Ions, tetrahedral coordination

An unusual and unexpected feature of NOS is the presence of a Zn ion tetrahedrally coordinated to pairs of symmetry-related Cys residues along the dimer interface 81, 82) (Fig. 4). The original structure of the mouse iNOS dimer did not have the Zn but instead two symmetry-related Cys residues formed a disulfide bond 80). The structure of the human iNOS heme domain also had a disulfide, but the disulfide could readily be broken and Zn reconstituted to give a ZnS4 center indistinguishable from that found in eNOS (S3). An independent structure... 

Other multinuclear protein-zinc interactions may be implicated for the GAL4 protein in vivo. For example, since the GAL4 dimer binds to palindromic DNA sequences (Giniger et al, 1985), one possibility is that the dimer interface of the intact protein could comprise three zinc ions tetrahedrally coordinated by the 12 cysteines of two GAL4 monomers... 

The active site of rubredoxin (Rd), (1) (Figure 1), consists of one Fe ion tetrahedrally coordinated by four cysteine thiolate ligands. The iron core serves as the electron-transport site of rubredoxin proteins. 

In the crystal structure of garnet, with the general formula A3B2C3O12 [A = Ca, Fe +, Mg, Mn"+, R B = A1, Cr, Fe +, Mn +, Ti, V -", Zr C = Si (including H4 instead of Si) Al, Ti], there are three types of sites which are filled with metal ions tetrahedrally coordinated sites filled with C cations, octahedrally coordinated sites... 

Figure 1. Schematic drawing of smectite clay f) octahedrally coordinated ion ( tetrahedrally coordinated ion O oxygen, OH group.

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

Schematic representation of defect clusters in Fei- jO. The normal NaCl-type structure (a) has Fe (small open circles) and O (large dark circles) at alternate comers of the cube. In the 4 1 cluster (h), four octahedral Fe" sites are left vacant and an Fe" ion (grey) occupies the cube centre, thus being tetrahedrally coordinated by the 40. In (c) a more extended 13 4 cluster is shown in which, again, all anion sites are occupied but the 13 octahedral Fe sites are vacant and four Fe occupy a tetrahedral array of cube centres.

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

The distortion arises because the bidentate nitrate ions are coordinated asymmetrically to such an extent that the stereochemistry may alternatively be regarded as approaching tetrahedral (p. 1217). 

The acceleration by anions under both conditions was attributed to displacement of one of the water molecules presumed to be tetrahedrally coordinated with the mercuric ion, the subsequent reaction being then envisaged as displacement of the anion or water molecule by the aromatic the anions which cause reaction to take place more slowly were presumed to be more tightly bound to the mercuric ion than water. It has, however, been pointed out that less tightly bound anions would be unlikely to displace the more tightly bound water molecules in the first place438. 

In aqueous solution, water competes effectively with bromide ions for coordination to Cir+ ions. The hexaaquacopper(II) ion is the predominant species in solution. However, in the presence of a large concentration of bromide ions, the solution becomes deep violet. This violet color is due to the presence of the tetrabromocuprate(Il) ions, which are tetrahedral. This process is reversible, and so the solution becomes light blue again on dilution with water, (a) Write the formulas of the two complex ions of copper(II) that form, (b) Is the change in color from violet to blue on dilution expected Explain your reasoning. 

In this younger field of chemistry, some examples of anions may be still missing. One possible candidate is the [BN4] ion with a tetrahedral coordination of boron as in cubic BN, others may be adopted from oxoborates. Another interesting feature is the existence of structures with condensed nitridoborate anions derived from portions of BN structures. Until now the only example with a condensed anion structure is U(BN), containing kinked B-B bonded chains of (BN)x with B-B distances near 188 pm, slightly longer than in [B2N4] . ... 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

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