Complementarity and Preorganization

It is not surprising that one ether oxygen should interact favorably with K The lone pairs on oxygen or, alternatively, the dipole associated with a C-O-C unit, will have a significant electrostatic attraction to a cation (see the ion-dipole interaction in Section 3.2.2). The same forces are involved when a simple salt dissolves in water. What, then, is special about the crown ether  

Several views of representative crown ethers binding alkali metal ions, along with two views of 18-crown-6. 

A key advance in the crown ether field was the development of a range of crowns, such as 12-crown-4,15-crown-5, etc. Table 4.1 shows a series of binding constants between various metals and crowns. Now the concept of complementarity is evident. There is a trend, but not a perfect one, that the smaller crowns have selectivity for the smaller cations, while the larger crowns target the larger cations preferentially. 

Using the Helical Structure of Peptides and the Complexation Power of Crowns to Create an Artificial Transmembrane Channel 

The predictable helical nature of many peptides leads to the possibility of using an a-helix as a scaffold to create chemical devices. One such device builds upon the molecular recognition properties of crown ethers discussed here. When every third amino acid in a 21 amino acid long peptide has an appended benzo-21-crown-7 entity, the crown ethers stack to form a column. This structure aligns within lipid bilayers spanning one side to another. The affinity of the crown ethers for K leads to conduction of this cation from one side of the membrane to the other via migration through the channel created by the stacked crowns. 

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From a practical viewpoint, extraction of (protonated) amines with crown ethers enjoys greater selectivity and efficacy than with pure active solvent, due to the complementarity and preorganization of binding sites. The applications of these advantages are now numerous. 

There are many examples of complementarity and preorganization. Essentially every article in this encyclopedia contains examples. After all, complementarity and preorganization are precepts to supramolecular chemistry, and thus, these concepts are integral to all supramolecular assemblies. 

Charts I, II, III and IV illustrate the kinds of hosts that have been designed and synthesized and the relationships between their structures and abilities to bind lithium, sodium, potassium and cesium picrates, respectively. The differences in structural recognition by a particular guest in complexing a variety of possible hosts are measured by the differences in the - AG values listed below each structure. For lithium picrate, the values range from >23 to <6 kcal mol for sodium picrate, from 19.2 to < 6 kcal mol" for potassium picrate, from 17.1 to < 6 kcal mol" for cesium picrate, from 13.1 to < 6 kcal mol The principles of complementarity and preorganization are illustrated repeatedly in the structure-binding relationships found in these charts [3]. Only a few relationships will be discussed. 

This discussion section first presents the concepts of complementarity and preorganization from a conceptual point of view. Subsequently, each of these concepts is elaborated upon in a tutorial fashion by the presentation of a series of classical and contemporary examples from the literature. Finally, we introduce some related concepts that arise from current research in supramolecular chemistry. 

Wadsoe, I. "Trends in Isothermal Microcalorimetry." Chem. Soc. Rev., 26,78-86 (1997). Complementarity and Preorganization... 

Enzymes possess binding sites that are complementary to their substrates using the same principles of complementarity and preorganization introduced for... 

These two properties (complementarity and preorganization) lead to great efficiency and selectivity by favouring the distribution of caesium cations over that of sodium cations. Table II summarizes some extraction results obtained with calixcrown compounds and classical crown ethers on simulating synthetic MLW solutions. Extraction experiments were performed by mixing equal volumes (5 to 7 mL) of aqueous and organic solutions in sealed tubes for an hour. Aliquots of each phase were analyzed by 7 spectrometry after centrifugation. The selectivity of the tested compounds towards caesium, in the presence of sodium, is expressed as the ratio of the distribution coefficients obtained separately for both cations ... 

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