Dinuclear site Energy

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. 

Combination of both of the above elements in a single molecule such as Os(CO)4(H)CH3 gives rise to facile dinuclear elimination. The dihydride is capable of dinuclear elimination but must rely on the comparatively hign-energy process of carbonyl dissociation to provide the necessary vacant coordination site. The dimethyl compound has the necessary vacant site easily available but no hydride to interact with it. The hydridomethyl compound has both elements and is uniquely unstable. 

Three Mn catalases have been purified and characterized, and all appear to have similar Mn structures (17). The Mn stoichiometry is ca. 2 Mn/subunit, suggesting a dinuclear Mn site. The optical spectrum of the as-isolated enzyme has a broad weak absorption band at ca. 450-550 nm in addition to the protein absorption at higher energies. This spectrum is similar to those observed for Mn(III) superoxide dismutase and for a variety of Mn(III) model complexes, thus implying that at least some of the Mn in Mn catalase is present as Mn(III). In particular, the absorption maximum at ca. 500 nm is similar in energy and intensity to the transitions seen for oxo-carboxylato-bridged Mn dimers, suggesting that a similar core structure may be seen for Mn catalase (18). 

Figure 1 Potential wells for an electron trapped by polarization of solvent or ligand environment in a dinuclear complex (A- B). V = electron potential energy, r = distance along the A-B axis, (a) Ground state p(A+- B) with electron localized mainly at site B, excited state s (A - B ) with electron mainly transferred to site A. (b) The energy wells adjusted to equal depth by solvent and ligand motion. The electron is delocalized with two energy levels separated by 2Hab- (c) The reverse polarization, ground state i(A- B ), excited state -B)...

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. 

We can thus draw a general conclusion the interaction of a M(bpy)2T component (where T is the bpy-type coordinating site of the tripod ligands 1 and 2 ) with any other (homo- or hetero-metallic) component which is present in the dinuclear and trinuclear supramolecular species is, at most, weak. As we will see later, the results obtained from electronic energy transfer experiments show that some electronic interaction does occur (at least between excited state and ground state components) and decreases with the increasing size of the spacer in the tripod ligand. 

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