Dimanganese model complexes

Fig. 44. Structural data for bridged dimanganese model complexes.

The redox properties of the dimanganese model complexes are listed in Table IX. In general, three redox states can be accessed electrochemically. For oxo-bridged complexes, the Mn, Mn" Mn, and Mn " can be attained with the Mn2 /Mn" Mn couple at about 1-1.8 V vs. NHE and the Mn "Mn /MnJ" couple 0.6-0.9 V lower. The phenoxo-bridged complexes, on the other hand, access the Mn , Mn"Mn ", and MnJ states in the same potential range. This positive shift in potential when the oxo bridge... 

Early proposals suggested [72] that catalase contains a p-oxo-bis(p-carboxylato)-dimanganese core. The UV-Vis spectra of this structural motif present in model complexes exhibit 480-520 nm d-d absorptions [73] similar to the UV-Vis absorption bands of manganese catalases. The EPR studies of oxidized T. ther-mophilus catalase [74] also suggested a MnIIIMnI" p-oxo-bis(p-carboxy 1 ato) core as a possible structural motif for the active site. 

The electronic spectrum of the dimanganese(III) form of catalase from Thermus thermophilus shows an intense absorption at 450 nm with a shoulder at 500 nm. EPR studies show that Mn Mn , Mn Mn , and Mn Mn forms are also accessible and the Mn Mn protein exhibits a 16-line spectrum, characteristic of a p-oxo bridged system 202). Comparison with the model complexes leads to prediction of p-oxo-bis(/[i-carboxylato)dimanganese(III) structure for the catalase. X-ray crystal data (203) have indicated an Mn—Mn distance of 3.6 3 A for the enzyme this appears to be inconsistent with the above proposal but, as the X-ray data are at rather low (3 A) resolution, this may be the least reliable piece of data. 

The dimanganese systems constitute one of the unusual cases in which the data for the relatively sophisticated model complexes contrast with the rather sparse information available for most of the enzyme active sites. The properties of the synthetic systems are being used in a predictive fashion to assign structures to the enzymes as spectropic data are obtained. 

EPR, MAGNETIC SUSCEPTIBILITY AND ENDOR STUDIES OF THE WATER OXIDIZING COMPLEX AND DIMANGANESE MODELS CONSEOUENCES OF CL- EXCHANGE AND CA2+DEPLETION. 

A similarity in the visible spectra of some /i-oxo-di-/i-acetato Mn(III) dinuclear complexes to the spectrum of Mn catalase has been noted (168-174). Reaction of hydrotris(l-pyrazolyl)borate, [HB(pz)3], or 1,4,7-triazacyclononane (TACN) with Mn(0Ac)3 2H20 results in the formation of the dinuclear complexes [Mn2in0(0Ac)2(HB(pz)3)2] (168) and [Mn/1 0(0Ac)2(TACN)2]2+ (169). Selected structural parameters are given in Table VII. The dimanganese cores of the complexes are essentially identical to those of some /i-oxo-di-/i-carboxylato diiron(III) complexes (177,178), which have been shown to be excellent structural models of the diiron site in methemerythrin (Fig. 8) (179). 

There is not much work thus far modeling the reaction chemistry of the dimanganese proteins more will undoubtedly be published in the near future. Dismukes and his co-workers (249) have found that [Mn2(HPTB)Cl3] decomposes H2O2 catalytically with an initial rate proportional to [H202] [complex], while mononuclear Mn(II) is ineffective. It is proposed that the reaction proceeds through the initial formation of a Mn "( x-0)... 

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