Vibrational Analysis of Water Network Around the Mn Cluster

Photosynthetic water oxidation takes place at the Mn cluster in photosystem II. During this process, several water molecules including four water ligands form a hydrogen bond network around the Mn cluster. To better elucidate the role of this network in the mechanisms underlying water oxidation, we examined the vibrational structure of the water molecules coupled with the Mn cluster using quantum mechanics/molecular mechanics (QM/MM) calculations. The OH vibrations of these water molecules in the hydrogen-bonding network between YZ and D1-D61 were simulated by the QM/MM simulations. The normal mode analysis showed that a broad positive feature at 2500–3200 cm−1 in the experimental S2-minus-S1 difference spectrum was attributed to the OH stretching vibrations with the strong hydrogen bond interaction of water molecules around the Mn cluster, including those of water ligands coordinating with Mn and the in-phase coupled vibration among water molecules in the hydrogen bond network around the Mn cluster. In contrast, the bands in higher frequency (3500–3700 cm−1) region were assigned to the OH stretching vibrations with weaker hydrogen bond interaction of water molecules in the network. These assignments strongly suggested that the in-phase mode among several water molecules may have a function to facilitate rapid proton transfer along the vibration direction using the Grotthuss mechanism. Thus, we proposed that the hydrogen bond network formed by several water molecules around the Mn cluster plays a key role in proton transfer during the water oxidation process.