Dinuclear site analysis

The nature of the Ngose reaction is described with respect to electron donation, energy requirement, and reduction characteristics, with particular analysis of the seven classes of substrates reducible by N20se, a complex of a Mo-Fe and Fe protein. Chemical and physical characteristics of Fe protein and crystalline Mo-Fe protein are summarized. The two-site mechanism of electron activation and substrate complexation is further developed. Reduction may occur at a biological dinuclear site of Mo and Fe in which N2 is reduced to NH3 via enzyme-bound diimide and hydrazine. Unsolved problems of electron donors, ATP function, H2 evolution and electron donation, substrate reduction, N20se characteristics and mechanism, and metal roles are tabulated, Potential utilities of N2 fixation research include in-creased protein production and new chemistry of nitrogen. 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

The above results and analysis on the low A in mononuclear and dinuclear cupredoxins have also been supported by numerous structural analyses of both oxidized and reduced cupredoxins. Both X-ray crystallography and XAS " of the metal-binding sites in the cupredoxins clearly show minimal difference between the oxidized and reduced proteins. Finally, in addition to the unique geometry and valence delocalization discussed above, which affect inner-sphere reorganization energy, other factors that may influence outer-sphere reorganization energy may also play an important role in cupredoxins. The factors include exclusion of water or solvent from the copper center in the folded proteins. This factor rules out electron transfer as a putative function for the red copper protein nitrosocyanin because its copper center is solvent accessible. 

X-ray analysis of both copper(I) (e.g. 9) and copper(II) complexes, independent synthesis, and ammonolysis to the free ligands 35 and 36, confirmed the hydroxylation pathway.42 02-binding and hydroxylation in complex 10 were shown to be sensitive to electronic effects of the para-substituent (X = OMe, Me, C02Me, N02>. Tyrosinase, which contains a dinuclear copper active site strongly resembling the hemocyanine active site, binds O2 reversibly and activates O2 for arene hydroxylation (stoichiometry Cu O2 = 2 These are key features observed in the dinuclear copper complexes shown in... 

In addition to the monograph on the iron-sulphur proteins, there have been two reviews on this important class of macromolecule, and the lUPAC-IUB commission on biochemical nomenclature has published recommendations on their nomenclature. A method has been described for the interpretation of e.p.r. spectra of reduced dinuclear iron-sulphur proteins which will allow both the symmetry and the extent of covalency at the paramagnetic site to be parameterized. The parameters can then be related to the chemical composition of the paramagnetic centre, the protein-dependent charge delocalization of the unpaired electron, and the geometrical arrangement at the reduced iron atom - an analysis which, it is hoped, will ultimately be useful in rationalizing the redox behaviour of these important metalloproteins. 

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