51 state, oxygen-evolving complex

Fig. 19. One of the structures included in an extensive evaluation study of magnetic and spectroscopic properties for models of the S2 state of the oxygen evolving complex.

Kulik LV, Epel B, Lubitz W, Messinger J. Electronic structure of the MnRJxCa cluster in the So and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn ENDOR and EPR spectroscopy. J Am Chem Soc. 2007 129(44) 13421-35. 

Styring S, Rutherford AW. In the oxygen-evolving complex of photosystem II the So state is oxidized to the Si state by D+ (signal IIsiow). Biochemistry. 1987 26(9) 2401-5. 

Ames W, Pantazis DA, Krewald V, et al. Theoretical evaluation of structural models of the S2 state in the oxygen evolving complex of photosystem II protonation states and magnetic interactions. J Am Chem Soc. 2011 133(49) 19743-57. 

Robblee, JH, Messinger, J, Cinco, RM, et al. The Mn cluster in the So state of the oxygen-evolving complex of photosystem II studied by EXAFS spectroscopy are there three Di-p-oxo-bridged Mn2 moieties in the tetranuclear Mn complex J Am Chem Soc. 2002 124(25) 7459-71. 

Liang WC, Roelofs TA, Cinco RM, et al. Structural change of the Mn cluster during the S2 —> S3 state transition of the oxygen-evolving complex of photosystem II. Does it reflect the onset of water/substrate oxidation Determination by Mn X-ray absorption spectroscopy. J Am Chem Soc. 2000 122(14) 3 399 412. 

Pantazis DA, Ames W, Cox N, Lubitz W, Neese F. Two interconvertible structures that explain the spectroscopic properties of the oxygen evolving complex of photosystem II in the S2 state. Angew Chem Int Ed 2012 51(39) 9935-40. 

The S states represent the accumulation of positive charges on the oxygen-evolving complex ((TEC), and O2 is evolved only when 4 charges are accumulated. Starting with dark-adapted chloroplasts (or intact photosynthetic cells), O2 evolution is maximal at the third flash, then proceeds with a periodicity of 4, because the state Si is the most abundant at equilibrium in the dark. After a number of cycles of the system, the periodicity tends to disappear due to misses and double hits , which finally randomize the PS II units into the 4 S states [8,9]. 

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