Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.
Phys Chem Chem Phys. 2021 Mar 28;23(12):7407-7417. doi: 10.1039/d1cp01011e. Epub 2021 Mar 23.
Photosynthetic processes are driven by sunlight. Too little of it and the photosynthetic machinery cannot produce the reductive power to drive the anabolic pathways. Too much sunlight and the machinery can get damaged. In higher plants, the major Light-Harvesting Complex (LHCII) efficiently absorbs the light energy, but can also dissipate it when in excess (quenching). In order to study the dynamics related to the quenching process but also the exciton dynamics in general, one needs to accurately determine the so-called spectral density which describes the coupling between the relevant pigment modes and the environmental degrees of freedom. To this end, Born-Oppenheimer molecular dynamics simulations in a quantum mechanics/molecular mechanics (QM/MM) fashion utilizing the density functional based tight binding (DFTB) method have been performed for the ground state dynamics. Subsequently, the time-dependent extension of the long-range-corrected DFTB scheme has been employed for the excited state calculations of the individual chlorophyll-a molecules in the LHCII complex. The analysis of this data resulted in spectral densities showing an astonishing agreement with the experimental counterpart in this rather large system. This consistency with an experimental observable also supports the accuracy, robustness, and reliability of the present multi-scale scheme. To the best of our knowledge, this is the first theoretical attempt on this large complex system is ever made to accurately simulate the spectral density. In addition, the resulting spectral densities and site energies were used to determine the exciton transfer rate within a special pigment pair consisting of a chlorophyll-a and a carotenoid molecule which is assumed to play a role in the balance between the light harvesting and quenching modes.
光合作用过程由阳光驱动。阳光太少，光合作用机制就无法产生还原力来驱动合成代谢途径。阳光过多，机器就会受损。在高等植物中，主要的光捕获复合物（LHCII）有效地吸收光能，但在过量时也可以将其耗散（猝灭）。为了研究与猝灭过程相关的动力学，以及一般的激子动力学，人们需要准确地确定所谓的光谱密度，该密度描述了相关色素模式与环境自由度之间的耦合。为此，采用基于密度泛函的紧束缚（DFTB）方法的量子力学/分子力学（QM/MM）方式对基态动力学进行了 Born-Oppenheimer 分子动力学模拟。随后，采用长程校正 DFTB 方案的时间相关扩展对 LHCII 复合物中单个叶绿素-a 分子的激发态进行了计算。对该数据的分析得出了光谱密度，其与该相当大的系统中的实验对应物惊人地吻合。与实验可观测结果的这种一致性也支持了本多尺度方案的准确性、稳健性和可靠性。据我们所知，这是首次针对如此大的复杂体系进行的理论尝试，旨在准确模拟光谱密度。此外，所得的光谱密度和位置能被用于确定由叶绿素-a 和类胡萝卜素分子组成的特殊色素对之间的激子转移速率，该色素对被认为在光捕获和猝灭模式之间的平衡中发挥作用。
