N.C.S.R. "Demokritos"

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Department of Materials Science

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Study of the water splitting process by Photosystem II

Photosystem II (PSII) catalyzes a fundamental reaction, the splitting of water which results in the evolution of molecular oxygen. The site of water oxidation, called the oxygen-evolving complex (OEC), contains a tetranuclear Mn cluster and a Ca2+ ion. The catalytic cycle of the OEC involves four light-driven one-electron oxidation steps, S0->S1, S1->S2, S2->S3, S3->[S4]->S0, accompanied by the progressive removal of four protons from two bound water molecules. Concomitant to the oxidation of Mn, spin changes and magnetic interaction changes among the Mn atoms are observed.

Trapping and study of critical short-lived intermediates during the photosynthetic water splitting process. The laboratory of EPR spectroscopy is seminally involved in studying new chemical intermediates trapped at liquid He and N2 temperatures. These intermediates are assigned to a specific tyrosine radical (D1Tyr 161 or TyrZ) which interacts magnetically with the Mn cluster. They are termed SnTyrZ•, n=0,1,2, and considered part of the water splitting mechanism (metallo-radical mechanism).
The S3 state. Particular interest exists in S3, the state that precedes the final step of water oxidation, oxygen evolution. S3 is an integer spin state, and shows a very unusual property. Excitation of the Mn cluster with NIR light induces reversible spin transitions. Theoretical studies show that the EPR spectra of S3 obtained at X-band (9.4 GHz) and Q-band (34 GHz) microwave frequencies can be fitted well with spin S=3.

Department of Materials Science, N.C.S.R. "Demokritos", 153 10 Aghia Paraskevi, Attiki, phone: +30 210 6503381, fax: +30 210 6519430
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