Spectroscopic probes of hydrogen bonding networks and proton transfer in photosynthetic water oxidation
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In the water oxidizing enzyme, photosystem II (PSII), a hydrogen-bonded network comprised of water molecules, amino acid side chains and peptide carbonyl groups functions to transfer protons. The network spans over 35 Å and is responsible for shuttling protons away from the catalytic oxygen-evolving Mn4CaO5 complex (OEC). Step-wise light induced oxidations of the OEC leads to the generation of molecular oxygen from water. The OEC transitions between four different oxidation states called the Sn states, where n represents the number of oxidizing equivalents stored on the OEC. These OEC reactions were mimicked in the laboratory using laser flashes. Reaction Induced Fourier Transform Infrared (RIFT-IR) spectroscopy can monitor long-lived PSII structural changes in response to the OEC oxidation. Experimental evidence has implicated the formation of protonated internal water clusters as necessary intermediates during two steps of the water oxidation reaction. Chloride ions regulate PSII activity by stabilizing the protonated water cluster based on the experimental data and theoretical calculations on a small model of the OEC. Using the spectral signatures of peptide carbonyl groups participating in the pathway, the water containing proton transfer network of PSII was demonstrated to be extremely robust and resilient to changes in pKa, ionic strength and hydrogen bonding. The formation of the internal protonated water cluster was found to be an important step in proton transfer. The alternative proton acceptor, acetate inhibited activity when substituted at the chloride site by accepting the extra proton from water and forming acetic acid.