Iron coordination

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

The crystallographic structure of rubredoxin from Clostridium pasteurianum at 2.5 A, a resolution sufficient to reveal the sequence of several of the bulky amino acid side chains, shows the iron coordinated to two pairs of cysteine residues located rather near the termini of the polypeptide chain (Fig. 1). A related rubredoxin, with a three times larger molecular weight, from Pseudomonas oleovorans is believed to bind iron in a similar fashion. This conclusion is based on physical probes, especially electron paramagnetic resonance spectroscopy, all of which indicate that the iron is in each case situated in a highly similar environment however, the proteins display some specificity in catalytic function. 

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

In this classical Haber-Weiss cycle iron is being reduced by superoxide anion radical (02T), ascorbic acid or glutathione and subsequently decomposes hydrogen peroxide - formed by spontaneous dismutation of 02T - in the Fenton reaction to produce 0H. This iron-driven 0H formation has a stringent requirement for an available iron coordination site, a sine qua non met not only by hexaaquoiron(III) but by most iron chelates (28). Thus, Fe-EDTA, -EGTA, and -ATP retain a reactive coordination site and catalyze the Haber-Weiss cycle. Phytic acid, however, occupies all available iron coordination sites and consequently fails to support 0H generation (Figure 6). 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

Not all cytochromes from sulfate-reducing bacteria reduce Fe(III) or other metals. D. vulgaris produces a cyt C553, which has a molecular mass of 9 kDa, midpoint redox potential of OmV, and a single heme and the iron atom is coordinated by histidine methionine. It is unclear at this time if the inability of this cyt C553 to reduce metals is due to lack of a bishistidinyl iron coordination or to some other factor, such as steric hinderance owing to orientation of heme in the protein. 

Fortunately, the structure of the fully reduced P. pantotrophus cytochrome cdi could be obtained, and this has provided important clues as to how the enzyme functions, as well as raising unexpected questions (12). First of all, the structure of the reduced enzyme reveals an nnprecedented switch of one axial ligand at the c heme from His to Met (Figs. 5a,b). The second important feature revealed by this structure was that the Tyr 25 had vacated the di heme iron coordination... 

Deoxvhemerythrin. Henrerythrin appears to retain its triply bridged crae structure in the deoxy form. A low (3.9 A) resoluticm difference electron density map of deoxyHr vs. metHr fi- n X-ray diffraction suggests that the iron atoms move slightly further apart in deoxyHr, but remain five and six coordinate, respectively (29). Confirmation of the iron coordination comes from near-IR absorption and circular dichroism spectra (40,46,47). Based on model high-spin ferrous complexes, the six coordinate iron is expected to give two of the three observed transitions near 10000 cm- while the five-coordinate iron accounts for the d-d transition at ca. 5000 cm-. ... 

Fig. 4. The structure of the cation of 4B, [Fe402(BICOH)2(BICO)2 (02CPh)4]showing the 40% probability thermal ellipsoids. Only the first atom of the phenyl rings is shown. BICOH stands for bis(N-methylimldazol-2-yl)carbinol. BICO is the alkoxide anion, (a) The Fe402 core iron coordination spheres and bridging benzoate groups. Continued on next page.

Fig. 6. The structure of the anion of 4D, [Fe402(OH)2(5-MeHXTA)2) . 5-MeHXTA stands for N,N -(2-hydroxy-5-methyl l,3-xylylene)bis(N-(carboxymethyl) glycine). Only the Iron coordination spheres are shown. (Reproduced from Ref. 10a. Copyright 1988 American Chemical Society.)...

There are four naturally occurring isotopes of iron ( Fe 5.82%, Fe 91.66%, Fe 2.19%, Fe 0.33%), and nine others are known. The most abundant isotope ( Fe) is the most stable nuclear configuration of all the elements in terms of nuclear binding energy per nucleon. This stability, in terms of nuclear equilibrium established in the last moments of supernova events, explains the widespread occurrence of iron in the cosmos. The isotope Fe has practical applications, most notably in Mossbauer spectroscopy, which has been widely exploited to characterize iron coordination complexes. 

Selected entries from Methods in Enzymology [vol, page(s)] Application in fluorescence, 240, 734, 736, 757 convolution, 240, 490-491 in NMR [discrete transform, 239, 319-322 inverse transform, 239, 208, 259 multinuclear multidimensional NMR, 239, 71-73 shift theorem, 239, 210 time-domain shape functions, 239, 208-209] FT infrared spectroscopy [iron-coordinated CO, in difference spectrum of photolyzed carbonmonoxymyo-globin, 232, 186-187 for fatty acyl ester determination in small cell samples, 233, 311-313 myoglobin conformational substrates, 232, 186-187]. 

Applications in infrared spectroscopy A and B bands, iron-coordinated CO, 232, 186-187 application to allosteric mechanisms, 249, 566 bacteriorhodopsin, 246, 9, 380-381 caged compounds, 246, 6, 520-521 DNA [base pair formation, 246, 506 conformation, 246, 506-507 denaturation thermodynamics, 246, 506 ligand interactions, 246, 6, 507 sample requirements, 246, 506] fatty acyl ester determination in small cell samples, 233,... 

The high specificity of siderophore iron coordination has been extensively explored in iron-chelation therapy for various medical applications, including iron overload diseases, control of iron in specific brain tissues , arresting the growth and proliferation of malaria parasite within their host , as well as arresting the proliferation of cancer cells . Other directions for metal ligation involve enzyme inhibition, which have been demonstrated by the inhibition of urease by coordination of hydroxamate ligand to nickel ions and zinc coordination in matrix metalloprotease (MMP) inhibition by primary hydroxamates. ... 

The close proximity of the drug to the iron coordination site, and hence to the receptor recognition domain, casts some doubts as to the generality of this approach and to the scope of utilizable drugs. 

Electrophilic aromatic substitution of 708 with the iron-coordinated cation 602 afforded the iron-complex 714 quantitatively. The iron-mediated quinone imine cyclization of complex 714, by sequential application of two, differently activated, manganese dioxide reagents, provided the iron-coordinated 4b,8a-dihydrocarbazole-3-one 716. Demetalation of the iron complex 716 with concomitant... 

Non-heme iron is defined as iron coordinated by ligands other than... 

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