Plants possess an essential ability to rapidly down-regulate light-harvesting in response to high light. This photoprotective process involves the formation of energy-quenching interactions between the chlorophyll and carotenoid pigments within the antenna of Photosystem II (PSII). The nature of these interactions is currently debated, with, among others, 'incoherent' or 'coherent' quenching models (or a combination of the two) suggested by a range of time-resolved spectroscopic measurements. In 'incoherent quenching', energy is transferred from a chlorophyll to a carotenoid and is dissipated due to the intrinsically short excitation lifetime of the latter. 'Coherent quenching' would arise from the quantum mechanical mixing of chlorophyll and carotenoid excited state properties, leading to a reduction in chlorophyll excitation lifetime. The key parameters are the energy gap, [Formula: see text] and the resonance coupling, J, between the two excited states. Coherent quenching will be the dominant process when [Formula: see text] i.e., when the two molecules are resonant, while the quenching will be largely incoherent when [Formula: see text] One would expect quenching to be energetically unfavorable for [Formula: see text] The actual dynamics of quenching lie somewhere between these limiting regimes and have non-trivial dependencies of both J and [Formula: see text] Using the Hierarchical Equation of Motion (HEOM) formalism we present a detailed theoretical examination of these excitation dynamics and their dependence on slow variations in J and [Formula: see text] We first consider an isolated chlorophyll-carotenoid dimer before embedding it within a PSII antenna sub-unit (LHCII). We show that neither energy transfer, nor the mixing of excited state lifetimes represent unique or necessary pathways for quenching and in fact discussing them as distinct quenching mechanisms is misleading. However, we do show that quenching cannot be switched 'on' and 'off' by fine tuning of [Formula: see text] around the resonance point, [Formula: see text] Due to the large reorganization energy of the carotenoid excited state, we find that the presence (or absence) of coherent interactions have almost no impact of the dynamics of quenching. Counter-intuitively significant quenching is present even when the carotenoid excited state lies above that of the chlorophyll. We also show that, above a rather small threshold value of [Formula: see text]quenching becomes less and less sensitive to J (since in the window [Formula: see text] the overall lifetime is independent of it). The requirement for quenching appear to be only that [Formula: see text] Although the coherent/incoherent character of the quenching can vary, the overall kinetics are likely robust with respect to fluctuations in J and [Formula: see text] This may be the basis for previous observations of NPQ with both coherent and incoherent features.

中文翻译：

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叶绿素-类胡萝卜素天线系统中的激发猝灭：“相干”或“不相干”。

植物具有基本功能，可以根据强光迅速下调采光。这种光保护过程涉及在Photosystem II（PSII）天线内的叶绿素和类胡萝卜素颜料之间形成能量猝灭相互作用。这些相互作用的性质目前正在争论中，其中包括一系列时间分辨光谱测量建议的“非相干”或“相干”猝灭模型（或两者的组合）。在“非相干猝灭”中，能量从叶绿素转移到类胡萝卜素，并由于其固有的短激发寿命而耗散。“相干淬灭”将来自叶绿素和类胡萝卜素激发态性质的量子机械混合，导致叶绿素激发寿命缩短。关键参数是两个激发态之间的能隙（公式：见正文）和共振耦合J。当[分子式：见正文]时，相干淬灭将是主要过程，即当两个分子共振时，而当[分子式：见正文]时，淬灭在很大程度上将是非相干的。人们会期望淬火在能量上不利于[分子式：见正文]。正文]淬火的实际动力学过程介于这些限制范围之间，并且与J和[公式]均具有非平凡的依存关系。它们对J和[公式：我们先考虑一个孤立的叶绿素-类胡萝卜素二聚体，然后将其嵌入PSII天线亚基（LHCII）中。我们表明，能量转移和激发态寿命的混合都不能代表淬灭的独特或必要途径，实际上，将它们作为独特的淬灭机理进行讨论是令人误解的。但是，我们确实表明，无法通过在共振点附近微调[公式：参见文本]来“打开”和“关闭”淬灭，由于类胡萝卜素激发态的重组能量很大，我们发现相干相互作用的存在（或不存在）对淬灭动力学几乎没有影响。即使类胡萝卜素的激发态高于叶绿素的激发态，也会存在反直觉的猝灭。我们还表明，超过[公式]的较小阈值，淬火对J变得越来越不敏感（因为在[公式]的窗口中，整个寿命与其无关）。淬灭的要求似乎仅是[公式：参见文本]尽管淬灭的相干/非相干特性可以变化，但整体动力学可能相对于J和[J]的波动具有稳健性。 NPQ既具有相干特征又具有非相干特征的先前观察的基础。