Binding metal cations

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

The recent detailed investigations on Zn(II) and Pb(II) have advanced our understanding of heavy metal cation binding to C-S-H. However there are still a number of important questions that require attention. We need to understand the relationship, if any between Ca Si ratio and metal uptake. We also need to understand how to interpret the sorption process, whether as a solid solution (Tommaseo Kersten 2002) or as precipitation within the C-S-H particles or as a sorption process suggested by Glasser (1993) to the surfaces of the crystalline domains. 

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. 

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. 

Fig. 16 Adaptation to shape change by component selection. Switching between W and U shapes of a component, under metal cation binding and removal, induces interconversion between polymer and macrocycle states with selection of the preferred component...

M. Albrecht, H. Rottele, P. Burger, Alkali-Metal Cation Binding by Self-Assembled Cryptand-Type Supermolecules , Chem. Em. J., 2, 1264 (1996)... 

The next two articles cover the chemistry of phosphorus containing macrocycles. The phosphorus containing cahxarenes have attracted much attention in recent years due to their various functions such as metal cations binding, catalysis, molecular recogination, and bioactivity. Likewise, other phosphorus-containing macrocycles, cryptands, and dendrimers find various uses in analytical chemistry and biochemistry. 

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