Alkali metal cations selective binding

There are two general classes of naturally-occurring antibiotics which influence the transport of alkali metal cations through natural and artificial membranes. The first category contains neutral macrocyclic species which usually bind potassium selectively over sodium. The second (non-cyclic) group contains monobasic acid functions which help render the alkaline metal complexes insoluble in water but soluble in non-polar solvents (Lauger, 1972 Painter Pressman, 1982). The present discussion will be restricted to (cyclic) examples from the first class. 

The 2-(AuC1)4 and 2-(PtCl2SMe2)4 complexes (see above), show extractability properties vs. alkali metal ions, with a greater affinity for than for other alkali metal ions [48]. No structural data were available and the nature of the binding in the formation of these complexes was not investigated. Similarly, the anionic complexes [2-Cu4(/t-Cl)4(/t3-Cl)] and [2-Ag4(/t-Cl)4(/t4-Cl)] have been shown to act as host for the selective binding of alkali metal cations and divalent metal ions like or Pb. Both complexes... 

The field, as we now know it, started with the selective binding of alkali metal cations by natural [1.21-1.23] as well as by synthetic macrocyclic and macropoly-cyclic ligands, the crown ethers [1.24,1.25] and the cryptands [1.26,1.27]. The out-... 

Examples such as this show the importance of alkali metal cation binding and transport in biochemistry, and a great deal of effort has been expended in supramolecular chemistry in attempts to understand natural cation binding and transport of the ionophore and channel type and to develop artificial systems capable of similar selectivities and reactivities. We will take a close look at many of these compounds in Chapter 3. 

Box 3.1 Selective binding of Alkali Metal Cations Properties of alkali metal cations ... 

A related reduced series (Figure 5.5) was also reported J74 Selectivity of these crowned arborols (100-102) towards alkali metal cation binding was examined and allosteric as well as conformational binding effects were studied. Also they showed that the sterically less crowded arborols (e.g., 102) are better for the dissolution of myoglobin in organic solvents, such as DMF. This example affords an interesting entrance to internal metal ion complexation at a specific loci see Section 8.3. 

Another class of macrocyclic molecules for binding with alkali metal cations is cahx[n]arene (n = 4,6,8) and its derivatives. They are made up of phenol and methylene units which provide a rigid arrangement of various number of oxygen donor atoms for metal complexation. Some of them are proved to display exceptional selectivity for K+ and Cs+ 131 Further details of this developing field will be found in Macrocyclic Ligands. 

The ditopic receptor 119 has been shown to bind both inorganic anions and alkali metal cations simultaneously (233). Selectivity was observed for HS04-over CP and the binding of alkali metal cations in the crown ethers was proven to substantially enhance the magnitude of anion binding (through electrostatic interaction). 

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