Magnetic properties of manganese in the photosynthetic O2- evolving complex

Julio C. De Paula, Gary W Brudvig

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Abstract

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 (P1/2 = 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 (P1/2 = 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 (P1/2 > 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.

Original languageEnglish
Pages (from-to)2643-2648
Number of pages6
JournalJournal of the American Chemical Society
Volume107
Issue number9
Publication statusPublished - 1985

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ASJC Scopus subject areas

  • Chemistry(all)

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