Effects of Protons and Small Alcohols on the Oxygen-Evolving Complex of Photosystem II

Abstract: Higher plants and algae use oxygenic photosynthesis to convert solar energy to reducing equivalents and chemical energy. The ultimate electron donor to oxygenic photosynthesis is water. Oxidation of water to oxygen occurs in the oxygen-evolving complex (OEC) in Photosystem II (PSII). The OEC is composed of four Mn atoms, Ca2+ and Cl-. The OEC cycles through five states (S0 - S4) during oxidation of water to oxygen. The OEC can be studied with Electron Paramagnetic Resonance (EPR) spectroscopy. The shape of the EPR signals depends on the internal magnetic couplings between the Mn atoms and their ligand environment. To detect the EPR signal from the S0 state it is necessary to add a few percent of methanol. The saturation of the S0 multiline signal amplitude with increasing methanol concentration was compared to the saturation of the S2 multiline signal amplitude. The S0 and S2 multiline signals show the same saturation behaviour. It was concluded that methanol affects the S0 and S2 states in a similar way. The effect of methanol has given new spectroscopic probes of the Mn cluster of the OEC. These can be used to further understand how the OEC functions. The second modification of the OEC was a change of the ambient pH. The pH is important for efficient electron transfer through PSII and optimal oxygen evolution. Acidic and alkaline pH-values were shown to decrease the amplitudes of the S0 and S2 multiline signals. The decrease of the amplitudes is reversible. The alkaline pH-treatment also induced a signal from the OEC in the formal S3 state. The signal arises from the magnetic interaction between the Mn cluster in the S2 state and the YZ-radical. It was concluded that at acidic pH the Mn cluster or a ligand to the Mn cluster becomes protonated and this changes the magnetic couplings of the Mn cluster. In the S0 and S2 states, either the Mn cluster or YZ lowers the redox potential by the alkaline pH-treatment. In the S3 state, it is only YZ that lowers the redox potential. It is proposed that the difference between the S-states is due to the accessibility of protons. This shows that YZ has a very central role in the oxidation of water. The investigations presented in this thesis also shows that both the Mn cluster and YZ are solvent accessible.