Fe coordinated

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

The active-site Fe ion also eluded detection hy either EPR (9) (and references therein) or Mdsshauer (88, 89) spectroscopy, suggesting that it is low spin and diamagnetic at least in those states that have been studied. This is consistent with the presence of one CO and two CN s, which are strong ligands, in the active-site Fe coordination... 

When one of the Fe-coordinating Ns of the porphyrin is made inequivalent to the others, for example, by pulling on it, or by putting a protein structure around the cofactor, then the molecular x axis and y axis become inequivalent, and the axial EPR spectrum turns into the rhombic spectrum in trace d with derivative trace e (see also Table 5.4). There are now three features in the spectrum a peak, a zero crossing, and a negative peak, and their field positions closely (exactly for zero linewidth) correspond to those of the g-values, gx, gy, and gz. Finally, in trace f of Figure 5.4, which is the experimental X-band spectrum of cytochrome c, it can be seen that not only the g-value (peak position) but also the linewidth is frequently found to be anisotropic. This extra complication will be discussed extensively in Chapter 9. 

As the electron orbitals of the isolated Fe coordination polyhedra must be converted to the multielectron states found in the actual oxides,... 

The situation becomes more complicated, however, when each oxygen belonging to an Fe coordination polyhedron is bound to different populations of next-nearest cations in their own coordination sites as a result of extensive atomic substitution of Fe2, Fe3, Ti4, etc., for Mg and Al. Such chemical and bonding variations for different mineral structures and the variability of inductive effects from adjacent cations are probably sufficiently strong to account for large ranges of isomer shifts exhibited within a specific coordination symmetry in silicate minerals. 

Before describing the electronic structures of Fe coordination sites, it is worthwhile to first outline the crystal chemistry of clay minerals and describe the kinds of coordination environments that Fe3 and Fe2 cations may enter. 

Nicolet Y, de Lacey AL, Vernede X, Fernandez VM, Hatchikian EC, Fontecilla-Camps JC. Crystallographic and FTIR spectroscopic evidence of changes in Fe coordination upon reduction of the active site of the Fe-only hydrogenase from Desulfovibrio desulfur-icans. J Am Chem Soc. 2001 123(8) 1596-601. 

Fe-4S] clusters, all with cysteinyl-S completing tetrahedral Fe coordination (Figure 1) and the function was exclusively electron transport. New types of electron transfer Fe-S clusters involving cubane-type [3Fe S] clusters and double-cubane-type [8Fe-7S] clusters have subsequently been characterized (Figure 1). Moreover, the known functions of Fe-S centers have now expanded to include coupling... 

Sherman, D. M. (1985a). The electronic structures of Fe + coordination sites in iron oxides applications to spectra, bonding, and magnetism. Phys. Chem. Mineral. 12, 161-75. 

The coordination of transition metals is known to influence the keto-enol tautomerism in the condensed phase" . The effect of coordination of bare Fe+ ions on the keto-enol equilibrium of phenol was investigated by means of generation of various cyclic [Fe,Cg, He, 0]+-isomers. These isomers were characterized by collisional activation (CA) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry" . It was shown that the energy difference between the phenol-iron complex 65 and the keto isomer 66 is not perturbed by the presence of the iron cation in comparison with the uncom-plexed isomers 3 and 4 (equation 25). Thus, the energy difference for both the neutral and the Fe+-coordinated systems amounts to ca 30 kJ moC in favor of the phenolic tautomer. 

Montmorillonite, a bentonite sample finm Linden/Bavaria, was used for the introduction of the binuclear complexes by cation exchange as previously reported [7]. The complexes can be removed fi om the interlayers only by highly concentrated solutions of NaC104 or KCIO4. As inferred fi om TGA when the clay sample is heated in an Ar/02 atmosphere, the Fe coordinated nitrate is removed between 160 and 260°C and the ligand is cracked between 350 and 450°C. 

Fe to shift into the ring center. The proximal histidine in the Fe coordination sphere (see Fig, lb) also moves and acts as a means of communicating between sub-units. Another feature of hemoglobin is designed to lessen its affinity for other ligands. The position of the distal histidine shown in Fig. lb forces coordination in a nonlinear geometry. This is favorable for 02 but not for strongly... 

Myoglobin contains a heme prosthetic group, which binds oxygen. Heme consists of a single atom of Fe coordinated within a porphyrin. Oxygen binds to myoglobin reversibly this... 

All three mechanisms begin with initial coordination of the substrate to the divalent ion. Pathway a shows nucleophilic attack by the Fe -coordinated hydroxide. Another proposed pathway is subsequent bidentate coordination, followed by nucleophilic attack of the bridging hydroxide (pathway b). Subsequent addition of water then gives the bidentately coordinated product shown. A third possibility is a reaction of the mono-dentately coordinated substrate in which the Fe " -bound hydroxide acts as a general base for nucleophilic attack by water (pathway c). 

Scheme 3 Three possible roles for metal-assisted phosphoryl transfer to water by PAPs." In (a), a Fe +-coordinated hydroxide is the nucleophile. In (b), a bridging hydroxide (or oxide) is the nucleophile, forming an intermediate that is subsequently opened by attack of water. In (c), a Fe -coordinated hydroxide acts as a general base to facilitate attack by water.

The deprotonation of the nucleophile is believed to be responsible for pA i in the pH profde. The identity of the divalent metal ion affects pA"ai, consistent with the proposal that the nucleophilic hydroxide bridges the metals, as in pathway b of Scheme 3. An electron paramagnetic resonance (EPR) study with the Fe " —Zn form of bovine spleen PAP showed that the spectroscopic characteristics of the Fe ion are unaffected by the addition of phosphate at pH 6.5, the optimal pH for suggesting that phosphate binds only to the spectroscopically silent Zn. In contrast, phosphate does perturb the Fe site at lower pH, suggesting that the mode of phosphate binding is pH dependent. At lower pH, the Fe-coordinated ligand would be the more easily displaced water rather than hydroxide, thus facilitating the conversion of the monodentate into the bidentate-bound phosphate, such as shown in the first step of pathway b in Scheme 3. 

Figure 17-11 Two-step mechanism for phosphate ester hydrolysis. In the first step, substrate binds to the Fe ion, thereby displacing the bond from the -hydroxo ligand to this metal ion. The resulting Fe -coordinated hydroxide serves as the nucleophile in the hydrolysis reaction. The resulting species is a phosphate bridge, the same species observed in x-rays studies of product-inhibited enzyme.

While Fe(SCys)4, [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters all function as one-electron donors or acceptors, the more complex double-cubane [8Fe-7S] cluster that is found only in nitrogenases (see Nitrogenase Catalysis Assembly) has the potential to mediate two-electron transfer processes.Three methods have been employed to functionalize Fe-S centers for substrate binding and activation. The first involves having an accessible Fe coordination site as in the mononuclear Fe centers of nitrile hydratase and SOR, and the [4Fe-4S] clusters at the active sites of hydratases/dehydratases and radical-5-adenosylmethionine (SAM) enzymes.Indeed the recent recognition of the importance of the superfamily of radical-SAM enzymes in initiating radical reactions, via cluster-mediated reductive cleavage of SAM to yield a... 

Fe coordination sphere. For myoglobin, the Fe weighted density of states peaks near 30 and qualitatively... 

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