Manganese cluster ions

The Mn C [Mn6(OH)3Cl3(hmp)9]6 2+ (31) complex, where hmp is 2-hydroxymethylpyridine, is structurally very close to these two manganese rings. It is a mixed-valence manganese cluster with a central Mn2+ ion. A Cl- counterion is hydrogen-bonded to the p3-OH groups which bridge the six Mn ions. 

Figure 23-35 Proposed sequence of S-states of the manganese cluster of photosystem II. The successive states as two molecules of H20 (green oxygen atoms) are converted to 02 is shown with the successive states S0-S4 labeled. To save space and possible confusion tyrosine 161 (Yz) and the nearby His 190 are shown only by S4. The Yz radical is thought to remove a hydrogen atom or H+ from one bound H20 and an electron from one Mn ion at each of the four S-states S0-S3 functioning in each case to eject a proton into the thylakoid lumen and to transfer an electron to P+ of the reaction center. However, the exact sequence of e transfer and H+ release may not be shown correctly. After Hoganson and Babcock.392 3923...

The other cofactor often neglected in discussions of the OEC is chloride. Again, EXAFS data have been obtained to address whether or not chloride is bound to the manganese cluster (33, 358, 359). These experiments are technically very difficult. There is the possibility that chloride (or bromide in bromide-substituted samples) may be observed in the EXAFS spectrum. However, we believe that it is unlikely that one can resolve this issue, because the signal-to-noise ratio may not be sufficient (156) to observe only one chloride per four manganese ions. Very recently, chloride EXAFS of manganese model complexes has been reported (360). This new application of EXAFS may be able to resolve this issue. 

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