Photosystem I (PSI), found in all oxygenic photosynthetic organisms, uses solar energy to drive electron transport with nearly 100% quantum efficiency, thanks to fast energy transfer among antenna chlorophylls and charge separation in the reaction center. There is no complete consensus regarding the kinetics of the elementary steps involved in the overall trapping, especially the rate of primary charge separation. In this work, we employed two-dimensional coherent electronic spectroscopy to follow the dynamics of energy and electron transfer in a monomeric PSI complex from Synechocystis PCC 6803, containing only subunits A–E, K, and M, at 77 K. We also determined the structure of the complex to 4.3 Å resolution by cryoelectron microscopy with refinements to 2.5 Å. We applied structure-based modeling using a combined Redfield–Förster theory to compute the excitation dynamics. The absorptive 2D electronic spectra revealed fast excitonic/vibronic relaxation on time scales of 50–100 fs from the high-energy side of the absorption spectrum. Antenna excitations were funneled within 1 ps to a small pool of chlorophylls absorbing around 687 nm, thereafter decaying with 4–20 ps lifetimes, independently of excitation wavelength. Redfield–Förster energy transfer computations showed that the kinetics is limited by transfer from these red-shifted pigments. The rate of primary charge separation, upon direct excitation of the reaction center, was determined to be 1.2–1.5 ps^–1. This result implies activationless electron transfer in PSI.

中文翻译：

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最小光系统 I 复合物的二维电子光谱揭示了初级电荷分离的速率

由于天线叶绿素之间的快速能量转移和反应中心的电荷分离，在所有含氧光合生物中都发现了光系统 I (PSI)，它利用太阳能以近 100% 的量子效率驱动电子传输。关于整体捕获中涉及的基本步骤的动力学，特别是初级电荷分离的速率，没有完全共识。在这项工作中，我们采用二维相干电子光谱来跟踪来自集胞藻属的单体 PSI 复合物中的能量和电子转移动力学。PCC 6803，仅包含亚基 A-E、K 和 M，温度为 77 K。我们还通过冷冻电子显微镜确定了复合物的结构，分辨率为 4.3 Å，改进为 2.5 Å。我们使用组合 Redfield-Förster 理论应用基于结构的建模来计算激发动力学。吸收性二维电子光谱显示，从吸收光谱的高能侧开始，在 50-100 fs 的时间尺度上快速激子/振动弛豫。天线激发在 1 ps 内汇集到吸收约 687 nm 的小叶绿素池中，此后以 4-20 ps 的寿命衰减，与激发波长无关。Redfield-Förster 能量转移计算表明，动力学受到这些红移颜料转移的限制。在直接激发反应中心时，初级电荷分离的速率，^–1。该结果意味着 PSI 中的非活化电子转移。