Department of Chemistry, Princeton University, Washington Road, Princeton, New Jersey 08544, USA.
Department of Biology, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
Nat Chem. 2018 Feb;10(2):177-183. doi: 10.1038/nchem.2910. Epub 2018 Jan 15.
Femtosecond pulsed excitation of light-harvesting complexes creates oscillatory features in their response. This phenomenon has inspired a large body of work aimed at uncovering the origin of the coherent beatings and possible implications for function. Here we exploit site-directed mutagenesis to change the excitonic level structure in Fenna-Matthews-Olson (FMO) complexes and compare the coherences using broadband pump-probe spectroscopy. Our experiments detect two oscillation frequencies with dephasing on a picosecond timescale-both at 77 K and at room temperature. By studying these coherences with selective excitation pump-probe experiments, where pump excitation is in resonance only with the lowest excitonic state, we show that the key contributions to these oscillations stem from ground-state vibrational wavepackets. These experiments explicitly show that the coherences-although in the ground electronic state-can be probed at the absorption resonances of other bacteriochlorophyll molecules because of delocalization of the electronic excitation over several chromophores.
飞秒脉冲激发光捕获复合物会在其响应中产生振荡特征。这一现象激发了大量旨在揭示相干拍频起源及其对功能可能产生影响的研究。在这里，我们利用定点突变来改变 Fenna-Matthews-Olson (FMO) 复合物中的激子能级结构，并使用宽带泵浦探测光谱比较相干性。我们的实验在 77 K 和室温下均探测到两种具有皮秒时间尺度去相位的振荡频率。通过选择性激发泵浦探测实验研究这些相干性，其中泵浦激发仅与最低激子态共振，我们表明这些振荡的关键贡献源于基态振动波包。这些实验明确表明，尽管处于基态电子态，相干性仍可通过其他细菌叶绿素分子的吸收共振进行探测，因为电子激发在几个发色团之间离域。
