Host structure

Preparation of biohybrid enzyme-inorganic materials imposes strict chemical requirements in order that the chemical conditions of the biohybrid preparation respect the preservation of the biomolecule integrity. Knowing that enzymes are very 

Material class Materials Chemical formula Framework charge Electronic prop Ionic Exch. prop p.z.c Ref 

2-D layered double hydroxides [Mn1 xMIII40H)2][X 7,- nH20] + insulating anion variable [23] 

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For the alkali metal doped Cgo compounds, charge transfer of one electron per M atom to the Cgo molecule occurs, resulting in M+ ions at the tetrahedral and/or octahedral symmetry interstices of the cubic Cgo host structure. For the composition MaCgg, the resulting metallic crystal has basically the fee structure (see Fig. 2). Within this structure the alkali metal ions can sit on either tetragonal symmetry (1/4,1/4,1/4) sites, which are twice as numerous as the octahedral (l/2,0,0) sites (referenced to a simple cubic coordinate system). The electron-poor alkali metal ions tend to lie adjacent to a C=C double... 

It should be noted that the host structures were prepared from the simple monoazacrowns by acylating with a bis-acid chloride followed by hydride reduction. 

Intercalation in MWCNT should be similar to intercalation in HOPG or fibres. This means that one would expect in helical samples that the intercalate would be introduced between the layers with a dramatic increase of its diameter. For concentric CNTs, it would require a large amount of defects for intercalation to occur between the planes and an important distortion of the host structure. 

Lithium-ion cells operate during charge and discharge by a mechanism that involves the electrochemical insertion of lithium into, and extraction from, positive and negative electrode host structures. For example, in the well known Li tC6 / Li, tCo02 system, which is assembled in the discharged state, lithium ions are extracted from the metal oxide structure and... 

Interstitial Solid Solutions Interstitial solid solutions involve occupation of a site by introduced ions or atoms, which is normally empty in the crystal structure, and no ions or atoms are left out. Many metals form interstitial solid solutions in which small atoms (e.g., hydrogen, carbon, boron, nitrogen) enter empty interstitial sites within the host structure of the metal. Palladium metal is well known for its ability to absorb an enormous volume of hydrogen gas, and the product hydride is an interstitial sohd solution of formula PdH, 0 0.7, in which hydrogen atoms occupy... 

Cation Vacancies If the cation of the host structure has a lower charge than the cation that is replacing it, cation vacancies may be introduced for the preservation of electroneutrality. Alternatively, the substitution of an anion by one of lower charge may also achieve this in certain systems. For example, NaCl is able to dissolve a small amount of CaCl2, and the mechanism of solid-solution formation involves the replacement of two Na+ ions by one Ca ion, leaving one vacancy on the Na" sublattice, Nai 2xCa Cl (where x denotes a vacancy). 

Anion Vacancies If the cation of the host structure has a higher charge than the replacing cation, electroneutrality may be maintained by introducing vacancies into the anion sublattice. The best-known examples of anion vacancies occur in the stabilized zirconia, such as calcium- or yttrium-stabilized zirconia. The high-temperature... 

At present, intercalation compounds are used widely in various electrochemical devices (batteries, fuel cells, electrochromic devices, etc.). At the same time, many fundamental problems in this field do not yet have an explanation (e.g., the influence of ion solvation, the influence of defects in the host structure and/or in the host stoichiometry on the kinetic and thermodynamic properties of intercalation compounds). Optimization of the host stoichiometry of high-voltage intercalation compounds into oxide host materials is of prime importance for their practical application. Intercalation processes into organic polymer host materials are discussed in Chapter 26. 

In 1979 the bieyclic diol exo-2,ejco-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (i) was prepared and observed to co-crystallise with various solvents, including ethyl acetate, chloroform, toluene, dioxane, and acetone. A crystal structure determination of the ethyl acetate compound revealed the occurrence of a helical canal host structure, containing ethyl acetate as guest (with 3 1 diol ethyl acetate stoichiometry), and that spontaneous resolution had occurred on crystallisation of the multimolecular inclusion compound 6>. 

The melting point of the empty host crystals is 189-191 °C. Therefore it was apparent that this is an independently stable host structure containing large yet chemically inert guest canals, in which guest species cannot interfere with the strong hydrogen bonds which maintain the host network. 

Yet another type of canal structure has been reported for the urea inclusion compound of 1,4-dichlorobutane 51). Even though the canals are pseudo-hexagonal in dimension, there is a significant difference in their symmetry. The host lattice is orthorhombic, space group Pbcn. The difference lies in the directions of the six pseudo-3j helices of host molecules around the walls of each canal the sequence is cyc/o-RRRLLL, as opposed to cyc/o-RRRRRR in the hexagonal inclusion compounds and cvc/o-RLRLRL in the rhombohedral. This orthorhombic host structure probably occurs also in the urea inclusion compounds with 1,5-dichloropentane and 1,6-dibromohexane 51). 

Fig, 11. Diagrammatic representation of a planar intercalating guest molecule complexed between adjacent base pairs of the double helical DNA host structure. The base pairs and intercalator are represented by stippled rods. Note the increased base pair separation caused by complexation with the guest. 

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

A series of enzyme and proteins (met-myoglobin, lysozyme, met-hemoglobin, glucose oxidase, a-chymotrypsin) was also immobilized in a-ZrP by Kumar et al. [134]. Binding constant values clearly confirm the high affinity of the various proteins with the host structure (Table 15.4). 

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