Anion exchangers local structure

Oxides are widely exploited as catalysts for the selective oxidation of hydrocarbons. They provide lattice oxygen in selective oxidation reactions and exchange it with oxygen gas (e.g. from air in the reactant stream). The periodic lattice oxygen loss for the hydrocarbon oxidation occurs because of reducing gases, despite the presence of gas phase oxygen in the reactant stream. This results in the formation of anion vacancies, local non-stoichiometry and defect structures as discussed in chapter 1. 

The function of spectrin superfamily proteins is particularly evident when taken in context of their cellular localization. They often form flexible links or structures that allow interactions with the cellular cyto-skeletal architecture and the membrane. In both spectrin and dystrophin, such a function is performed, but the spectrin repeats of these molecules are also able to interact with actin and contribute to binding. A portion of the dystrophin rod domain that spans residues 11-17 contains a number of basic repeats that allow a lateral interaction with filamentous actin (Rybakova et al., 2002). The homologous utrophin can also interact laterally with actin. This interaction is distinct from that of dystrophin, as the utrophin rod domain lacks the basic repeat cluster and associates with actin via the first ten spectrin repeats (Rybakova et al., 2002). /3-Spectrin also exhibits an extended contact with actin via the first spectrin repeat. In this situation, it was found that the extended contact increased the association of the adjacent ABD with actin (Li and Bennett, 1996). In conjunction with this interaction, it has been found that the second repeat is also required for maximal interaction with adducin (Li and Bennett, 1996), a protein localized at the spectrin-actin junction that is believed to contribute to the assembly of this structure in the membrane skeletal network (Gardner and Bennett, 1987). In the erythrocyte cytoskeletal lattice, /3-spectrin interacts with ankyrin, which in turn binds to the cytoplasmic domain of the membrane-associated anion exchanger. This indirect link to the cellular membrane occurs via repeat 15 of /3-spectrin (Kennedy et al., 1991) and is largely responsible for the attachment of the spectrin-actin network to the erythrocyte membrane (reviewed in Bennett and Baines, 2001). A much larger number of direct links to transmembrane proteins have been determined for the spectrin repeats of o-actinin (reviewed in Djinovic-Carugo et al, 2002). 

Okada and Haroda [25] studied the local structures of chloride and bromide within an anion-exchange resin by means of X-ray adsorption fine structure (XAFS). Water molecules are coordinated with the paired anion within the resin, up to an average hydration number of 3. However, the anions are not as highly hydrated within the resin as they are in the external solution. An average of 2.1 water molecules are stripped off in transfer of chloride from the bulk solution to the resin. An average of 2.6 water molecules are stripped off bromide in the same transfer. 

To explain the local structural variation of these mixtures, two factors can be proposed here (i) the balance of coulombic interactions between cation and anion and (ii) the number density of fluoride ion. From the first point, as shown in EXAFS results, Na+ makes the local structure stabilised around Th much more than Li" " does. Since the ionic radius of Na+ (0.99 A) is larger than that of Li" " (0.59 A) [13], Li" " can thus approach much closer to the fluorides coordinated around Th" +. This leads to a large exchange rate of fluoride around the Th" " " coordination sphere when Li+ is added to the mixture. To help the discussion from the second point, the number densities of all these mixtures are plotted in Figure 6.7.3a which was derived by MD. With decreasing concentration of ThF4, the number density decreases, ft means that the smaller number density... 

The most studied of all variables is the coimterion. Numerous workers " have shown that the counterion incorporated has a dramatic effect on the conductivity of the polymer. For a given counterion, the concentration employed also affects the conductivity of the resultant polymer. Maddison and Jenden even showed that counterion exchange after synthesis has an effect on the polymer conductivity. Conductivity decreases as the electron affinity of the counterion is increased. It has been reported that the degree of oxidation of the polymer does not vary appreciably as the coimterion is varied. The trend in conductivity is related to the nucleophilicity of the coimterion employed this may be due to some sort of anion-induced localization of the radical cation in the polymer. The effect of even slight changes in the molecular structure of the counterion on the conductivity of a range of PPys has been studied (Table 3.1). In some cases, the use of mixed counterion systems also has a marked effect on conductivity (Table 3.2). 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

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