Oxygen Evolving Complex Sample Clauses

Oxygen Evolving Complex. The photosynthetic oxygen evolution occurs at a specialized site of PSII called the oxygen-evolving complex (OEC) [5, 11]. To couple the single-electron photochemistry of the PSII reaction centre to the four- electron oxidation of two water molecules to molecular oxygen, the OEC cycles through five intermediate states, denoted Si, where the index i indicates the number of positive charges accumulated at the OEC and runs from 0 to 4. The S4 state decays spontaneously back to the S0 state, releasing an oxygen molecule. The catalytic centre of the OEC is a Mn3O4Ca cubane with a fourth manganese atom outside the cube. The detailed structure of the OEC has been matter of debate during the last decade, with the most accepted structures being the Xxxxxxxx and the Loll model [12, 13]. However, more recently a higher resolution structure has been obtained from X-Ray data by Xxxxx et al. [14]. Moreover, there is increasing converging evidence that the four oxidizing equivalents are not accumulated, instead they are used throughout the S-state cycle to extract electrons and protons from two water molecules bound to the MnCa cluster in the S0 state. In addition, it is known that calcium and chloride ions are required to perform the water-oxidation catalysis and it is thought that their location is close to the MnCa cluster [5]. The radical states formed during the reaction on the substrate water intermediates are stabilized by increasing the oxidation state of the MnCa cluster. During each step of the S0-S4 cycle one proton and one electron are released, the system remains electroneutral, and no coulombic restriction is imposed to the system by the increase of the oxidation state of the manganese atoms. However, there is considerable discussion about the mechanistic details of this reaction [15-17].
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