Su Ji-Hu, Lubitz Wolfgang, Messinger Johannes
Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
J Am Chem Soc. 2008 Jan 23;130(3):786-7. doi: 10.1021/ja076620i.
In the oxygen-evolving complex (OEC) of photosystem II (PSII) molecular oxygen is formed from two substrate water molecules that are ligated to a mu-oxo bridged cluster containing four Mn ions and one Ca ion (Mn4OxCa cluster; Ox symbolizes the unknown number of mu-oxo bridges; x >or= 5). There is a long-standing enigma as to when, where, and how the two substrate water molecules bind to the Mn4OxCa cluster during the cyclic water-splitting reaction, which involves five distinct redox intermediates (Si-states; i = 0,...,4). To address this question we employed hyperfine sublevel correlation (HYSCORE) spectroscopy on H217O-enriched PSII samples poised in the paramagnetic S2 state. This approach allowed us to resolve the magnetic interaction between one solvent exchangeable 17O that is directly ligated to one or more Mn ions of the Mn4OxCa cluster in the S2 state of PSII. Direct coordination of 17O to Mn is supported by the strong (A approximately 10 MHz) hyperfine coupling. Because these are properties expected from a substrate water molecule, this spectroscopic signature holds the potential for gaining long-sought information about the binding mode and site of one of the two substrate water molecules in the S2 state of PSII.
在光系统II（PSII）的析氧复合物（OEC）中，分子氧由两个底物水分子形成，这两个底物水分子与一个包含四个锰离子和一个钙离子的μ-氧桥簇（Mn4OxCa簇；Ox表示未知数量的μ-氧桥；x≥5）相连。关于在循环水分解反应过程中，两个底物水分子何时、何地以及如何与Mn4OxCa簇结合，存在一个长期未解之谜，该反应涉及五个不同的氧化还原中间体（Si态；i = 0，...，4）。为了解决这个问题，我们对处于顺磁S2态的富含H217O的PSII样品采用了超精细亚能级相关（HYSCORE）光谱法。这种方法使我们能够解析在PSII的S2态中，一个与Mn4OxCa簇的一个或多个锰离子直接相连的可溶剂交换的17O之间的磁相互作用。17O与锰的直接配位由强（A约10 MHz）超精细耦合支持。因为这些是底物水分子预期的性质，这种光谱特征有可能获得关于PSII的S2态中两个底物水分子之一的结合模式和位点的长期寻求的信息。
