Oxo-bridged clusters

For the purposes of this chapter, we limit the discussion to /a-oxo bridged clusters. Model compounds of binuclear octahedral iron without oxo bridges have electromagnetic properties different from the related clusters with ju-oxo bridges (Surerus et al. (1989) only the fi-oxo bridged clusters have been found in proteins so far. 

A series of triangular, oxo-bridged cluster compounds, Ru30(CH3-C02)eL3 (L is a neutral ligand), was prepared by Spencer and Wilkinson... 

However, there has been progress in metal-mediated N2O activation. Oxo-transfer from N2O can lead to formation of metal-oxo complexes or oxidation of ligands contained in the metal complex. In pioneering research of N2O reactivity, Bottomley and coworkers [188-192] utihzed N2O in the preparation of a series of unusual oxo-bridged clusters with titanium, vanadium and chromium metal ions M-(N20)-M-bridged intermediates were suggested to form. 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

Fig. 21. The bismuth citrate cluster (NH4)12[Bi1208(cit)8] 10 H20 showing citrate and oxo bridging. Adapted from (454).

It seems probable that other redox centres contain this binuclear iron structure, but that this has not yet been recognized. For example, a non-heme iron protein of the methane monooxygenase from Methylococcus capsulatus (Bath), which functions as the oxygenase in equation (28), has been described as having a novel iron centre which is not an iron-sulfur cluster. This may well be an oxo-bridged system. Analysis suggests 2.3 Fe per molecule of protein. 

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. 

Fox, B. G., Surerus, K. K., M,nck, E., and Lipscomb, J. D., 1988, Evidence for a p-oxo-bridged binuclear iron cluster in Oie hych oxylase component of methane monooxygenase. MYs-bauer and EPR studies, J. Biol. Chem. 263 10553nl0556. 

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