Fenna-Matthews-Olson复合体中的动态量子相干解纠缠

H. Duan, A. Jha, Lipeng Chen, Vandana Tiwari, R. Cogdell, K. Ashraf, V. Prokhorenko, M. Thorwart, R. Miller
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摘要

在自然光收集的第一步中,太阳光子的能量在天线叶绿素中被捕获为光激发的电子-空穴对或激子。它向储存化学势的有效转化发生在特殊的对反应中心,这必须通过下坡超快激发态能量输运才能达到。这个过程的关键是叶绿素发色团之间的相互作用程度,这可能导致空间离域和量子相干效应。量子对能量传输的重要性取决于叶绿素与周围蛋白质基质或基质的波动强度和重组动力学之间的相对耦合。后者诱导位点能量的不相关调制,导致量子退相干和激子空间范围的局域化。目前的共识是,在生理条件下,量子退相干发生在10fs时间尺度上,量子相干对观测到的皮秒能量传递动力学作用很小。在这项工作中,我们从不同的角度重申了这一点,发现重要的电子量子相干的真正开始只发生在~ 20k的极低温度下。我们利用FMO配合物的二维(2D)电子能谱在广泛的温度范围内直接确定了激子相干时间。在20 K时,我们发现电子相干持续到200 fs(靠近天线),在反应中心侧略微高达500 fs。随着温度的适度升高,电子相干性衰减明显加快,在150 K以上变得无关紧要。这种温度依赖性还允许解开先前报道的被认为是电子量子相干性贡献的证据的长寿命跳动。我们证明了它们是由电子基态中的混合振动相干引起的。我们还揭示了激发态之间的相关电子相干性,并检查了转移动力学的温度依赖的非马尔可夫性,以表明即使在低温下,该浴也涉及不相关的运动。观察到的温度依赖性允许将脆弱的电子相干性与强大的振动相干性明确分离。通过基于测量的浴槽参数的理论模型来处理关键浴槽相互作用的具体细节,该模型再现了温度依赖的动力学。通过这一点,我们提供了一个完整的浴槽相互作用的画面,它将这些系统置于强浴槽耦合的状态。我们相信这一主要结论对光收集系统是普遍有效的。这一原理使得该系统对脆弱的量子效应具有鲁棒性,正如强温度依赖性所证明的那样。我们得出的结论是,即使在生物过程的最快时间尺度上,大自然也明确地利用工程现场能量的退相干或耗散来产生下坡能量梯度到准确无误的直接能量。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Disentangling Dynamical Quantum Coherences in the Fenna-Matthews-Olson Complex
In the primary step of natural light-harvesting, the energy of a solar photon is captured in antenna chlorophyll as a photoexcited electron-hole pair, or an exciton. Its efficient conversion to stored chemical potential occurs in the special pair reaction center, which has to be reached by down-hill ultrafast excited state energy transport. Key to this process is the degree of interaction between the chlorophyll chromophores that can lead to spatial delocalization and quantum coherence effects. The importance of quantum contributions to energy transport depends on the relative coupling between the chlorophylls in relation to the intensity of the fluctuations and reorganization dynamics of the surrounding protein matrix, or bath. The latter induce uncorrelated modulations of the site energies, resulting in quantum decoherence, and localization of the spatial extent of the exciton. The current consensus is that under physiological conditions quantum decoherence occurs on the 10 fs time scale, and quantum coherence plays little role for the observed picosecond energy transfer dynamics. In this work, we reaffirm this from a different point of view by finding that the true onset of important electronic quantum coherence only occurs at extremely low temperatures of ~20 K. We have directly determined the exciton coherence times using two-dimensional (2D) electronic spectroscopy of the Fenna-Matthew-Olson (FMO) complex over an extensive temperature range. At 20 K, we show that electronic coherences persist out to 200 fs (close to the antenna) and marginally up to 500 fs at the reaction-center side. The electronic coherence is found to decay markedly faster with modest increases in temperature to become irrelevant above 150 K. This temperature dependence also allows disentangling the previously reported long-lived beatings thought to be evidence for electronic quantum coherence contributions. We show that they result from mixing vibrational coherences in the electronic ground state. We also uncover the relevant electronic coherence between excited electronic states and examine the temperature-dependent non-Markovianity of the transfer dynamics to show that the bath involves uncorrelated motions even to low temperatures. The observed temperature dependence allows a clear separation of the fragile electronic coherence from the robust vibrational coherence. The specific details of the critical bath interaction are treated through a theoretical model based on measured bath parameters that reproduces the temperature dependent dynamics. By this, we provide a complete picture of the bath interaction which places these systems in the regime of strong bath coupling. We believe this main conclusion to be generically valid for light harvesting systems. This principle makes the systems robust against otherwise fragile quantum effects as evidenced by the strong temperature dependence. We conclude that nature explicitly exploits decoherence or dissipation in engineering site energies to yield downhill energy gradients to unerringly direct energy, even on the fastest time scales of biological processes.
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