Excitation dynamics of the light-harvesting complex 2 from Thermochromatium Tepidum

Purple photosynthetic bacterium Thermochromatium (Tch.) tepidum is a moderate thermophile growing in an optimal temperature range of 48-50 °C Its light-harvesting complex 2 (LH2) possesses heterogeneous compositions of apoprotein and carotenoid (Car), but the high-resolution crystallographic structure remains unknown. We have attempted an ultrafast time-resolved spectroscopic study of the isolated LH2 complex from Tch. tepidum. The spectral dynamics and population kinetics of n-dodecyl-β-D-moltoside (DDM) and lauryldimethylamine oxide (LDAO) preparations of LH2 reveal efficient S₂-state mediated Car-to-Car and Car-to-bacteriochlorophyll (BChl) singlet excitation energy transfer (EET) occurring in a time scale of ~100 fs, as well as the Qy-state mediated B800-to-B850 singlet EET for the DDM preparation proceeding with a time constant of ~1.2 ps. These ultrafast EET processes suggest that the Cars with 11 and 12 conjugated C=C double bonds (NC=C) coexist in the LH2 complex, and that the B800-B850 mutual orientation in LH2 differs considerably from those in the LH2s from some of the previously investigated bacterial species. In addition, anhydrorhodovibrin (N_C=C=12) as a minor Car composition is found to act as an efficient trap of excitation energy, which is considered to be an important photoprotection mechanism. Furthermore, based on the results of ultrafast Car band shift in response to BChl excitation, we propose that, compared to other Car compositions, (OH-)spirilloxanthin (N_C=C=13) locates in closer proximity to BChl. Our results may facilitate to understand the light-harvesting and the photoprotection mechanisms of Tch. tepidum living under harsh natural conditions.