Bound Fe ions

For example, the inverse spinel structure of magnetite (see Chap. 2) results from the fact that the CFSE of Fe is greater for octahedral than for tetrahedral coordination, so Fe preferentially occupies octahedral sites. For Fe the CFSE is zero for both octahedral and tetrahedral coordination, so that this ion has no preference for either type of coordination. 

Molecular orbital description of bonding in iron oxides 

Interactions between Fe and the oxide ions in the iron oxides have been described using the molecular orbital method. This approach regards the electrons as being subject to the influence of all the nuclei in the entity under consideration. The mole- 

1) The letters used to designate the single and multielectronic states are obtained using group theory (Figgis, 1966). 

These three types of clusters all involve one electron orbital. They provide a basis for the description of the d-d (i. e. ligand field) transitions and the ligand to metal charge transfer transitions which are responsible for most of the UV-visible spectra and opti- 

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FIGURE 15.27 The displaceiTient of the Fe ion of the heme of deoxymyoglobin from the plane of the porphyrin ring system by the pull of His F8. In oxymyoglobin, the bound O9 counteracts this effect. 

Solution Phase Free trace element ion > Soluble chelated complexes (inorganic / small organic molecular complexes) > Strongly complexed species (insoluble macroorganic molecular complexes) Solid Phase Exchangeable > Organically bound > Carbonate bound, Fe/Mn oxide bound... 

At this point it is not known if the proton taken up during reduction of Ni is still present. Note that the CN bands in the FTIR spectra do not shift, suggesting no changes in electron density on the Fe ion. The CO band, however, shifts 18 cm to higher frequency. Thus far there is no reasonable explanation for this behaviour we do not yet know where H2 (presumably as H /H+) is precisely bound. As mentioned above, in the presence of mediating dyes, an H2-producing reaction... 

Thus, we assume that Ni is trivalent in this state, as suggested earlier by other workers. We also assume that a hydride is bound at or near the Fe ion in this state. Upon illumination the bond between the hydride and the Fe is broken, whereby the... 

However, near-stoichiometric Fe " ion binding to NifU-1 or NifU was observable only in experiments conducted at 2°C in anaerobic samples that had been pretreated with dithiothreitol to ensure reduction of any intrasubunit or intersubunit disulfides. At room temperature, <10% of the NifU-1 or NifU was in a Fe bound form, and colorimetric analysis indicates that the remainder of the Fe is in solution was in the form of free Fe " ion. Hence this mononuclear Fe -bound species is more likely to be an intermediate in the reduction of Fe ion by NifU or NifU-1 rather than an initial step in cluster assembly on the NifU-1 domain of NifU. In this connection, it is important to note that Fe is rapidly reduced to Fe by cysteine in aqueous solution (Schubert, 1932). The physiological significance (if any) of the apparent ferric reductase activity associated with the NifU-1 domain of NifU remains to be established. 

Amine-terminated, full-generation PAMAM and PPI dendrimers, as well as carboxylate-terminated half-generation PAMAM dendrimers, can directly bind metal ions to their surfaces via coordination to the amine or acid functionality. A partial hst of metal ions that have been bound to these dendrimers in this way includes Na+, K+, Cs+, Rb+, Fe +, Fe +, Gd +, Cu+, Cu +, Ag+, Mn +, Pd, Zn, Co, Rh+,Ru +,andPt + [18,19,27,36,54,82-96]. Tuxro et al.have also shown that the metal ion complexes, such as tris(2,2 -bipyridine)ruthenium (Rulbpylj), can be attached to PAMAM dendrimer surfaces by electrostatic attraction [97]. A wide variety of other famihes of dendrimers have also been prepared that bind metal ions to their periphery. These have recently been reviewed [3]. Such surface-bound metal ions can be used to probe dendrimer structure using optical spectroscopy, mass spectrometry, and electron paramagnetic resonance (EPR) [86-88,90,97-99]. 

Many proteins, including many enzymes, contain hghtly bound metal ions. These may be inhmately involved in enzyme catalysis or may serve a purely structural role. The most common tightly bound metal ions found in metalloproteins include copper (Cu+ and Cu +), zinc (Zn +), iron (Fe + and Fe +), and manganese (Mn +). Other proteins may contain weakly bound metal ions that generally serve as modulators of enzyme activity. These include sodium (Na+), potassium (K+), calcium (Ca +), and magnesium (Mg +). There are also exotic cases for which enzymes may depend on nickel, selenium, molybdenum, or silicon for activity. These account for the very small requirements for these metals in the human diet. 

Metalloenrymes have tightly bound metal ions, such as Zn " or Fe ", that serve as metal ion bridges between the enzyme and substrate. 

The Fe-Fe interactions vary sensitively with the ratio of the radial extension of the n-bonding orbitals to the Fe-Fe separation. This ratio is significantly greater for the minority-spin electron at an Fe " ion than for the majority-spin electrons because the minority-spin electron of a high-spin 3 d configuration is more weakly bound to the iron nucleus (by some 3 eV for octahedral-site iron in rutile) than are the 3 d majority-spin electrons of the same symmetry. Therefore Fe-Fe interactions are strongest for the couple. 

Many aminopeptidases are metalloenzymes.437 Most studied is the cytosolic leucine aminopeptidase which acts rapidly on N-terminal leucine and removes other amino acids more slowly. Each of the subunits of the hexameric enzyme contains two divalent metal ions, one of which must be Zn2+ or Co2+438/439 A methionine aminopeptidase from E. colt contains two Co2+ ions440/441 and a proline-specific aminopeptidase from the same bacterium two Mn2+.442 In all of these enzymes the metal ions are present as dimetal pairs similar to those observed in phosphatases and discussed in Section D,4 and to the Fe-Fe pairs of hemerythrin and other diiron proteins (Fig. 16-20). A hydroxide ion that bridges the metal ions may serve as the nucleophile in the aminopeptidases.438 A bound bicarbonate ion may assist.4383... 

For CO dehydrogenase no crystal structure has been published. On the basis of a combination of spectroscopies an active-center structure has been proposed that features a [4Fe-4S] cubane bridged through an unknown ligand to a protein-bound nickel ion. This would seem to be a situation analogous to that discussed for sulfite reductase. On the basis of resonance Raman spectroscopic experiments it has been concluded that the substrate CO binds directly to one of the Fe ions of the cubane [54] however, this claim has recently been retracted [55], The reader is referred to Chapter 9 on nickel enzymes. 

The catalytic dismutation of superoxide is actually more complicated in E. coli [42] and B. thermophilus [43] Mn-SODs than that of either Cu or Fe proteins since it may involve an inactive form of the enzyme. The inactive form is believed [44] to contain a Mnm-side-on peroxo unit (of the type shown in Figure 29) formed within the hydrophobic environment of MnSOD, in the absence of H+, by the oxidative addition of the superoxide ion to the Mn11 center. When H+ ions are present, an active, end-on peroxo complex forms, yielding successively a bound hydroperoxide ion and free dihydrogen peroxide (cf. Figure 3). Thus, the key parameter that turns the reaction off or on may be the absence or presence of a H+ ion [44],... 

Metalloenzymes contain a bound metal ion as part of their structure. This ion can either partidpate directly in the catalysis, or stabilize the active conformation of the enzyme. In Lewis acid catalysis (typically with zinc, vanadium, and magnesium), the M"+ ion is used instead of H+. Many oxidoreductases use metal centers such as V, Mo, Co, and Fe in much the same way as homogeneous catalysis uses ligand-metal complexes. Figure 5.7 shows a simplified mechanism for the halide oxidation readion catalyzed by vanadium chloroperoxidase. The vanadium atom ads as a Lewis add, activating the bound peroxide [30]. 

An additional family of organometallic materials is the cyanometallates, which are Prussian blue analogues. These are microporous materials, similar to zeolites, with relatively large adsorption space and small access windows [237-241], These Prussian blue analogues develop zeolite-like structures based upon a simple cubic (T[M(CN)6]) framework, in which octahedral [M(CN)6]" complexes are linked via octahedrally coordinated, nitrogen-bound Tm+ ions [237], In the prototypic compound, that is, Prussian blue, specifically (Fe4[Fe(CN)6]3 14H20), charge balance with the Fe3+ ions conducts to vacancies at one-quarter of the [Fe(CN)6]4 complexes [242],... 

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