Metal cation specific carriers

Computer-Aided Design of Metal Cation-Specific Carriers... 

Thus effective combinations of carrier chemistry with computer chemistry have established a useful guideline for the design of new, metal cation-specific carriers. We have many options in the selection of carrier structures from a variety of acyclic podands, cyclic crown ethers, bicyclic cryptands and related derivatives. When we adjust a carrier structure for a target guest, we can consult the computer and decide on the proper carrier structure in a non-empirical fashion. Although there are still many problems to be resolved, computer chemistry can be a good partner with carrier chemistry. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

Other areas of specific interest include the role of the IA cations as charge carriers in the transmission of nerve impulses, as stabilizers of a variety of structures, and as activators of a large number of enzymes. The first topic is outside the scope of this chapter, although progress is being made in the study of metal-selective channels in nerve cells. 

Kirch and Lehn have studied selective alkali metal transport through a liquid membrane using [2.2.2], [3.2.2], [3.3.3], and [2.2.C8] (146, 150). Various cryptated alkali metal picrates were transported from an in to an out aqueous phase through a bulk liquid chloroform membrane. While carrier cation pairs which form very stable complexes display efficient extraction of the salt into the organic phase, the relative rates of cation transport were not proportional to extraction efficiency and complex stability (in contrast to antibiotic-mediated transport across a bulk liquid membrane). Thus it is [2.2.Ca] which functions as a specific potassium ion carrier, while [2.2.2] is a specific potassium ion receptor (Table VI). 

In looking at the various physical properties of these cations (cf. Table 1) one realizes especially the differences in size, which manifest themselves clearly in energy parameters. However, the effective sizes of the solvated ions — i. e. the Stokes radii — apart from showing a reversed order do not differ greatly. Any simple model based on Brownian motion of totally solvated metal ions under the influence of an electric field gradient could not explain pronounced specificities of electric transport across membranes. In realizing this fact physiologists proposed the idea of a carrier, which via its peculiar molecular architecture could specifically interact with an ion of a... 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

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