Magnetic properties of manganese in the photosynthetic O2- evolving complex

We report the continuous power saturation and temperature dependence of three EPR signals which are generated by low-temperature illumination of dark-adapted Photosystem II (PSII) membranes and are associated with the S₂ state of the O₂-evolving complex of photosynthesis. PSII membranes which are dark-adapted for 4 h at 0°C and illuminated at 200 K for 2 min exhibit a S₂ state EPR signal which saturates easily (P_1/2 = 3.7 mW at 6.0 K) and has an intensity maximum at 6.9 K under nonsaturating conditions. The S₂ state EPR signal obtained from 6-min dark-incubated samples illuminated at 160 K exhibits no intensity maximum in the 4.0-16.0 K range under nonsaturating conditions and saturates at higher microwave powers (P_1/2 = 37.1 mW at 6.0 K). Finally, a third signal produced by 170 K illumination of 6-min dark-adapted membranes shows an intensity maximum at 5.9 K under nonsaturating conditions and is not saturated with our current experimental setup (P_1/2 > 156 mW at 6.0 K). We conclude that each EPR spectrum originates from a thermally excited state of one of three distinct configurations of the manganese complex which is believed to make up the active site. The temperature dependence data are fitted to a model in which two paramagnetic sites are ferromagnetically exchange coupled. All three sets of data can be accounted for by varying the magnitude of the superexchange coupling constant.