Institut für Theoretische Physik, Johannes Kepler Universität Linz, Altenberger Str. 69, 4040 Linz, Austria.
J Chem Phys. 2020 Dec 7;153(21):215103. doi: 10.1063/5.0027994.
The intermolecular contribution to the spectral density of the exciton-vibrational coupling of the homotrimeric Fenna-Matthews-Olson (FMO) light-harvesting protein of green sulfur bacteria P. aestuarii is analyzed by combining a normal mode analysis of the protein with the charge density coupling method for the calculation of local transition energies of the pigments. Correlations in site energy fluctuations across the whole FMO trimer are found at low vibrational frequencies. Including, additionally, the high-frequency intrapigment part of the spectral density, extracted from line-narrowing spectra, we study intra- and intermonomer exciton transfer. Whereas the intrapigment part of the spectral density is important for fast intramonomer exciton relaxation, the intermolecular contributions (due to pigment-environment coupling) determine the intermonomer exciton transfer. Neither the variations of the local Huang-Rhys factors nor the correlations in site energy fluctuations have a critical influence on energy transfer. At room temperature, the intermonomer transfer in the FMO protein occurs on a 10 ps time scale, whereas intramonomer exciton equilibration is roughly two orders of magnitude faster. At cryogenic temperatures, intermonomer transfer limits the lifetimes of the lowest exciton band. The lifetimes are found to increase between 20 ps in the center of this band up to 100 ps toward lower energies, which is in very good agreement with the estimates from hole burning data. Interestingly, exciton delocalization in the FMO monomers is found to slow down intermonomer energy transfer, at both physiological and cryogenic temperatures.
采用蛋白质的正则模态分析与发色团局部跃迁能量计算的电荷密度耦合方法相结合，分析了海洋聚球藻（P. aestuarii）同三聚体 Fenna-Matthews-Olson（FMO）光捕获蛋白中激子-振动耦合的分子间贡献。在低振动频率下，发现整个 FMO 三聚体中存在着在各发色团位置能量涨落的相关性。此外，还包括从线宽变窄谱中提取的光谱密度的高频率内部分。我们研究了内、外分子间激子转移。虽然光谱密度的内部分对于快速的分子内激子松弛很重要，但分子间贡献（由于发色团-环境耦合）决定了分子间激子转移。局部 Huang-Rhys 因子的变化和位置能量涨落的相关性都不会对能量转移产生关键影响。在室温下，FMO 蛋白中的分子间转移发生在 10 ps 的时间尺度上，而分子内激子平衡则快两个数量级。在低温下，分子间转移限制了最低激子带的寿命。发现该寿命在该带中心从 20 ps 增加到 100 ps，而能量越低寿命越大，这与孔烧蚀数据的估计非常吻合。有趣的是，在生理和低温条件下，FMO 单体中的激子离域都会减缓分子间的能量转移。
