Exploring the Deprotonation Process during Incorporation of a Ligand Water Molecule at the Dangling Mn Site in Photosystem II

The Mn4CaO5 cluster, featuring four ligand water molecules (W1 to W4), serves as the water-splitting site in photosystem II (PSII). X-ray free electron laser (XFEL) structures exhibit an additional oxygen site (O6) adjacent to the O5 site in the fourth lowest oxidation state, S3, forming Mn4CaO6. Here, we investigate the mechanism of the second water ligand molecule at the dangling Mn (W2) as a potential incorporating species, using a quantum mechanical/molecular mechanical (QM/MM) approach. Previous QM/MM calculations demonstrated that W1 releases two protons through a low-barrier H-bond toward D1-Asp61 and subsequently releases an electron during the S2 to S3 transition, resulting in O•– at W1 and protonated D1-Asp61. During the process of Mn4CaO6 formation, O•–, rather than H2O or OH–, best reproduced the O5···O6 distance. Although the catalytic cluster with O•– at O6 is more stable than that with O•– at W1 in S3, it does not occur spontaneously due to the significantly uphill deprotonation process. Assuming O•– at W2 incorporates into the O6 site, an exergonic conversion from Mn1­(III)­Mn2­(IV)­Mn3­(IV)­Mn4­(IV) (equivalent to the open-cubane S2 valence state) to Mn1­(IV)­Mn2­(IV)­Mn3­(IV)­Mn4­(III) (equivalent to the closed-cubane S2 valence state) occurs. These findings provide energetic insights into the deprotonation and structural conversion events required for the Mn4CaO6 formation.锰4钙氧5团簇（Mn4CaO5 cluster）包含四个配体水分子（记为W1至W4），是光系统II（PSII）中的水分解位点。X射线自由电子激光器（XFEL）结构显示，在第四低氧化态S3中，O5位点旁存在额外的氧位点O6，进而形成锰4钙氧6团簇（Mn4CaO6）。本研究采用量子力学/分子力学（QM/MM）方法，探究了悬垂锰位点处的第二个配体水分子（W2）作为潜在掺入物种的作用机制。过往的QM/MM计算研究表明，W1通过朝向D1-天冬氨酸61（D1-Asp61）的低势垒氢键释放两个质子，并在S2到S3的氧化态转变过程中释放一个电子，最终使W1位点形成氧自由基阴离子（O•–），同时D1-Asp61被质子化。在锰4钙氧6团簇的形成过程中，氧自由基阴离子（O•–）相较于水分子（H2O）或氢氧根离子（OH–），最能准确复现O5与O6之间的距离。尽管在S3氧化态下，O6位点带有O•–的催化团簇比W1位点带有O•–的团簇更稳定，但由于去质子化过程为显著的 uphill（自由能升高）过程，该过程无法自发发生。假设W2位点的O•–掺入O6位点，则会发生从Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)（对应开立方烷型S2价态）到Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)（对应闭立方烷型S2价态）的放能转变过程。本研究结果为锰4钙氧6团簇形成所需的去质子化与结构转变过程提供了热力学层面的深入解析。