Photosynthesis produces the oxygen necessary for all aerobic life. During this process, the manganese-containing oxygen evolving complex (OEC) in photosystem II (PSII), cycles through five oxidation states, S0-S4. One of these, S2, is known to be paramagnetic and gives rise to electron paramagnetic resonance (EPR) signals used to probe the catalytic structure and function of the OEC. The S0 states has long been thought to be paramagnetic. We report here a Mn EPR signal from the previously EPR invisible S0 state. The new signal oscillates with a period of four, indicating that it originates from fully active PSII centers. Although similar to the S2 state multiline signal, the new signal is wider (2200 gauss compared with 1850 gauss in samples produced by flashing), with different peak intensity and separation (82 gauss compared with 89 gauss). These characteristics are consistent with the S0 state EPR signal arising from a coupled MnII-MnIII intermediate. The new signal is more stable than the S2 state signal and its decay in tens of minutes is indicative of it originating from the S0 state. The S0 state signal will provide invaluable information toward the understanding of oxygen evolution in plants.

During oxygen evolution, the Mn cluster in Photosystem II cycles through five oxidation states, S0-S4. S0 and S2 are paramagnetic, and can be monitored by electron paramagnetic resonance (EPR). Recently a new EPR signal from the S0 state was discovered [Ahrling et al. (1997) Biochemistry 36, 13148-13152, Messinger et al. (1997) J. Am. Chem. Soc. 119, 11349-11350]. Here, we present a well-resolved S0 spectrum, taken at high power and low temperature. The spectrum is wider and more resolved than previously thought, with structure over more than 2500 G, and appears to have at least 20 reproducible peaks on each side of g = 2. We also present the temperature dependence of the unsaturated S0 signal amplitude. A linear relationship was found between signal intensity and reciprocal temperature (1/T) in the region 5-25 K, clearly extrapolating to 0. This obeys the Curie law, indicating that the S0 state is a ground S = 1/2 state with no thermally accessible excited state. The data are consistent with a minimum energy gap of 30 cm-1 between the ground and first excited states.

Electron paramagnetic resonance (EPR) spectroscopy is one of the major techniques used to analyse the structure and function of the water oxidising complex (WOC) in Photosystem II. The discovery of an EPR signal from the S0 state has opened the way for new experiments, aiming to characterise the S0 state and elucidate the differences between the different S states. We present a review of the biochemical and biophysical characterisation of the S0 state multiline signal that has evolved since its discovery, and compare these results to previous and recent data from the S2 multiline signal. We also present some new data from the S2 state reached on the second turnover of the enzyme.

The Mn-derived electron paramagnetic resonance (EPR) multiline signal from the S(0) state of the water-oxidizing complex is observable only in the presence methanol. In the present study, we have characterized the effect of methanol on the EPR signals from the S(0) and S(2) states as well as on the EPR Signal II(slow) originating from the Tyrosine(D)(ox) radical. The amplitudes of the S(0) and S(2) multiline signals increase with the methanol concentration in a similar way, whereas the S(2) g=4.1 excited state signal amplitude shows a concomitant decrease. The methanol concentration at which half of the spectral change has occurred is approximately 0.2% and the effect is saturating around 5%. Methanol has an effect on the microwave power saturation of the S(2) multiline signal, as well. The microwave power at half saturation (P(1/2)) is 85 mW in the presence of methanol, whereas the signal relaxes much slower (P(1/2) approximately 27 mW) without. The relaxation of Signal II(slow) in the presence of methanol has also been investigated. The P(1/2) value of Signal II(slow) oscillates with the S cycle in a similar way as without methanol, but the P(1/2) values are consistently lower in the methanol-containing samples. From the results, we conclude that methanol modifies the magnetic properties of the S(0) and S(2) states in a similar way. The possible site and nature of methanol binding is discussed.

Single-crystal InP was grown by the synthesis solute diffusion (SSD) technique. The measured etch pit density of ∼10 cm−2, mobility (77 K) of 50 200 cm2 V−1 s−1 and carrier concentration (77 K) of 2.3×1014 cm−3 demonstrate the feasibility of SSD for the growth of high-quality bulk InP.