紫色光合细菌ThermochromatiumTepidum捕光天线复合物2的激发态动力学

Thermochromatium (Tch.) tepidum是一种中等嗜热的紫色光合细菌, 最佳生长温度为48-50 ℃; 其捕光天线复合物2 (LH2)含有非均一性脱辅基蛋白和类胡萝卜素(Car), 且高分辨率晶体结构未知. 我们通过超快光谱研究了分别采用去垢剂n-dodecyl-β-D-maltoside (DDM)和lauryldimethylamine oxide (LDAO)制备的LH2的激发态动力学, 观测到由细菌叶绿素(BChl)的Qy态介导的B800-to-B850单重态能量传递过程(时间尺度~1.2 ps, 用DDM制备的LH2), 以及由类胡萝卜素S2态介导的Car-to-Car和Car-to-BChl 单重态能量传递过程(~100 fs). 结果表明C=C共轭双键数目(NC=C)为11和12的两类Car共处于同一LH2复合物中; 相对于源自其它菌种、构成组分相对简单的LH2, Tch. tepidum的LH2中B800-B850的相对取向有较大差异. 本工作发现LH2中低含量类胡萝卜素组分anhydrorhodovibrin (NC=C=12)起着高效“能量陷阱”的作用, 可能是一种重要的光保护机制; 基于类胡萝卜素的超快谱带位移现象提出(OH-)spirilloxanthin(NC=C=13)距BChl分子可能比其它类胡萝卜素更近. 这些研究结果有助于进一步理解苛刻自然条件下生长的Tch. tepidum的捕光和光保护机制.

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℃.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 (Nc=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 (Nc=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.