Metal sites

Due to a number of puzzling observations one might ask whether the iron sites in the two protomers of R2 are identical. The purified protein from E. coli normally contains lnl.2 Tyr and ca 3 Fe per R2 as opposed to the expected integer values of 2 and 4, respectively. The crystal structure indicates at least 70n80% occupancy at the iron sites. In a recent publica- 

There are two main problems which EXAFS can address in catalytic systems. Firstly in some species the local environment ie the ligand donor set. at the metal centre can be probed. Alternatively the morphology and atomic packing in metal particles 

1 Models for metal sites in supported catalysts. - An example of this is the use of some model polynuclear complexes in an investigation into the structure of Co-Mo/AI2O3 

By using ab initio phase shifts and back-scattering factors a spherical wave analysis of the W L(III) edge EXAFS of a salt 

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Rate of polymerization. The rate of polymerization for homogeneous systems closely resembles anionic polymerization. For heterogeneous systems the concentration of alkylated transition metal sites on the surface appears in the rate law. The latter depends on the particle size of the solid catalyst and may be complicated by sites of various degrees of activity. There is sometimes an inverse relationship between the degree of stereoregularity produced by a catalyst and the rate at which polymerization occurs. 

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

Fig. 9. Catalyst pore and reaction. The CO diffuses into a precious metal site D reacts with O2 and leaves as CO2.

Fig. 10. Catalyst macropores showing D noble metal sites and (a) narrowed micropores after exposure to high temperatures where H represents thermally damaged noble metal sites and (b) pore mouth plugging from poisons where A, if aUowed, diffuses in to be converted to B.

RH Holm, P Kennepohl, El Solomon. Structural and functional aspects of metal sites in biology. Chem Rev 96 2239-2341, 1996. 

RB Yelle, BW Beck, JB Koerner, CA Sacksteder, T Ichiye. Influence of the metal site on the structure and solvation of nibredoxm and its analogs A molecular dynamics study. Proteins accepted. 

Q Zeng, ET Smith, DM Kurtz, RA Scott. Protein determinants of metal site reduction potentials. Site directed mutagenesis studies of Clostridium pasteurianum laibredoxin. Inorg Chim Acta 242 245-251, 1996. 

In a sense, a superconductor is an insulator that has been doped (contains random defects in the metal oxide lattice). Some of the defects observed via neutron diffraction experiments include metal site substitutions or vacancies, and oxygen vacancies or interstituals (atomic locations between normal atom positions). Neutron diffraction experiments have been an indispensable tool for probing the presence of vacancies, substitutions, or interstituals because of the approximately equal scattering power of all atoms. 

FIGURE 21.19 The binuclear center of cytochrome oxidase. A ligand, L (probably a cysteine S), is shown bridging the Cng and Fe L metal sites. 

The radius of the 24-coordinate metal site in MBs is too large (215-225 pm) to be comfortably occupied by the later (smaller) lanthanide elements Ho, Er, Tm and Lu, and these form MB4 instead, where the metal site has a radius of 185-200 pm. The structure of MB4 (also formed by Ca, Y, Mo and W) consists of a tetragonal lattice formed by chains of Bs octahedra linked along the c-axis and joined laterally by pairs of B2 atoms in the xy plane so as to form a 3D skeleton with tunnels along the c-axis that are filled by metal atoms (Fig. 6.11). The pairs of boron atoms are thus surrounded by trigonal prisms of... 

TetrakisidithioacetatoivanadiumCIV) was originally classified as dodecahedral. Re-examination has shown that its unit cell in fact contains two independent metal sites. One is indeed dodecahedral but the other is square antiprismatic C. W. Haigh, Polyhedron 14, 2871-8 (1995). 

Azaferrocene reacts with aromatic hydrocarbons in the presence of aluminium chloride, giving rise to the cationic complexes of the type (Ti -arene)(Ti -cyclopenta-dienyl)iron(l+) isolated as BF4 salts [87JOM(333)71]. The complex 28 is obtained by reaction of the sulfane compound [Cp(SMc2)3Fe]BF4 with pentamethyl-pyrrole [88AG(E)579 88AG(E)1468 90ICA(170)155]. The metallic site in this center reveals expressed Lewis acidity (89CB1891). 

There is also evidence that the adsorbed gases may be charged this led Vlasenko and Uzefovich to develop a completely ionic mechanism (4) in which the bracket, [ ], represents a metallic site ... 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

Probably all adenylyl cyclases are inhibited competitively by substrate analogs, which bind at the site and to the enzyme configuration with which cation-ATP binds (cf Fig. 4). One of the best competitive inhibitors is (3-L-2, 3 -dideoxy adenosine-5 -triphosphate ( 3-L-2, 3 -dd-5 -ATP Table 4) [4], which allowed the identification of the two metal sites within the catalytic active site (cf Fig. 4) [3]. This ligand has also been labeled with 32P in the (3-phosphate and is a useful ligand for reversible, binding displacement assays of adenylyl cyclases [4]. The two inhibitors, 2, 5 -dd-3 -ATP and 3-L-2, 3 -dd-5 -ATP, are comparably potent... 

Two metal sites were identified in the structure solved of the chimeiic AC5Ci AC2C2 adenylyl cyclase in complex with 3-L-2, 3 -dd-5 -ATP (cf. Figs. 5 and 7) One is... 

Grafting from methods involve creation of metalated sites on a polymer backbone, and subsequent use of these sites to initiate the polymerization of a second monomer. Several examples of such grafting processes have been described 135 ,39>. However, no control of the number and length of the grafts is provided, since some sites may be inaccessible to the incoming monomer. As no accurate characterization... 

Similarly, boron stabilizes a ternary phase (Nioo.iBeo.37XBeo7Bo3)2, a MgCu2-type structure, by substitution of small metal atoms on a typical metal site. ... 

Figure 1. Relationship among the crystal structures of Re3B and Mo2lrB2. Full circles are atoms in 1/2 open circles are atoms in 0 the two metal sites in Re3B (8f and 4b) are differentiated by larger and smaller circles smallest circles are B atoms. The structure of Mo2lrB2 is generated by a shift of every second prism row of Rc3B (vector c/4).

The self-bonding tendency of B results in strong boron frameworks, which leads to predictions of defect structures that primarily involve changes in the fraction of the metal sites occupied. 

The size of the metallic site in the ThB4-type structure is relatively large (r = 1.91 X 10 pm for YB4) and accepts only large metal atoms, e.g., rare earths or actinides. Smaller metallic atoms, e.g.. Mg, Cr and Mn, form tetraborides having other structures described previously, in which the size of the metallic site is 1.58 X 10 pm for MgB4 and 1.32 X 10 pm for QB4. 

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