Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing Mn4CaO5 cluster upshifts the redox potential (Em) of the primary quinone electron acceptor QA by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the Mn4CaO5 cluster and QA in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the Mn4CaO5 cluster on Em(QA-/QA) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of QA at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the Mn4CaO5 cluster hardly affected the Em(QA-/QA) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent Em upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the Mn4CaO5 cluster does not directly regulate Em(QA-/QA) to control the redox reactions on the electron acceptor side of PSII.

中文翻译：

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水氧化性Mn4CaO5团簇是否调节光系统II中伯醌电子受体QA的氧化还原电位？傅里叶变换红外光谱电化学的研究。

长期以来，使用光系统II（PSII）的荧光测量进行氧化还原滴定表明，水氧化Mn4CaO5簇的损伤使伯醌电子受体QA的氧化还原电势（Em）上移了100 mV以上，这已被提议作为光保护剂。 PSII的机制。但是，仍无法解决Mn4CaO5团簇和QII中QA之间这种长距离相互作用的分子机理。在这项研究中，我们使用傅立叶变换红外（FTIR）光谱电化学技术重新研究了Mn4CaO5簇的耗尽对Em（QA- / QA）的影响，该光谱可直接在预期电势下监测QA的氧化还原状态。菠菜和嗜热嗜热球菌的完整和Mn耗尽的PSII制剂在一系列电极电势下的光致FTIR差异测量表明，Mn4CaO5簇的耗尽几乎不会影响Em（QA- / QA）值。相比之下，使用相同的PSII样品，电化学电池和氧化还原介体进行荧光光谱电化学测量时，Mn耗尽时表观Em发生了较大的上移，而当使用较弱的可见光进行荧光激发时，观察到的移位较小。因此，与红外光谱对FTIR测量中的PSII反应无干扰相比，暗示了荧光的测量光可能会干扰Mn贫化的PSII中的滴定曲线。根据这些结果，