and the BChl-to-BChl EET processes of isolated LH2 complex prepared with different surfactants. These ultrafast processes suggest that the Cars with N=11 and 12 coexistence in a LH2 complex and the Car with N=13 locates in closer proximity to BChl with reference to other Cars. In addition, the B800-to-B850 molecular orientation is proposed to differ considerably from that of other extensively investigated bacterial species. Importantly, Cars with N=12 as a minor Car composition is found to be an efficient trap of excitation energy, which may contribute to the photoprotection of the LH2.4. Effect of Aggregation on the Excitation Dynamics of Solubilized and Chromatophore-Embedded LH2 complexes of Tch. tepidumInfluence of detergent concentration on the aggregation of LH2, and the intra- and inter-complex excitation dynamics of solubilized and chromatophore-embedded LH2 complexes were investigated. The intra-LH2 EET dynamics, i.e. B800→B800, B800→B850 and B850→B850, as well as the inter-complex EET dynamics, i.e. LH2→LH2, LH2→LH1and LH1→RC, were examined. The intra-LH2 B800→B850 energy transfer time for mono-dispersed LH2 was determined to be～1.32 ps, and the inter-complex LH2(B850)→LH1(B915) transfer time was found to be 6.62 ps, both of which are considerably longer than the values documented for other bacterial species. Furthermore, the depolarization of B850* excitation among LH2s showed a decay time constant of 21 ps, which corresponds to a free path of spatial migration of 30~40 nm (3~4 LH2s). The results are helpful in understanding the roles of LH2 clustering in native photosynthetic membrane.