Light-harvesting complex II (LHCII) is the major antenna complex in higher plants and green algae. It has been suggested that a major part of the excited state energy dissipation in the so-called "non-photochemical quenching" (NPQ) is located in this antenna complex. We have performed an ultrafast kinetics study of the low-energy fluorescent states related to quenching in LHCII in both aggregated and the crystalline form. In both sample types the chlorophyll (Chl) excited states of LHCII are strongly quenched in a similar fashion. Quenching is accompanied by the appearance of new far-red (FR) fluorescence bands from energetically low-lying Chl excited states. The kinetics of quenching, its temperature dependence down to 4 K, and the properties of the FR-emitting states are very similar both in LHCII aggregates and in the crystal. No such FR-emitting states are found in unquenched trimeric LHCII. We conclude that these states represent weakly emitting Chl-Chl charge-transfer (CT) states, whose formation is part of the quenching process. Quantum chemical calculations of the lowest energy exciton and CT states, explicitly including the coupling to the specific protein environment, provide detailed insight into the chemical nature of the CT states and the mechanism of CT quenching. The experimental data combined with the results of the calculations strongly suggest that the quenching mechanism consists of a sequence of two proton-coupled electron transfer steps involving the three quenching center Chls 610/611/612. The FR-emitting CT states are reaction intermediates in this sequence. The polarity-controlled internal reprotonation of the E175/K179 aa pair is suggested as the switch controlling quenching. A unified model is proposed that is able to explain all known conditions of quenching or non-quenching of LHCII, depending on the environment without invoking any major conformational changes of the protein.

中文翻译：

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荧光叶绿素电荷转移态作为光捕获络合物II激发态猝灭中的中间体的表征。

采光复合体II（LHCII）是高等植物和绿藻中的主要天线复合体。已经提出，所谓的“非光化学猝灭”（NPQ）中的激发态能量耗散的主要部分位于该天线复合体中。我们已经对与聚集和结晶形式的LHCII中的淬灭有关的低能荧光状态进行了超快动力学研究。在这两种样品类型中，LHCII的叶绿素（Chl）激发态都以相似的方式被强烈淬灭。淬灭伴随着来自能量低的Chl激发态的新的远红（FR）荧光带的出现。在LHCII聚集体和晶体中，淬灭动力学，其温度依赖性低至4 K以及FR发射态的性质都非常相似。在未淬灭的三聚体LHCII中未发现此类FR发射状态。我们得出的结论是，这些状态代表弱发射Chl-Chl电荷转移（CT）状态，其形成是淬灭过程的一部分。最低能量激子和CT状态的量子化学计算（明确包括与特定蛋白质环境的耦合）提供了对CT状态化学性质和CT猝灭机理的详细了解。实验数据与计算结果相结合强烈表明，淬灭机理由两个质子耦合电子转移步骤组成，涉及三个淬灭中心Chls 610/611/612。发射FR的CT状态是该序列中的反应中间体。建议将E175 / K179氨基酸对的极性控制的内部质子化作为控制猝灭的开关。提出了一个统一的模型，该模型能够根据环境解释LHCII淬灭或非淬灭的所有已知条件，而无需调用蛋白质的任何主要构象变化。