Energy transfer between electronically coupled photosynthetic light-harvesting antenna pigments is frequently assisted by protein and chromophore nuclear motion. This energy transfer mechanism usually occurs in the weak or intermediate system-bath coupling regime. Redfield theory is frequently used to describe the energy transfer in this regime. Spectral densities describe vibronic coupling in visible transitions of the chromophores and govern energy transfer in the Redfield mechanism. In this work, we perform finely sampled broadband pump–probe spectroscopy on the phycobilisome antenna complex with sub-10-fs pump and probe pulses. The spectral density obtained by Fourier transforming the pump–probe time-domain signal is used to perform modified Redfield rate calculations to check for vibrational enhancement of energy transfer in a coupled chromophore dimer in the C-phycocyanin protein of the phycobilisome antenna. We find two low-frequency vibrations to be in near-resonance with the interexcitonic energy gap and a few-fold enhancement in the interexcitonic energy transfer rate due to these resonances at room temperature. Our observations and calculations explain the fast downhill energy transfer process in C-phycocyanin. We also observe high-frequency vibrations involving chromophore–protein residue interactions in the excited state of the phycocyanobilin chromophore. We suggest that these vibrations lock the chromophore nuclear configuration of the excited state and prevent the energetic relaxation that blocks energy transfer.

中文翻译：

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C-藻蓝蛋白发色团二聚体中下坡能量转移的共振振动增强


电子耦合光合光捕获天线色素之间的能量转移通常由蛋白质和发色团核运动辅助。这种能量传递机制通常发生在弱或中间系统-浴耦合状态中。红场理论经常被用来描述这种状态下的能量转移。光谱密度描述了发色团可见转变中的振动耦合，并控制 Redfield 机制中的能量传递。在这项工作中，我们对具有低于 10 fs 泵浦和探测脉冲的体胆小体天线复合物进行了精细采样的宽带泵浦-探针光谱。通过傅里叶变换泵浦-探针时域信号获得的光谱密度用于执行修改后的 Redfield 速率计算，以检查藻胆体天线的 C-藻蓝蛋白中偶联发色团二聚体中能量传递的振动增强。我们发现两个低频振动与激子间能隙近共振，并且由于室温下的这些共振，激子间能量传递速率提高了几倍。我们的观察和计算解释了 C-藻蓝蛋白的快速下坡能量转移过程。我们还观察到在藻蓝蛋白发色团的激发态下涉及发色团-蛋白质残基相互作用的高频振动。我们认为这些振动锁定了激发态的发色团核构型，并阻止了阻止能量转移的能量弛豫。