Hydration of the oxygen-evolving complex in photosystem II probed by water proton NMR (spin-lattice) relaxation
To understand the basic mechanism of water oxidation during photosynthetic oxygen evolution one needs to know the structure of the oxygen-evolving complex (OEC) of photosystem II (PSII) that contains a Mn 4 O 5 Ca cluster, the Mn oxidation states and the mode of substrate water binding in the different oxidation states of this reaction cycle (S states). Because for PSII the substrate is identical to the solvent, the study of substrate binding is more complicated than for other enzymes. The identification of the bound water species (terminal, bridging; oxo, hydroxo, water) and the determination of the redox states of the Mn to which they binding is very difficult. Much of what is currently known about substrate binding has come from measurements of the oxygen evolving complex (OEC) using time resolved membrane inlet mass spectrometry, electron paramagnetic resonance (EPR), FTIR difference spectroscopy, X-ray crystallography and theoretical models. However, many questions remain to be answered.
In our project the hydration of OEC is monitored by studying the solvent water proton nuclear magnetic relaxation dispersion (NMRD) under ambient conditions. This approach relies on the well established NMR proton relaxation enhancement (NMR-PRE) effect, which is governed by the exchange of bulk-water protons with protons bound at or near a fast relaxing, paramagnetic centre, which in our case is the Mn 4 O 5 Ca cluster of the OEC.