Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371.
Biological Research Centre, Szeged, Temesvári Körút 62, Szeged 6726, Hungary.
J Chem Phys. 2019 Nov 28;151(20):205101. doi: 10.1063/1.5125744.
Energy equilibration in light-harvesting antenna systems normally occurs before energy is transferred to a reaction center. The equilibration mechanism is a characteristic of the excitation energy transfer (EET) network of the antenna. Characterizing this network is crucial in understanding the first step of photosynthesis. We present our phenomenology-based analysis procedure and results in obtaining the excitonic energy levels, spectral linewidths, and transfer-rate matrix of Light-Harvesting Complex II directly from its 2D electronic spectra recorded at 77 K with waiting times between 100 fs to 100 ps. Due to the restriction of the models and complexity of the system, a unique EET network cannot be constructed. Nevertheless, a recurring pattern of energy transfer with very similar overall time scales between spectral components (excitons) is consistently obtained. The models identify a "bottleneck" state in the 664-668 nm region although with a relatively shorter lifetime (∼4-6 ps) of this state compared to previous studies. The model also determines three terminal exciton states at 675, 677-678, and 680-681 nm that are weakly coupled to each other. The excitation energy equilibration between the three termini is found to be independent of the initial excitation conditions, which is a crucial design for the light-harvesting complexes to ensure the energy flow under different light conditions and avoid excitation trapping. We proposed two EET schemes with tentative pigment assignments based on the interpretation of the modeling results together with previous structure-based calculations and spectroscopic observables.
在能量转移到反应中心之前，光捕获天线系统中的能量通常会达到平衡。平衡机制是天线激发能量转移（EET）网络的特征。表征该网络对于理解光合作用的第一步至关重要。我们提出了基于唯象学的分析程序和结果，直接从在 77 K 下记录的 2D 电子光谱中获得了光捕获复合物 II 的激子能级、光谱线宽和转移率矩阵，记录时的等待时间从 100 fs 到 100 ps。由于模型的限制和系统的复杂性，无法构建唯一的 EET 网络。然而，始终可以获得具有非常相似的光谱成分（激子）之间整体时间尺度的能量转移重复模式。尽管该状态的寿命（约 4-6 ps）比以前的研究短，但模型确定了在 664-668nm 区域的“瓶颈”状态。该模型还确定了三个末端激子态，分别在 675nm、677-678nm 和 680-681nm，它们彼此之间弱耦合。发现三个末端之间的激发能平衡与初始激发条件无关，这是光捕获复合物的一个关键设计，可确保在不同光照条件下的能量流动并避免激发捕获。我们提出了两种 EET 方案，并基于对建模结果的解释以及以前基于结构的计算和光谱可观测值，提出了暂定的色素分配。
