Photosynthetic Oxygen Evolution - EPR studies of the manganese cluster of photosystem II

In this thesis, a new electron paramagnetic resonance (EPR) signal from photosystem II is presented. The signal is shown to originate from the S0 oxidation state of the oxygen-evolving complex (OEC). It is a Mn-derived signal displaying clear hyperfine structure, centered at g=2, and is significantly broader than the well-known multiline signal from the S2 state, indicating the presence of MnII in the S0 state. A few percent of methanol in the sample buffer are required in order to observe the S0 EPR signal. The effects of this solvent on the OEC poised in the S0 and S2 states were explored further, and it was found that the ground state multiline signals from these states are enhanced in a similar way by the presence of methanol. Together with previous reports on the effect of methanol on various S-states, this indicates that the action of methanol is to enhance the exchange couplings within the Mn cluster, increasing the energy gap between the ground state and the first excited state of the system. The temperature dependence of the S0 signal showed that it arises from a ground state with S=1/2, with no thermally accessible excited states, while the S2 multiline signal has a lower lying excited state. The microwave power saturation of the two signals was also recorded as a function of temperature, and found to differ significantly. The relaxation behavior of the two signals combined with the temperature dependence of the nonsaturated signal amplitudes is interpreted in terms of different spin-lattice relaxation mechanisms, and it is suggested that both signals relax via a Raman mechanism. New results are also presented from the S2 state as it reappears on the second oxygen-evolving cycle after dark-adaptation. Magnetic differences were found between the S2 state from the second-cycle and first-cycle samples. This difference between the first and second turn-over was found to have effects in the S0 and S1 states as well, and the phenomenon was interpreted as a tuning of the magnetic couplings within the OEC through the first few enzymatic cycles after dark-adaptation, optimizing it for continuous water splitting. Furthermore, field swept electron spin echo (ESE) spectra from samples poised in all the different S-states, covering more than one full S-cycle, are presented. The pulsed data also represent a change of the Mn cluster over the first S-cycle that is not reversed upon the rereduction of the OEC, corroborating the notion of a light-adaptation of the OEC.