Atomically Resolved Dynamics Department, Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany.
I. Institut für Theoretische Physik, Universität Hamburg, 20355 Hamburg, Germany.
Proc Natl Acad Sci U S A. 2017 Aug 8;114(32):8493-8498. doi: 10.1073/pnas.1702261114. Epub 2017 Jul 25.
During the first steps of photosynthesis, the energy of impinging solar photons is transformed into electronic excitation energy of the light-harvesting biomolecular complexes. The subsequent energy transfer to the reaction center is commonly rationalized in terms of excitons moving on a grid of biomolecular chromophores on typical timescales [Formula: see text]100 fs. Today's understanding of the energy transfer includes the fact that the excitons are delocalized over a few neighboring sites, but the role of quantum coherence is considered as irrelevant for the transfer dynamics because it typically decays within a few tens of femtoseconds. This orthodox picture of incoherent energy transfer between clusters of a few pigments sharing delocalized excitons has been challenged by ultrafast optical spectroscopy experiments with the Fenna-Matthews-Olson protein, in which interference oscillatory signals up to 1.5 ps were reported and interpreted as direct evidence of exceptionally long-lived electronic quantum coherence. Here, we show that the optical 2D photon echo spectra of this complex at ambient temperature in aqueous solution do not provide evidence of any long-lived electronic quantum coherence, but confirm the orthodox view of rapidly decaying electronic quantum coherence on a timescale of 60 fs. Our results can be considered as generic and give no hint that electronic quantum coherence plays any biofunctional role in real photoactive biomolecular complexes. Because in this structurally well-defined protein the distances between bacteriochlorophylls are comparable to those of other light-harvesting complexes, we anticipate that this finding is general and directly applies to even larger photoactive biomolecular complexes.
在光合作用的初始步骤中，入射太阳光子的能量被转化为光捕获生物分子复合物的电子激发能量。随后，能量向反应中心的转移通常可以用激子在典型时间尺度[Formula: see text]100 fs 上在生物分子发色团网格上移动的方式来合理化。今天对能量转移的理解包括激子在几个相邻位点上离域的事实，但量子相干性的作用被认为与转移动力学无关，因为它通常在几十飞秒内衰减。这种关于共享离域激子的少数几个色素团簇之间的非相干能量转移的正统观点，已经受到 Fenna-Matthews-Olson 蛋白超快光学光谱实验的挑战，在这些实验中，报告并解释了长达 1.5 ps 的干涉振荡信号，作为电子量子相干性异常长寿命的直接证据。在这里，我们表明，在水环境中，该复合物的室温 2D 光子回波光谱并不能提供任何长寿命电子量子相干性的证据，而是证实了电子量子相干性在 60 fs 的时间尺度上迅速衰减的正统观点。我们的结果可以被认为是通用的，并且没有暗示电子量子相干性在实际的光活性生物分子复合物中发挥任何生物功能作用。由于在这个结构上定义良好的蛋白质中，细菌叶绿素之间的距离与其他光捕获复合物的距离相当，我们预计这一发现是普遍的，并且直接适用于甚至更大的光活性生物分子复合物。
