Sohail Sara H, Sohoni Siddhartha, Ting Po-Chieh, Fantz Lexi R, Abdulhadi Sami M, MacGregor-Chatwin Craig, Hitchcock Andrew, Hunter C Neil, Engel Gregory S, Massey Sara C
Department of Chemistry, Institute for Biophysical Dynamics, the James Franck Institute, and the Pritzker School for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, United States.
Laboratory of Chemical Physics, National Institute of Diabetes, and Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, United States.
J Phys Chem B. 2025 Mar 27;129(12):3191-3197. doi: 10.1021/acs.jpcb.5c00198. Epub 2025 Mar 18.
External stressors modulate the oligomerization state of photosystem I (PSI) in cyanobacteria. The number of red chlorophylls (Chls), pigments lower in energy than the P reaction center, depends on the oligomerization state of PSI. Here, we use ultrafast transient absorption spectroscopy to interrogate the effective connectivity of the red Chls in excitonic energy pathways in trimeric PSI in native thylakoid membranes of the model cyanobacterium sp. PCC 6803, including emergent dynamics, as red Chls increase in number and proximity. Fluence-dependent dynamics indicate singlet-singlet annihilation within energetically connected red Chl sites in the PSI antenna but not within bulk Chl sites on the picosecond time scale. These data support picosecond energy transfer between energetically connected red Chl sites as the physical basis of singlet-singlet annihilation. The time scale of this energy transfer is faster than predicted by Förster resonance energy transfer calculations, raising questions about the physical mechanism of the process. Our results indicate distinct strategies to steer excitations through the PSI antenna; the red Chls present a shallow reservoir that direct excitations away from P, extending the time to trapping by the reaction center.
外部应激源可调节蓝藻中光系统I（PSI）的寡聚化状态。红色叶绿素（Chls）的数量，即能量低于P反应中心的色素，取决于PSI的寡聚化状态。在此，我们使用超快瞬态吸收光谱来研究模式蓝藻 sp. PCC 6803天然类囊体膜中三聚体PSI激子能量途径中红色Chls的有效连接性，包括随着红色Chls数量和间距增加而出现的动力学。光通量依赖性动力学表明，在皮秒时间尺度上，PSI天线中能量相连的红色Chl位点内存在单重态-单重态湮灭，但在大量Chl位点内不存在。这些数据支持能量相连的红色Chl位点之间的皮秒能量转移作为单重态-单重态湮灭的物理基础。这种能量转移的时间尺度比福斯特共振能量转移计算预测的要快，这引发了对该过程物理机制的质疑。我们的结果表明了通过PSI天线引导激发的不同策略；红色Chls呈现出一个浅库，将激发从P引开，延长了反应中心捕获的时间。
