Structure of an active water molecule in the water-oxidizing complex of photosystem II as studied by FTIR spectroscopy.
Structure of an active water molecule in the water-oxidizing complex of photosystem II as studied by FTIR spectroscopy.
复制标题
通过 FTIR 光谱研究光系统 II 的水氧化复合物中活性水分子的结构。
DOI：
10.1021/bi001040i
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发表时间：
2000
期刊：
影响因子：
2.9
通讯作者：
M. Sugiura
中科院分区：
文献类型：
--
作者：
T. Noguchi;M. Sugiura
首次利用傅里叶变换红外光谱法（FTIR）检测到光系统II水氧化复合物（WOC）中水分子的振动。在S（1）-S（2）跃迁的闪光诱导FTIR差谱中，分别在3618和3585 cm（-1）处观察到一对正带和负带，并且在H（2）（18）O取代H（2）（16）O后，这两个带均显示出12 cm（-1）的下移。D（2）O取代后，谱带大幅度下移至2681和2652 cm（-1）。这些观测结果表明，3618和3585 cm（-1）处的谱带来自水分子（可能是底物水）的O-H伸缩振动，该水分子分别与处于S（2）和S（1）态的Mn团簇耦合。光谱频率表明O-H基团形成弱氢键，且这种氢键在S（2）形成后变得更弱。通过H（2）O/D（2）O（1：1）混合物的解耦实验，研究了该水分子与其它O-H振动的分子内耦合。对于3618（S（2））和3585 cm（-1）（S（1））带，通过去耦的下移估计为4和12 cm（-1），这两个带都比水蒸气中的52 cm（-1）小得多，表明观察到的水具有相当不对称的结构;即，其中一个O-H基团是弱键合的，另一个是强键合的。S（2）态的耦合比S（1）态的耦合小，这意味着这种氢键不对称性在S（2）形成时变得更加突出。这种结构变化可以通过降低势垒来促进在后面的步骤中发生的质子释放反应。本研究表明，FTIR检测O-H振动是一种有用的和有前途的方法，直接监测基质水的化学反应，阐明光合水氧化的分子机制。
The vibrations of a water molecule in the water-oxidizing complex (WOC) of photosystem II were detected for the first time using Fourier transform infrared (FTIR) spectroscopy. In a flash-induced FTIR difference spectrum upon the S(1)-to-S(2) transition, a pair of positive and negative bands was observed at 3618 and 3585 cm(-1), respectively, and both bands exhibited downshifts by 12 cm(-1) upon replacement of H(2)(16)O by H(2)(18)O. Upon D(2)O substitution, the bands largely shifted down to 2681 and 2652 cm(-1). These observations indicate that the bands at 3618 and 3585 cm(-1) arise from the O-H stretching vibrations of a water molecule, probably substrate water, coupled to the Mn cluster in the S(2) and S(1) states, respectively. The band frequencies indicate that the O-H group forms a weak H-bond and this H-bonding becomes weaker upon S(2) formation. Intramolecular coupling with the other O-H vibration of this water molecule was studied by a decoupling experiment using a H(2)O/D(2)O (1:1) mixture. The downshifts by decoupling were estimated to be 4 and 12 cm(-1) for the 3618 (S(2)) and 3585 cm(-1) (S(1)) bands, both of which were much smaller than 52 cm(-1) of water in vapor, indicating that the observed water has a considerably asymmetric structure; i.e., one of the O-H groups is weakly and the other is strongly H-bonded. The smaller coupling in the S(2) than the S(1) state means that this H-bonding asymmetry becomes more prominent upon S(2) formation. Such a structural change may facilitate the proton release reaction that takes place in the later step by lowering the potential barrier. The present study showed that FTIR detection of the O-H vibrations is a useful and promising method to directly monitor the chemical reactions of substrate water and clarify the molecular mechanism of photosynthetic water oxidation.