Manganese cluster

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. 

The conversion of hydroperoxide/peroxide to superoxide is a one-electron redox reaction and requires the presence of transition metals having accessible multiple oxidation states as in biological iron or manganese clusters (e.g., Fe(II, III, IV) clusters of monooxygenase or the Mn(II, HI, IV) clusters of photosystems). Ti is usually not reduced at ambient temperatures. The various possibilities that could facilitate the transformation of hydroperoxo/peroxo to superoxo species are as follows ... 

Manganese Clusters and Manganese Cluster Carbides with Hydrogen. The Mn-CH3 Bond Energy. 

We begin this overview of manganese biochemistry with a brief account of its role in the detoxification of free radicals, before considering the function of a dinuclear Mn(II) active site in the important eukaryotic urea cycle enzyme arginase. We then pass in review a few microbial Mn-containing enzymes involved in intermediary metabolism, and conclude with the very exciting recent results on the structure and function of the catalytic manganese cluster involved in the photosynthetic oxidation of water. 

This initial process finishes with the participation of manganese-containing system connected with proteins of reaction center PS-2. Structural reconstruction can take place in manganese cluster (two-nucleus or four-nucleus) under the action of radiation [8, 9] from univalent state (4.9369 eV - this is similar to initial values of Mg PE-parameter) to bivalent (9.9414 eV) and further - to quadrivalent state (17.518 eV). 

Figure 5 Tetranuclear manganese cluster core structures.

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

Much of the impetus for the investigation of polynuclear manganese complexes has come from a desire to produce structural and/or functional models of the manganese cluster of the OEC. Many of the results of these studies have been described in Section 5.1.2 and in reviews. ... 

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...

Hiickel method ( Section 4.4). In the hands of Hoffmann, to whom we owe the EHM in its current form [112], extended Hiickel calculations have given powerful insight into the structures of these compounds. Wide-ranging corroboration of this assertion is seen in Hoffmann s Nobel lecture [100]. Some other examples are a polymeric rhenium compound [113], manganese clusters [114], and iridium [115] and nickel [116] coordination compounds. 

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