Role of a Water Network around the Mn₄CaO₅ Cluster in Photosynthetic Water Oxidation: A Fourier Transform Infrared Spectroscopy and Quantum Mechanics/Molecular Mechanics Calculation Study

Photosynthetic water oxidation takes place at the Mn₄CaO₅ cluster in photosystem II. Around the Mn₄CaO₅ cluster, a hydrogen bond network is formed by several water molecules, including four water ligands. To clarify the role of this water network in the mechanism of water oxidation, we investigated the effects of the removal of Ca²⁺ and substitution with metal ions on the vibrations of water molecules coupled to the Mn₄CaO₅ cluster by means of Fourier transform infrared (FTIR) difference spectroscopy and quantum mechanics/molecular mechanics (QM/MM) calculations. The OH stretching vibrations of nine water molecules forming a network between D1-D61 and Y_Z were calculated using the QM/MM method. On the the calculated normal modes, a broad positive feature at 3200-2500 cm^-1 in an S₂-minus-S₁ FTIR spectrum was attributed to the vibrations of strongly hydrogen-bonded OH bonds of water involving the vibrations of water ligands to a Mn ion and the in-phase coupled vibration of a water network connected to Y_Z, while bands in the 3700-3500 cm^-1 region were assigned to the coupled vibrations of weakly hydrogen-bonded OH bonds of water. All the water bands were lost upon Ca²⁺ depletion and Ba²⁺ substitution, which inhibit the S₂ → S₃ transition, indicating that a solid water network was broken by these treatments. By contrast, Sr²⁺ substitution slightly altered the water bands around 3600 cm^-1, reflecting minor modification in water interactions, consistent with the retention of water oxidation activity with a decreased efficiency. These results suggest that the water network around the Mn₄CaO₅ cluster plays an essential role in the water oxidation mechanism particularly in a concerted process of proton transfer and water insertion during the S₂ → S₃ transition.