Diiron system

Recently, an IR spectroscopic study showed that the diiron system [111] corresponding to 128 does not undergo such photoisomerization, because this system under irradiation only generates the respective syw-biradical and not a triplet species necessary for the formation of a photoproduct corresponding to 130 [136]. Actual publications report on the application of the system for solar energy storage [137]. 

Two different types of zinc-porphyrins coordinated diiron complex act as catalysts for the photochemical reduction hydrogen evolution from water. In this system... 

Under conditions of copper deficiency, some methanotrophs can express a cytosolic, soluble form of MMO (sMMO) (20-23), the properties of which form the focus of the present review. The sMMO system comprises three separate protein components which have all been purified to homogeneity (24,25). The hydroxylase component, a 251 kD protein, contains two copies each of three subunits in an a 82y2 configuration. The a subunit of the hydroxylase houses the dinuclear iron center (26) responsible for dioxygen activation and for substrate hydroxylation (27). The 38.6 kD reductase contains flavin adenine dinucleotide (FAD) and Fe2S2 cofactors (28), which enable it to relay electrons from reduced nicotinamide adenine dinucleotide (NADH) to the diiron center in the... 

As indicated by the negative shifts in the reduction potentials of Hox, protein B can interact with the diiron center in Hmv from both MMO systems (63, 64). Consistent with this interpretation are EPR studies of Hmv from both organisms which indicate that, in the presence of protein B, the EPR signal moves from gav 1.83 to gav 1.75 (48, 66). 

Fig. 4. Substrate first binds to the complete system containing all three protein components. Addition of NADH next effects two-electron reduction of the hydroxylase from the oxidized Fe(III)Fe(III) to the fully reduced Fe(II)Fe(II) form, bypassing the inactive Fe(II)Fe(III) state. The fully reduced hydroxylase then reacts with dioxygen in a two-electron step to form the first known intermediate, a diiron(III) peroxo complex. The possibility that this species itself is sufficiently activated to carry out the hydroxylation reaction for some substrates cannot be ruled out. The peroxo intermediate is then converted to Q as shown in Fig. 3. Substrate reacts with Q, and product is released with concomitant formation of the diiron(III) form of the hydroxylase, which enters another cycle in the catalysis.

Physical studies of the hydroxylase have established the structural nature of the diiron core in its three oxidation states, Hox, Hmv, and Hred. Although the active site structures of hydroxylase from M. tri-chosporium OB3b and M. capsulatus (Bath) are similar, some important differences are observed for other features of the two MMO systems. The interactions with the other components, protein B and reductase, vary substantially. More structural information is necessary to understand how each of the components affects the others with respect to its physical properties and role in the hydroxylation mechanism and to reconcile the different properties seen in the two MMO systems. The kinetic behavior of intermediates in the hydroxylation reaction cycle and the physical parameters of intermediate Q appear similar. The reaction of Q with substrate, however, varies. The participation of radical intermediates is better established with the M. triehosporium... 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

A number of molecular mechanics studies of metal-cyclopentadienyl complexes have been reported recently. The systems studied include linear metallocenes (in particular ferrocene), ferrocene derivatives (such as complexes with substituted cy-clopentadienyl ligands, bis(fulvalene)diiron complexes, ferrocenophanes and mixed-ligand complexes with carbonyls and phosphines), and nonlinear cyclopentadienyl complexes 8,153,221 231]. 

FIGURE 2.4 Spin expectation values of the lowest spin level of the 5 = 2 system of Figure 2.3. The curve labeled (Sy) was obtained by applying B along the y-axis of the ZFS tensor. The break in the curve for (Sz) is due to level crossing for BZ = 4.6T, the first excited state becomes the ground state. This diagram is relevant for the diiron(IV) complex of Section. ... 

The relatively simple active site of [Fe]H2ase and the limited involvement of the protein as ligands in the first coordination sphere has appealed to computational chemists as an appropriate system to explore by Density Functional Theory (12,28-32). The calculations published to date have focused on correlating v(CO)/v(CN) vibrational frequencies of the different redox levels of the diiron active site with... 

Tabushi, I. Kugimiya, S. and Sasaki, T., 1985. Artificial allosteric systems. 3. Cooperative carbon monoxide binding to Diiron (II)Gable Porphyrin-Diimidazolylmethane complexes, J. Am. Chem. Soc., 107. 5159-5163. 

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